what does the presentation layer of the osi model do

  Layer 6 Presentation Layer

De/Encryption, Encoding, String representation

The presentation layer (data presentation layer, data provision level) sets the system-dependent representation of the data (for example, ASCII, EBCDIC) into an independent form, enabling the syntactically correct data exchange between different systems. Also, functions such as data compression and encryption are guaranteed that data to be sent by the application layer of a system that can be read by the application layer of another system to the layer 6. The presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable for both systems data format, the ASN.1 (Abstract Syntax Notation One) used.

OSI Layer 6 - Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer. In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format. Wikipedia

• Data conversion
• Character code translation
• Compression
• Encryption and Decryption

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

SASE specific application service element

Layer 7   Application Layer

Layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

The OSI Model – The 7 Layers of Networking Explained in Plain English

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

• Basic familiarity with common networking terms (explained below)
• A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

• What the OSI model is
• The purpose of each of the 7 layers
• The problems that can happen at each of the 7 layers
• The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

• Please | Physical Layer
• Do | Data Link Layer
• Not | Network Layer
• Tell (the) | Transport Layer
• Secret | Session Layer
• Password (to) | Presentation Layer
• Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

• Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
• Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
• Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
• Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
• Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
• Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
• Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

• Defunct cables, for example damaged wires or broken connectors
• Broken hardware network devices, for example damaged circuits
• Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Flow control. How much data should be transmitted?
• Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

• Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
• Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

• Header: typically includes MAC addresses for the source and destination nodes.
• Body: consists of the bits being transmitted.
• Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

• All the problems that can occur on Layer 1
• Unsuccessful connections (sessions) between two nodes
• Sessions that are successfully established but intermittently fail
• Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

• All the problems that can crop up on previous layers :)
• Faulty or non-functional router or other node
• IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

• Blocked ports - check your Access Control Lists (ACL) & firewalls
• Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

• Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
• Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

• Servers are unavailable
• Servers are incorrectly configured, for example Apache or PHP configs
• Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

• American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
• Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
• Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

• Non-existent or corrupted drivers
• Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

• All issues on previous layers
• Incorrectly configured software applications
• User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

• What is the OSI model?
• What are each of the layers?
• How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
• Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
• The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
• OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

Read more posts .

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What is the OSI model? How to explain and remember its 7 layers

A tutorial on the open systems interconnection (osi) networking reference model plus tips on how to memorize the seven layers..

The Open Systems Interconnect (OSI) model is a conceptual framework that describes networking or telecommunications systems as seven layers, each with its own function.

The layers help network pros visualize what is going on within their networks and can help network managers narrow down problems (is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Tech vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”.

The 7 layers of the OSI model

The layers (from bottom to top) are: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

It wasn’t always this way. Conceived in the 1970s when computer networking was taking off, two separate models were merged in 1983 and published in 1984 to create the OSI model that most people are familiar with today. Most descriptions of the OSI model go from top to bottom, with the numbers going from Layer 7 down to Layer 1.

The layers, and what they represent, are as follows:

Layer 7: Application

The Application Layer in the OSI model is the layer that is the “closest to the end user”. It receives information directly from users and displays incoming data to the user. Oddly enough, applications themselves do not reside at the application layer. Instead the layer facilitates communication through lower layers in order to establish connections with applications at the other end. Web browsers (Google Chrome, Firefox, Safari, etc.) TelNet, and FTP, are examples of communications that rely on Layer 7.

Layer 6: Presentation

The Presentation Layer represents the area that is independent of data representation at the application layer. In general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission; this happens at Layer 6.

Layer 5: Session

When two computers or other networked devices need to speak with one another, a session needs to be created, and this is done at the Session Layer . Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

Layer 4: Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

Layer 3: Network

Here at the Network Layer is where you’ll find most of the router functionality that most networking professionals care about and love. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers . You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2: Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sublayers exist here as well–the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2. But it’s not that simple. Some switches also operate at Layer 3 in order to support virtual LANs that may span more than one switch subnet, which requires routing capabilities.

Layer 1: Physical

At the bottom of our OSI model we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in a Wi-Fi network), as well as the layout of pins, voltages, and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example.

Why you need to know the 7 OSI layers

Most people in IT will likely need to know about the different layers when they’re going for their certifications, much like a civics student needs to learn about the three branches of the US government. After that, you hear about the OSI model when vendors are making pitches about which layers their products work with.

In a Quora post  asking about the purpose of the OSI model, Vikram Kumar answered this way: “The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.”

While some people may argue that the OSI model is obsolete (due to its conceptual nature) and less important than the four layers of the TCP/IP model, Kumar says that “it is difficult to read about networking technology today without seeing references to the OSI model and its layers, because the model’s structure helps to frame discussions of protocols and contrast various technologies.”

If you can understand the OSI model and its layers, you can also then understand which protocols and devices can interoperate with each other when new technologies are developed and explained.

The OSI model remains relevant

In a post on GeeksforGeeks, contributor Vabhav Bilotia argues several reasons why the OSI model remains relevant, especially when it comes to security and determining where technical risks and vulnerabilities may exist.

For example, by understanding the different layers, enterprise security teams can identify and classify physical access, where the data is sitting, and provide an inventory of the applications that employees use to access data and resources.

“Knowing where the majority of your company’s data is held, whether on-premises or in cloud services, will help define your information security policy,” writes Bilotia. “You can invest in the correct solutions that provide you data visibility within the proper OSI layers once you have this knowledge.”

In addition, the OSI model can be used to understand cloud infrastructure migrations, particularly when it comes to securing data within the cloud.

And because the model has been around for so long and understood by so many, the uniform vocabulary and terms helps networking professionals understand quickly about the components of the networking system “While this paradigm is not directly implemented in today’s TCP/IP networks, it is a useful conceptual model for relating multiple technologies to one another and implementing the appropriate technology in the appropriate way,” Bilotia writes. We couldn’t agree more.

How to remember the OSI Model 7 layers: 8 mnemonic tricks

If you need to memorize the layers for a college or certification test, here are a few sentences to help remember them in order. The first letter of each word is the same as the first letter an OSI layer.

From Application to Physical (Layer 7 to Layer 1): 

• All People Seem To Need Data Processing
• All Pros Search Top Notch Donut Places
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The first gadget Keith Shaw ever wanted was the Merlin, a red plastic toy that beeped and played Tic-Tac-Toe and various other games. A child of the '70s and teenager of the '80s, Shaw has been a fan of computers, technology and video games right from the start. He won an award in 8th grade for programming a game on the school's only computer, and saved his allowance to buy an Atari 2600.

Shaw has a bachelor's degree in newspaper journalism from Syracuse University and has worked at a variety of newspapers in New York, Florida and Massachusetts, as well as Computerworld and Network World. He won an award from the American Society of Business Publication Editors for a 2003 article on anti-spam testing, and a Gold Award in their 2010 Digital Awards Competition for the "ABCs of IT" video series.

Shaw is also the co-creator of taquitos.net , the crunchiest site on the InterWeb, which has taste-tested and reviewed more than 4,000 varieties of snack foods.

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The OSI Model’s 7 Layers, Explained

The seven layers in the Open Systems Interconnection (OSI) model each serve a specific function and work together to create an efficient network communication system.

The Open Systems Interconnection (OSI) model is a framework in network communication that simplifies complex network interactions into a structured format. 

What Is the OSI Model?

The Open Systems Interconnection model is a framework in network communication designed to simplify complex network interactions into a structured format. This architecture has seven layers, each of which serves a specific function. All seven layers work together to create a robust and efficient network communication system.

Each of its seven layers has a distinct role, ensuring efficient data transfer from one device to another . The OSI model is essential for understanding how data is transmitted in a network and is also a practical guide for network protocol design and problem solving.

learn more about cybersecurity An Introduction to Microsegmentation in Network Security

The OSI model, developed by the International Organization for Standardization , outlines the essential functions of networking and telecommunications systems for practical application. It plays a crucial role in telecommunications, where vendors use it to define the features and capabilities of their products and services.

This approach allows for a detailed explanation of different aspects of network communication, including transport protocols, addressing schemes and data packaging methods. As a result, the OSI model resolves the complexities of network communication and fosters a more integrated and coherent digital world .

The 7 Layers of the OSI Model

Each layer of the OSI model serves a specific function, yet they work in harmony to create a robust and efficient network communication system. Understanding these layers provides valuable insights into the complexities of network design and operation, showcasing the intricate nature of modern digital communication.  

Layer 7: Application Layer

Functionality: The Application Layer is the closest to the end user. It facilitates user interaction with networked systems, providing interfaces and protocols for web browsers, email clients and other applications.

Key protocols: Protocols like HTTP, FTP and SMTP operate at this layer, enabling services such as web browsing, file transfers and email communications.

Layer 6: Presentation Layer

Role: The Presentation Layer acts as a translator, converting data formats from the application layer into a network-compatible format and vice versa. It ensures that data sent from one system is readable by another.

Data formatting: This layer is responsible for data encryption and compression, playing a significant role in maintaining data privacy and efficient transmission.

Layer 5: Session Layer

Managing sessions: It establishes, manages and terminates sessions between applications. This layer ensures that sessions are maintained for the duration of the communication.

Coordination: The Session Layer coordinates communication between systems, managing dialogues and synchronizing data exchange.

Layer 4: Transport Layer

Data segmentation and control: The Transport Layer is crucial for segmenting data into smaller packets. It ensures end-to-end data integrity and delivery, managing flow control, error correction and sequencing.

Protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are key protocols in this layer, differing in their approach to data transmission.

Layer 3: Network Layer

Routing and addressing: This layer is responsible for logical addressing and routing data packets across different networks. It determines the best path for data to travel from source to destination.

Internet protocol: The Internet Protocol (IP), fundamental for internet data exchange, operates at this layer.

Layer 2: Data Link Layer

Framing and MAC addressing: The Data Link Layer frames data into packets. It handles physical addressing through MAC addresses, ensuring that data is directed to the correct hardware.

Error detection: This layer is also involved in error detection and handling, improving overall data transmission reliability.

Layer 1: Physical Layer

Physical transmission: The Physical Layer deals with the physical aspects of data transmission, including cable types, electrical signals and data rates.

Hardware components: It involves hardware components like cables, switches and network interface cards, forming the foundation of network communication.

How Data Flows in the OSI Model

Understanding this data flow process is crucial for professionals, as it aids in diagnosing and troubleshooting network issues, designing efficient network solutions and ensuring robust data security and management.

Encapsulation Process

When data is sent, it begins at the Application Layer and moves down through the layers. At each stage, it is encapsulated with the necessary headers, trailers, and other control information relevant to that layer. For instance, at the Transport Layer, data is segmented and encapsulated with port numbers, while at the Network Layer, IP addresses are added.

Each layer plays a role in preparing the data for transmission. The Presentation Layer may encrypt the data for security, while the Data Link Layer ensures it is formatted into frames suitable for physical transmission.

Data Transmission Across the Network

The Physical Layer transmits the raw bits over a physical medium, such as a cable or wireless network. This transmission is the actual movement of data across the network. In cases where data must move across different networks, the Network Layer’s routing functionalities become crucial. It ensures that data packets find the most efficient path to their destination.

Decapsulation Process

Upon reaching the destination, the data moves up the OSI model, with each layer removing its respective encapsulation. The Data Link Layer, for instance, removes framing, and the Transport Layer checks for transmission errors and reassembles the data segments. Once the data reaches the Application Layer, it is in its original format and ready to be used by the receiving application, whether it’s an email client, a web browser or any other networked software.

Seamless Data Flow

The OSI model ensures that each layer only communicates with its immediate upper and lower layers, creating a seamless flow. This layered approach means changes in one layer’s protocols or functionalities can occur without disrupting the entire network.

OSI Model Advantages

The OSI model is a cornerstone in network architecture for several reasons:

Simplification of network design

The OSI model’s layered approach breaks down complex network processes, making design and operation more manageable. Each layer focuses on a specific aspect of communication, allowing for independent development and easier troubleshooting.

Standardization and interoperability

It establishes universal standards for network communication, enabling different technologies to interact seamlessly. This interoperability is crucial for the efficient functioning of diverse network devices and applications.

Flexibility and Scalability

Adaptable to technological advancements, the OSI model allows individual layers to evolve without overhauling the entire system. This scalability makes it suitable for various network sizes and types.

Enhanced Security

Security measures are integrated at multiple layers, providing a robust defense against threats. Each layer can address specific security concerns, leading to comprehensive network protection.

Real-World Applications of the OSI Model

The OSI model’s influence extends well beyond theoretical concepts, playing a crucial role in various practical aspects of networking:

Network Design and Protocol Development

Network professionals use the OSI model as a blueprint for structuring and developing robust networks. It guides the creation of new protocols, ensuring seamless integration and functionality across different network layers.

Efficient Troubleshooting and Management

In troubleshooting, the OSI model provides a systematic approach for identifying issues, from physical connectivity to application-level errors. It also aids in network maintenance and performance optimization, addressing each layer to enhance overall efficiency.

Cybersecurity Strategy

The model is foundational in crafting layered security strategies . By implementing security measures at different layers, it offers comprehensive protection against various cyber threats. Understanding the OSI layers is key in detecting and mitigating attacks targeting specific network segments.

Educational and Training Tool

It serves as an essential framework in networking education, helping students and professionals alike understand complex network operations. The OSI model is a cornerstone in training programs , emphasizing the intricacies of network architecture and security.

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OSI Model vs. TCP/IP Model

While the OSI model offers a detailed conceptual framework, the TCP/IP model is recognized for its practical application in today’s internet-driven world.

Structural Differences

OSI model : Introduced as a comprehensive, protocol-independent framework, the OSI model details seven distinct layers, offering a more granular approach to network communication.

TCP/IP model : Developed earlier by the U.S. Department of Defense, the TCP/IP model consists of four layers (Application, Transport, Internet and Network Access), combining certain OSI layers.

Theoretical vs. Practical Approach

OSI model : Developed as a theoretical and universal networking model, it’s used more for educational purposes to explain how networks operate.

TCP/IP model : This model is designed around specific standard protocols, focusing on solving practical communication issues. It leaves sequencing and acknowledgment functions to the transport layer, differing from the OSI approach.

Adoption and Use

OSI model: While not widely implemented in its entirety, the OSI model’s clear layer separation is influential in protocol design and network education; simpler applications in the OSI framework may not utilize all seven layers, with only the first three layers (Physical, Data Link, and Network) being mandatory for basic data communication.

TCP/IP model : The dominant model used in most network architectures today, especially in internet-related communications. In TCP/IP, most applications engage all layers for communication.

Frequently Asked Questions

Why is the osi model important.

The OSI model is crucial for standardizing network communication and ensuring interoperability between various devices and systems. It simplifies network design and troubleshooting and serves as a fundamental educational tool in networking.

What are the 7 layers of the OSI model?

Layer 1: Physical Layer — Transmits raw data.

Layer 2: Data Link Layer — Manages direct links and framing.

Layer 3: Network Layer — Handles addressing and routing.

Layer 4: Transport Layer — Ensures reliable data transfer.

Layer 5: Session Layer — Manages connections.

Layer 6: Presentation Layer — Translates data formats.

Layer 7: Application Layer — Interfaces with applications.

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What is OSI Model? – Layers of OSI Model

OSI stands for Open Systems Interconnection , where open stands to say non-proprietary. It is a 7-layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe. The OSI reference model was developed by ISO – ‘International Organization for Standardization ‘, in the year 1984.

The OSI model provides a theoretical foundation for understanding network communication . However, it is usually not directly implemented in its entirety in real-world networking hardware or software . Instead, specific protocols and technologies are often designed based on the principles outlined in the OSI model to facilitate efficient data transmission and networking operations

What is OSI Model?

• What are the 7 layers of the OSI Model?

Physical Layer – Layer 1

Data link layer (dll) – layer 2, network layer – layer 3, transport layer – layer 4, session layer – layer 5, presentation layer – layer 6, application layer – layer 7.

• What is the Flow of Data in OSI Model?

Advantages of OSI Model

• OSI Model in a Nutshell

OSI vs TCP/IP Model

The OSI model, created in 1984 by ISO , is a reference framework that explains the process of transmitting data between computers. It is divided into seven layers that work together to carry out specialised network functions , allowing for a more systematic approach to networking.

Data Flow In OSI Model

When we transfer information from one device to another, it travels through 7 layers of OSI model. First data travels down through 7 layers from the sender’s end and then climbs back 7 layers on the receiver’s end.

Data flows through the OSI model in a step-by-step process:

• Application Layer: Applications create the data.
• Presentation Layer: Data is formatted and encrypted.
• Session Layer: Connections are established and managed.
• Transport Layer: Data is broken into segments for reliable delivery.
• Network Layer : Segments are packaged into packets and routed.
• Data Link Layer: Packets are framed and sent to the next device.
• Physical Layer: Frames are converted into bits and transmitted physically.

Each layer adds specific information to ensure the data reaches its destination correctly, and these steps are reversed upon arrival.

Let’s look at it with an Example:

Luffy sends an e-mail to his friend Zoro.

Step 1: Luffy interacts with e-mail application like Gmail , outlook , etc. Writes his email to send. (This happens in Layer 7: Application layer )

Step 2: Mail application prepares for data transmission like encrypting data and formatting it for transmission. (This happens in Layer 6: Presentation Layer )

Step 3: There is a connection established between the sender and receiver on the internet. (This happens in Layer 5: Session Layer )

Step 4: Email data is broken into smaller segments. It adds sequence number and error-checking information to maintain the reliability of the information. (This happens in Layer 4: Transport Layer )

Step 5: Addressing of packets is done in order to find the best route for transfer. (This happens in Layer 3: Network Layer )

Step 6: Data packets are encapsulated into frames, then MAC address is added for local devices and then it checks for error using error detection. (This happens in Layer 2: Data Link Layer )

Step 7: Lastly Frames are transmitted in the form of electrical/ optical signals over a physical network medium like ethernet cable or WiFi.

After the email reaches the receiver i.e. Zoro, the process will reverse and decrypt the e-mail content. At last, the email will be shown on Zoro’s email client.

What Are The 7 Layers of The OSI Model?

The OSI model consists of seven abstraction layers arranged in a top-down order:

• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.  

Functions of the Physical Layer

• Bit Synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.
• Bit Rate Control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
• Physical Topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
• Transmission Mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.

Note: Hub, Repeater, Modem, and Cables are Physical Layer devices.  Network Layer, Data Link Layer, and Physical Layer are also known as Lower Layers or Hardware Layers .

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address .  The Data Link Layer is divided into two sublayers:  

• Logical Link Control (LLC)
• Media Access Control (MAC)

The packet received from the Network layer is further divided into frames depending on the frame size of the NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header. 

The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.  

Functions of the Data Link Layer

• Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
• Physical Addressing: After creating frames, the Data link layer adds physical addresses ( MAC addresses ) of the sender and/or receiver in the header of each frame.
• Error Control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
• Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.
• Access Control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

Note: Packet in the Data Link layer is referred to as Frame.   Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines.  Switch & Bridge are Data Link Layer devices.

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender & receiver’s IP address es are placed in the header by the network layer. 

Functions of the Network Layer 

• Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.
• Logical Addressing: To identify each device inter-network uniquely, the network layer defines an addressing scheme. The sender & receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

Note: Segment in the Network layer is referred to as Packet .  Network layer is implemented by networking devices such as routers and switches.  

The transport layer provides services to the application layer and takes services from the network layer. The data in the transport layer is referred to as Segments . It is responsible for the end-to-end delivery of the complete message. The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.

At the sender’s side:  The transport layer receives the formatted data from the upper layers, performs Segmentation , and also implements Flow and error control to ensure proper data transmission. It also adds Source and Destination port number s in its header and forwards the segmented data to the Network Layer. 

Note: The sender needs to know the port number associated with the receiver’s application.  Generally, this destination port number is configured, either by default or manually. For example, when a web application requests a web server, it typically uses port number 80, because this is the default port assigned to web applications. Many applications have default ports assigned. 

At the receiver’s side:  Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data. 

Functions of the Transport Layer 

• Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.
• Service Point Addressing: To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.

Services Provided by Transport Layer 

• Connection-Oriented Service
• Connectionless Service

1. Connection-Oriented Service: It is a three-phase process that includes:

• Connection Establishment
• Data Transfer
• Termination/disconnection

In this type of transmission, the receiving device sends an acknowledgment, back to the source after a packet or group of packets is received. This type of transmission is reliable and secure.

2. Connectionless service: It is a one-phase process and includes Data Transfer. In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service.

Note:   Data in the Transport Layer is called Segments .  Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls.  The transport layer is called as Heart of the OSI model.  Device or Protocol Use : TCP, UDP  NetBIOS, PPTP

This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.

Functions of the Session Layer

• Session Establishment, Maintenance, and Termination: The layer allows the two processes to establish, use, and terminate a connection.
• Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
• Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

Note: All the below 3 layers(including Session Layer) are integrated as a single layer in the TCP/IP model as the “Application Layer”.  Implementation of these 3 layers is done by the network application itself. These are also known as Upper Layers or Software Layers.   Device or Protocol Use :  NetBIOS, PPTP.

Let us consider a scenario where a user wants to send a message through some Messenger application running in their browser. The “ Messenger ” here acts as the application layer which provides the user with an interface to create the data. This message or so-called Data is compressed, optionally encrypted (if the data is sensitive), and converted into bits (0’s and 1’s) so that it can be transmitted.  

Communication in Session Layer

The presentation layer is also called the Translation layer . The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. 

Functions of the Presentation Layer

• Translation: For example, ASCII to EBCDIC .
• Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
• Compression: Reduces the number of bits that need to be transmitted on the network.

Note: Device or Protocol Use:  JPEG, MPEG, GIF.

At the very top of the OSI Reference Model stack of layers, we find the Application layer which is implemented by the network applications. These applications produce the data to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. 

Example : Application – Browsers, Skype Messenger, etc. 

Note: The application Layer is also called Desktop Layer.              Device or Protocol Use :   SMTP .

Functions of the Application Layer

The main functions of the application layer are given below.

• Network Virtual Terminal(NVT): It allows a user to log on to a remote host.
• File Transfer Access and Management(FTAM): This application allows a user to access files in a remote host, retrieve files in a remote host, and manage or control files from a remote computer.
• Mail Services: Provide email service.
• Directory Services: This application provides distributed database sources and access for global information about various objects and services.

Note:  The OSI model acts as a reference model and is not implemented on the Internet because of its late invention. The current model being used is the TCP/IP model. 

OSI Model – Layer Architecture

TCP/IP protocol ( Transfer Control Protocol/Internet Protocol ) was created by U.S. Department of Defense’s Advanced Research Projects Agency (ARPA) in 1970s.

Some key differences between the OSI model and the TCP/IP Model are:

• TCP/IP model consists of 4 layers but OSI model has 7 layers. Layers 5,6,7 of the OSI model are combined into the Application Layer of TCP/IP model and OSI layers 1 and 2 are combined into Network Access Layers of TCP/IP protocol.
• The TCP/IP model is older than the OSI model, hence it is a foundational protocol that defines how should data be transferred online.
• Compared to the OSI model, the TCP/IP model has less strict layer boundaries.
• All layers of the TCP/IP model are needed for data transmission but in the OSI model, some applications can skip certain layers. Only layers 1,2 and 3 of the OSI model are necessary for data transmission.

OSI vs TCP/IP

Why Does The OSI Model Matter?

Even though the modern Internet doesn’t strictly use the OSI Model (it uses a simpler Internet protocol suite), the OSI Model is still very helpful for solving network problems. Whether it’s one person having trouble getting their laptop online, or a website being down for thousands of users, the OSI Model helps to identify the problem. If you can narrow down the issue to one specific layer of the model, you can avoid a lot of unnecessary work.

Imperva Application Security

Imperva security solutions protect your applications at different levels of the OSI model. They use DDoS mitigation to secure the network layer and provide web application firewall (WAF), bot management, and API security to protect the application layer.

To secure applications and networks across the OSI stack, Imperva offers multi-layered protection to ensure websites and applications are always available, accessible, and safe. The Imperva application security solution includes:

• DDoS Mitigation: Protects the network layer from Distributed Denial of Service attacks.
• Web Application Firewall (WAF) : Shields the application layer from threats.
• Bot Management: Prevents malicious bots from affecting the application.
• API Security: Secures APIs from various vulnerabilities and attacks.

The OSI Model defines the communication of a computing system into 7 different layers. Its advantages include:

• It divides network communication into 7 layers which makes it easier to understand and troubleshoot.
• It standardizes network communications, as each layer has fixed functions and protocols.
• Diagnosing network problems is easier with the OSI model .
• It is easier to improve with advancements as each layer can get updates separately.

Disadvantages of OSI Model

• Complexity: The OSI Model has seven layers, which can be complicated and hard to understand for beginners.
• Not Practical: In real-life networking, most systems use a simpler model called the Internet protocol suite (TCP/IP), so the OSI Model isn’t always directly applicable.
• Slow Adoption: When it was introduced, the OSI Model was not quickly adopted by the industry, which preferred the simpler and already-established TCP/IP model.
• Overhead: Each layer in the OSI Model adds its own set of rules and operations, which can make the process more time-consuming and less efficient.
• Theoretical: The OSI Model is more of a theoretical framework, meaning it’s great for understanding concepts but not always practical for implementation.

In conclusion, the OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven distinct layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer has specific responsibilities and interacts with the layers directly above and below it, ensuring seamless communication and data exchange across diverse network environments. Understanding the OSI model helps in troubleshooting network issues, designing robust network architectures, and facilitating interoperability between different networking products and technologies.

Frequently Asked Questions on OSI Model – FAQs

Is osi layer still used.

Yes, the OSI model is still used by networking professionals to understand data abstraction paths and processes better.

What is the highest layer of the OSI model?

Layer 7 or Application layer is highest layer of OSI model.

What is layer 8?

Layer 8 doesn’t actually exist in the OSI model but is often jokingly used to refer to the end user. For example: a layer 8 error would be a user error.

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The OSI Model

What is the osi model.

How a single bit travels from one computer to the next is a complex concept. In 1984, the open systems interconnection (OSI) model was published as a framework for network communication. The model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. The message is built as it moves down the protocol stack. However, it is not sent to another network until it reaches the physical layer.

The model helps IT, computer science, and cybersecurity professionals understand how a single bit travels from one computer to the next by breaking the system into these layers.

From physical devices to user interfaces (UI), this model explains the communication role of each layer in overall computer networking. This article will start by introducing the Physical Layer (Layer 1).

Layer 1: the physical layer

The physical layer is where data moves across network interfaces as digital signals. Additionally, this is where the transmitting and receiving of network communication occurs. Starting with the Application Layer the message moves down the OSI model, and it eventually reaches the Physical Layer for transmission. When the message is received by the physical layer, the message will then move up the OSI layers until it reaches the final application layer.

Layer 2: data-link layer

Electrical signals received (or transmitted) to the physical layer are linked and translated to digital logic in the data-Link layer . Computer devices may be networked at the Data-Link layer, but only as a Local Area Network (LAN). Connecting a LAN to another LAN occurs at Layer 3.

Within Layer 2, the Protocol Data Unit (PDU) known as a frame consists of a header, footer, and data. Understanding how a frame is structured is important for network traffic analysis.

Additionally, within Layer 2, physical addresses are assigned and are also known as MAC addresses and/or hardware addresses in networking. MAC addresses are unique to each device on a local network. They are 48-bits in length and are assigned in hexadecimal characters.

Some other things to note about Layer 2 is that there are a few protocols that reside in it that we should know about:

• Ethernet : The most common type of LAN, Ethernet is the standard used to connect computing devices, routers, and switches in a wired network.
• IEEE 802.11 : “Wi-Fi” or “Wireless LAN.”
• Fiber Distributed Data Interface (FDDI) : Network standard for fiber optic LAN connections.
• Link Layer Discovery Protocol (LLDP) : A Link Layer protocol used for advertising neighbors, identity, and capabilities on a LAN.
• Address Resolution Protocol (ARP) : Converts and links Internet Protocol (IP) addresses to MAC addresses on a LAN.
• Cisco Discovery Protocol (CDP) : Similar to LLDP, but Cisco proprietary. The protocol collects neighbor information of directly connected LAN devices.

Additionally, Layer 2 is split into two sublayers:

• Logical Link Control (LLC) : Responsible for establishing the logical link between devices on a local network.
• Media Access Control (MAC) : Responsible for the procedures used by devices across a network medium.

Layer 3: network layer

When we think of the internet, we are thinking of interconnected networks. Interconnecting networks refer to a Local Area Network (LAN) connection to neighboring or remote networks. Layer 3 of the OSI model, the network layer , is where internetworking takes place and is where logical addresses are assigned to networked devices. A primary function of this layer is to route network packets from one LAN to another. Routing requires IP addresses and logical mapping of other networks across the internet to properly deliver messages. Another important function of Layer 3 is its ability to fragment and reassemble large communication. When Layer 3 passes a message down to Layer 2 for transmission, message length limits may be encountered in some cases.

Additionally, Layer 3 is the layer where the protocols used to route communication between networks reside. A few common network protocols are:

• Internet Protocol (IP) : IPv4 and IPv6 are two versions of IP, and IPv4 is the most common protocol of the Internet .
• Internet Protocol Secure (IPSec) : A more secure version of IP which leverages cryptography.
• Routing Information Protocol (RIP) : Distance-vector routing protocol that uses hop count as a metric of routing.
• Enhanced Interior Gateway Routing Protocol (EiGRP) : Cisco proprietary. A distance-vectoring protocol used for automating network configurations and routing decisions.
• Internet Control Message Protocol (ICMP) : Network protocol used for error reporting of network issues.
• Border Gateway Protocol (BGP) : A routing protocol designed to exchange routing information automatically on the internet.

Within Layer 3, the Protocol Data Unit (PDU) is the packet . Packets encapsulate data intended for transmission with header and footer data.

The IPv4 protocol encapsulates data with IPv4 header information necessary for delivery. For example, the 32-bit packet format contains the source address, the destination address, protocol, time-to-live (TTL), etc. in the IPv4 header data.

Layer 4: transport layer

The transport layer , Layer 4, is responsible for being the go-between the abstract layers of the OSI model (Layers 7-5) and the concrete communication layers (Layers 3-1).

Depending on the type of application, the transportation of that application’s communication will need to be handled in a specific way. For example, basic web browsing communication uses Hypertext Transfer Protocol (HTTP) . HTTP communicates via a specific connection service type and port. The transport layer is responsible for delivering/receiving the HTTP communication and maintaining the connection throughout the HTTP communication.

The Protocol Data Unit (PDU) at Layer 4 is known as a data segment . Segmentation is the process of dividing raw data into smaller pieces. Once the raw data is packaged from the higher application layers it is segmented at the transport layer before being passed to the Network Layer.

The transport layer protocols are divided into two categories depending on their connection service type:

Connection-oriented services

This connection type establishes a logical connection between two devices prior to beginning communication across a network. Connection-oriented protocols typically maintain service connection by following a set of rules that initiate, negotiate, manage, and terminate the communication. The Transport Layer protocols will also retransmit any data that is received without acknowledgment. The most common Connection-Oriented protocol is the Transmission Control Protocol (TCP) and its process to manage a connection between two devices is called the Three-Way Handshake . In TCP communication, the communicating devices typically share a client/server relationship where a client initiates communication with a service. The handshake involves the process of sending special TCP messages to synchronize a state of negotiated connection in communication.

Connectionless services

In connectionless communication, the protocol does not establish a connection between client and server. Instead, once a request is made to the server, the server sends all data without initiation, negotiation, or management of connection. Connectionless protocols also do not attempt to correct any interruptions in data transmission. Once the server sends the data, the server is not concerned if the client receives it.

When TCP or UDP are used to establish communication, the communication is assigned a port as the Layer 4 address. A port is a logical assignment given to processes and their respective application protocols on a computing system. A few important facts to memorize about ports are:

• There are 65,535 valid port numbers available to assign to a communication process.
• Ports 0 - 1023 are Well-Known Ports : Assigned to universal TCP/IP application protocols. These protocols are the most common such as HTTPS, SSH, FTP, DNS, and the list goes on. They are registered to these protocols by a global
• Ports 1024 - 49,151 are Registered Ports : Reserved for application protocols that are not specified as universal TCP/IP application protocols.
• Ports 49,152 - 65,535 are Private/Dynamic Ports : These ports may be used for any process without the need to register the port with the global assigning authority.
• When TCP and IP are used together, a Layer 4 port and a Layer 3 IP address are assigned to the connection. This is called a socket. For example, 8.8.8.8:443 is a socket indicating that communication to IP address 8.8.8.8 is to connect to port 443 on the server.

Layer 5: session layer

The session layer starts, manages, and terminates sessions between end-user application processes. Sessions are considered the persistent connection between devices. A session is application-focused; sessions are not concerned with layers 1-4. Instead, the session layer controls dialog between two networked devices. It is considered to facilitate host-to-host communication. Sessions dialog may be controlled through synchronization checkpoints, and through management of communication modes. There are two modes of communication permitted at Layer 5:

• Half-Duplex : Communication travels in both directions between sender and receiver, but only one device may transmit a message at a time.
• Full-Duplex : Communication travels in both directions between sender and receiver, and messages may be sent simultaneously in either direction.

The session layer resembles a phone conversation. For example, when a person picks up a phone and calls someone else a session is created. Once the communication on the call is completed, the session is terminated by hanging up the phone. In computing, software applications are making the phone call and establishing a session.

Two common Layer 5 protocols still used today are:

• Remote Procedure Call (RPC)

Layer 6: presentation layer

The presentation layer is primarily responsible for presenting data so that the recipient will understand the data. Data formatting and encoding protocols apply at Layer 6 to ensure data is legible and presented properly in the application receiving it. Data compression is also a function of Layer 6. If necessary, data may be compressed to improve data throughput over network communication.

Some common Layer 6 protocols are ASCII , JPEG , GIF , MPEG , and PNG .

Another main function of the presentation layer is the encryption and decryption of data sent across a network. Most encryption communication protocols straddle multiple layers of the OSI model, but the actual encryption function is Layer 6.

Two of the most common secure communication protocols are:

Transport Layer Security (TLS)

• Secure Socket Layer (SSL)

Layer 7: application layer

The topmost layer of the OSI model is the application layer . On computer systems, applications display information to the user via the UI.

Note : Software applications running on a computer are NOT considered to reside in the application layer. Instead, they leverage application layer services and protocols that enable network communication.

For example, the user can craft messages and access the network from the application layer. A web browser application allows a user to access a web page. The user may input information and receive information through the web browser. However, the application layer protocol HTTP performs the network communication function. The web browser and HTTP work closely together, and the distinction between the two may be subtle. Yet, HTTP is the web browsing protocol for all web browser applications. In contrast, no single web browser software exclusively utilizes HTTP.

HTTP is one of many common application layer protocols. Below are a few additional protocols to know. It is also good practice to memorize the associated port assigned to the protocols:

The OSI model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. Understanding the OSI model will help you communicate with other network technologists. Computer networking may seem complex, but, with a bit of study, you can gain this knowledge to become an effective Cybersecurity Analyst.

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OSI model - What's the presentation and session layer for?

So I feel I pretty well understand the application layer, and everything below (and including) the transport layer.

The session and presentation layers, though, I don't fully understand. I've read the simplistic descriptions in Wikipedia, but it doesn't have an example of why separating out those layers is useful.

• What is the session layer? What does it do, and under what circumstances is it better to have a session layer than simply talk to the transport with your app?
• What is the presentation layer? (same questions as above)

7 Answers 7

The session layer is meant to store states between two connections, like what we use cookies for when working with web programming.

The presentation layer is meant to convert between different formats. This was simpler when the only format that was worried about was character encoding, ie ASCII and EBCDIC. When you consider all of the different formats that we have today(Quicktime, Flash, Pdf) centralizing this layer is out of the question.

TCP/IP doesn't make any allocation to these layers, since they are really out of the scope of a networking protocol. It's up to the applications that take advantage of the stack to implement these.

The reasons there aren't any examples on wikipedia is that there aren't a whole lot of examples of the OSI network model, period.

OSI has once again created a standard nobody uses, so nobody really know how one should use it.

Layers 5-6 are not commonly used in today's web applications, so you don't hear much about them. The TCP/IP stack is slightly different than a pure OSI Model.

One of the reasons TCP/IP is used today instead of OSI is it was too bloated and theoretical, the session and presentation layer aren't really needed as separate layers as it turned out.

I think that presentation layer protocols define the format of data. This means protocols like XML or ASN.1. You could argue that video/audio codecs are part of the presentation layer Although this is probably heading towards the application layer.

I can't help you with the session layer. That has always baffled me.

To be honest, there are very vague boundaries in everything above the transport layer. This is because it is usually handled by a single software application. Also, these layers are not directly associated with transporting data from A to B. Layers 4 and below each have a very specific purpose in moving the data e.g. switching, routing, ensuring data integrity etc. This makes it easier to distinguish between these layers.

Presentation Layer The Presentation Layer represents the area that is independent of data representation at the application layer - in general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission - this happens at Layer 6.

Session Layer When two devices, computers or servers need to “speak” with one another, a session needs to be created, and this is done at the Session Layer. Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

For the presentation layer :because most of communication done between heterogeneous systems (Operating Systems,programing langages,cpu architectures)we need to use a unified idepedent specification .like ANS1 ans BRE.

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What is the OSI Model?

The OSI Model Defined, Explained, and Explored

The OSI Model Defined

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

Created at a time when network computing was in its infancy, the OSI was published in 1984 by the International Organization for Standardization (ISO). Though it does not always map directly to specific systems, the OSI Model is still used today as a means to describe Network Architecture.

The 7 Layers of the OSI Model

Physical layer.

The lowest layer of the OSI Model is concerned with electrically or optically transmitting raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find “physical” resources such as network hubs, cabling, repeaters, network adapters or modems.

Data Link Layer

At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer.

The data link layer encompasses two sub-layers of its own. The first, media access control (MAC), provides flow control and multiplexing for device transmissions over a network. The second, the logical link control (LLC), provides flow and error control over the physical medium as well as identifies line protocols.

Network Layer

The network layer is responsible for receiving frames from the data link layer, and delivering them to their intended destinations among based on the addresses contained inside the frame. The network layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to quite literally route information where it needs to go between networks.

Transport Layer

The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. One of the most common examples of the transport layer is TCP or the Transmission Control Protocol.

Session Layer

The session layer controls the conversations between different computers. A session or connection between machines is set up, managed, and termined at layer 5. Session layer services also include authentication and reconnections.

Presentation Layer

The presentation layer formats or translates data for the application layer based on the syntax or semantics that the application accepts. Because of this, it at times also called the syntax layer. This layer can also handle the encryption and decryption required by the application layer.

Application Layer

At this layer, both the end user and the application layer interact directly with the software application. This layer sees network services provided to end-user applications such as a web browser or Office 365. The application layer identifies communication partners, resource availability, and synchronizes communication.

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The Open Systems Interconnect (OSI) model is a seven-layer visual model that describes the networking system and how apps can communicate with each other. 

In the earlier days of computing, devices from diverse manufacturers were not interoperable. To facilitate that, the International Organization for Standardization created the OSI model in 1984, providing a set of common standards for different equipment to communicate with each other.

Starting from the bottom, the seven layers are:

• Physical layer
• Data link layer (DLL)
• Network layer
• Transport layer
• Session layer
• Presentation layer
• Application layer

Each of the seven layers performs a specific function and communicates with the layer above and below it.

Today, the OSI model is primarily used as a reference model to teach computer professionals the basics of computer networking.

Table of Contents

7 layers of the OSI model

Breaking up networking functions into layered functionalities helps network engineers understand the workings of their networks better and helps them zero in on the problems more quickly. Here’s how each layer serves that purpose.

Layer 1: Physical layer

Layer 1 is the physical layer and also the lowest layer of the OSI model. This layer transmits information in the form of bits (1s and 0s) from one node to the next. Components of the physical layer include cables, power plugs, connectors, network interface cards (NICs), and other hardware.

Layer 2: Data link layer (DLL)

The data link layer (DLL) is the second layer and handles the node-to-node transfer of data. Its primary function is to ensure that the transferred data is error-free. 

DLL comprises two sub-layers: logical link control (LLC) and media access control (MAC). LLC handles multiplexing and demultiplexing signals as well as node flow control and error management, while MAC handles hardware interactions with the network.

Layer 3: Network layer

The network layer is responsible for routing data packets from a source host to a destination host. It does so by selecting the shortest possible path. This layer is also responsible for packet forwarding and logical addressing. Necessary protocols used in this layer are ICMP , ARP, RIP, IPv4/v6, and IPsec.   

Layer 4: Transport layer

The transport layer is responsible for transferring data from hosts to users. The most frequently used protocols in the transport layer are User Datagram Protocol (UDP) and Transmission Control Protocol (TCP). While TCP enables data transfer between computing devices, UDP is designed for speedy data transmission.

Layer 5: Session layer

The session layer is responsible for controlling, managing, and terminating the connections between computers. Functions include token management, creating dialog units, synchronizing data flow, and efficiently using available network resources.

Layer 6: Presentation layer 

The presentation layer handles information related to data coding and encoding. It’s also called the syntax layer.

Layer 7: Application layer

The application layer is the one that is most familiar to end users, as it governs communication with software housed on their host PCs. Note that this layer doesn’t include the actual software or application itself, but only the protocols that manage them.

How the OSI model works

The OSI model works by segmenting all transmitted data through a seven-layer abstraction in order to more efficiently and securely parse and deliver the data to its destination.

To better understand this process, here’s an example of the OSI model’s abstraction process in operation. Let’s say you decide to send an email from computer (A) to computer (B).

Your email application client resides on layer 7. When you send the email, the layer 7 application protocol encapsulates or places a header on your messages and sends it to layer 6.

Layer 6 compresses the data and transfers it to layer 5. It also encrypts the data before sending it forward.

Layer 5 opens a session between your computer and the outgoing server. It also decides which data packet belongs to which file. Finally, it adds an appropriate header and transfers the data to layer 4.

When layer 4 receives data from layer 5, it segments it and assigns each segment a destination and source port number to ensure the data gets delivered to the correct service. Layer 4 also enforces security controls.   

Layer 3 breaks the data into packets and transmits it over multiple paths. The packets contain destination and source IP addresses for easier identification of end devices.  

Then the data is sent to layer 2, where the DLL adds MAC addresses to the packets, which are then grouped into frames. To deliver to the correct destination, the LLC sublayer adds control information to each frame.

Finally, the frames are transmitted to layer 1 in the form of 1s and 0s. That binary data is then transferred to device B either through electric pulses (Ethernet), radio waves (Wi-Fi), or light pulses (Fiber optics).

Layer 1 ensures bit synchronization so that, as the data reverses its abstraction through each layer at the destination, the end user receives the information in a readable format—just as it was intended.

Why the OSI model is still relevant

Although the TCP/IP model is the preferred model for IT professionals and is used in most modern computers, the OSI model remains relevant for its easy troubleshooting, flexible nature, and continued use as a teaching tool.

Easy troubleshooting

The OSI model enables IT teams to classify their asset inventories into easily digestible chunks at each layer. In case of a problem, spotting and identifying problems within the layers is relatively easy thanks to their specificity—especially at the top three layers, which are all collapsed into one in the TCP/IP model.

Flexibility

The OSI model supports both connectionless services as well as connection-oriented services, making it highly flexible in nature.

Acts as a teaching aid

The OSI model serves as an excellent teaching aid as it provides users with a clear understanding of how software and hardware work together. By breaking down networking concepts into layers, it removes ambiguity and provides network professionals with a clear picture of networking.

Drawbacks of the OSI model

Although the OSI model does have benefits, it does fall short in some areas, including practicality, popularity, and complexity.

Theoretical model

Compared to TCP/IP, OSI functions as more of a theoretical framework that doesn’t offer many solutions for practical implementation.

Less popular

While the OSI model was developed at the same time as the TCP/IP model, it couldn’t compete with the popularity of TCP/IP. IT professionals generally preferred TCP/IP protocols more; as a result, the OSI model fell increasingly out of favor.

More complex

The OSI model is quite complicated and thus has implementation problems. In contrast, the TCP/IP model is more effective and easier to operate.

Duplication

There’s a lot of duplication of services in the OSI model. For instance, both the transport and the data layer provide similar services. To add to it, several layers, like the presentation or the session layer, are barely used, which is in part why they were collapsed into the application layer in TCP/IP.

OSI vs. TCP/IP model

The Transmission Control Protocol/Internet Protocol (TCP/IP) is a communication protocol that shows how a particular computer can connect to another over the internet. The model has a four-layer architecture and is designed to ensure the transmission of error-free data in the form of packets across networks.

From top to bottom, the four layers of the TCP/IP model comprise the application layer, transport layer, internet layer, and network access layer. 

While both OSI and the TCP/IP model have a layered architecture and provide almost the same functionalities, they have several dissimilarities. They include:

• OSI is a conceptual framework, while TCP/IP is a connection-oriented protocol
• TCP/IP has 4 layers, while OSI has 7 layers
• OSI uses the session layer, presentation layer, and application layer to demarcate the upper layers, while TCP/IP uses only the application layer  
• Layers 1 and 2 are separate in the OSI model, but in TCP/IP they are combined
• The layers in the OSI model are highly interdependent. So, if the lower layer fails, there is a probability that the upper layers will also not work well either. In contrast, TCP/IP offers a more flexible architecture.

OSI and TCP/IP layers compared

The below table shows how the TCP/IP model simplifies the layering system compared to OSI’s more granular and complex model.

What does the future of the OSI model look like?

Given the strong preference of network administrators for the TCP/IP model and its massive usage, it may be tempting to assume OSI’s days are numbered. 

However, although the OSI model may no longer be popular, there’s no denying that it serves as a reference model for thousands of IT experts, aiding them in designing more efficient and reliable systems. 

In fact, even today, the OSI model is referenced in product manuals and certification exams. So even if it’s unlikely that the OSI model will experience a resurgence in practical usage, it’s equally unlikely to disappear from common theoretical application altogether.

Bottom line: How the OSI model is used today

Despite not being used much in practice today, network engineers and vendors still reference the OSI model to have a complete overview of computer networking and to learn about its functions. While the TCP/IP protocol is implemented more by modern networks, still the OSI model is used as a guide by networking professionals to get clarity on data abstraction paths and processes.

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Core Functions of the OSI Model

The seven layers of the osi model, the importance of the osi model, osi model vs. tcp/ip model, radware solutions for network and application security.

The OSI (Open Systems Interconnection) Model is a conceptual framework that standardizes and defines the functions of a telecommunication or networking system. It's a crucial tool in the world of networking and plays a fundamental role in understanding how data communication works across various network protocols and technologies.

The model was developed by the International Organization for Standardization (ISO) to promote interoperability and facilitate communication between different devices and systems, regardless of their manufacturers. The OSI Model is structured into seven distinct layers, each with a specific set of functions and responsibilities, as explained in the next section.

The OSI Model operates by allowing each of its seven layers to perform specific functions while relying on the services provided by the layer immediately below it. When data is transmitted from one device to another, it passes through all seven layers at the sender and then traverses all seven layers at the receiver. Here's how it works:

Data Encapsulation: Data from the application layer is encapsulated in a format specific to that layer. As it moves down the OSI Model, each layer adds its own header and possibly trailer information to the data.

Data Transmission: The data, now structured into frames or packets, is transmitted from the sender's physical layer over the physical medium to the receiver's physical layer.

Interoperability: The OSI Model promotes interoperability by providing a common framework that different vendors and technologies can adhere to. As long as devices and software follow the specifications of the OSI Model, they can communicate effectively, regardless of their origins.

Data De-encapsulation: At the receiver's end, the data goes through a process of de-encapsulation. Each layer removes its header and trailer, leaving the original data from the application layer.

Essentially, the OSI Model divides network communication into seven layers, each with specific responsibilities—as explained below—to ensure efficient data transmission and interoperability. Data passes through these layers, undergoing encapsulation and de-encapsulation, as it travels from sender to receiver, allowing for a structured and standardized approach to networking.

The OSI Model serves as a foundational framework in the field of networking, defining the fundamental functions and responsibilities necessary for successful data communication. Comprised of seven distinct layers, this model guides the way data is transmitted, received, and processed across networks. Each layer is designed with specific tasks, and collectively they work together to ensure reliable, efficient, and interoperable communication between devices and systems. In the following section, we will delve into each layer of the OSI Model, providing detailed insights into their functions, protocols, and contributions to the seamless operation of network communication.

Layer 1: Physical Layer

The Physical Layer serves as the foundation of the OSI Model, focusing on the actual transmission of data in its raw bit form over a physical medium. Its essential functions encompass:

Encoding and Signaling: Encoding bits into electrical or optical signals suitable for the chosen physical medium. For example, it decides how ‘0’ and ‘1’ bits are represented through variations in voltage levels (for copper cables) or light pulses (for optical fibers).

Bit Rate: Determining the bit rate, which is the speed at which bits are transmitted. It defines how fast data can move over the network, commonly measured in bits per second (bps).

Transmission Media: Managing the characteristics and properties of the physical medium used for data transmission. This medium can vary widely, including copper wires, fiber-optic cables, radio waves, or even satellite links.

Physical Connectors: Defining the types of physical connectors and interfaces needed for devices to connect to the network. This includes specifications for cables, plugs, and sockets.

Error Detection and Correction: Implementing basic error-detection mechanisms to ensure data integrity during transmission. While more advanced error correction is often handled by higher layers, the Physical Layer can include simple parity checks or checksums.

Transmission Modes: Determining how data is sent between devices—whether it's simplex (one-way), half-duplex (two-way but not simultaneously), or full-duplex (two-way simultaneously).

Transmission Distance: Setting limits on the distance data can travel over the physical medium before it needs to be boosted or regenerated. This is crucial for understanding the reach of a network.

Example Use Cases: The Physical Layer plays a pivotal role in various networking scenarios, including high-speed data centers, telecommunications networks, wireless communication systems, and home Wi-Fi networks. It ensures that data is transmitted reliably and efficiently over diverse physical media, optimizing network performance and integrity.

Layer 2: Data Link Layer

The Data Link Layer sits just above the Physical Layer and plays a pivotal role in network communication. Its primary responsibilities include framing data for transmission and managing MAC addresses for efficient and reliable data exchange.

Framing: One of the key functions of the Data Link Layer is to divide the stream of bits received from the Physical Layer into manageable frames. A frame is a structured unit of data, often with a header and trailer, that encapsulates the actual data along with control information. Framing helps in delineating the boundaries of data packets, allowing devices to recognize the start and end of each transmission. This process ensures that data is correctly segmented for transmission over the network.

MAC Addressing: Another vital aspect of the Data Link Layer is the management of MAC addresses. MAC addresses are unique identifiers assigned to network interface cards (NICs) or network adapters. These addresses are used to identify devices on a local network segment. When a device wants to send data to another device within the same network, it uses the recipient's MAC address to direct the frame to the correct destination. The Data Link Layer is responsible for adding the MAC address information to the frame's header.

Network Devices: The Data Link Layer is crucial for the operation of network devices like switches. Switches operate at this layer and use MAC addresses to determine how to forward frames. When a frame arrives at a switch, it examines the MAC address in the frame's header to make forwarding decisions. This capability enables switches to efficiently direct data only to the specific port where the destination device is connected, minimizing network congestion and enhancing overall network performance.

LLC (Logical Link Control): The Data Link Layer is further divided into two sub-layers: the Logical Link Control (LLC) sub-layer and the MAC sub-layer. The LLC sub-layer is responsible for managing flow control, error checking, and addressing within the Data Link Layer. It ensures that data is reliably transmitted between devices, handles error detection and correction, and manages communication between devices using various data-link protocols.

Example Use Cases: Use cases for the Data Link Layer include Ethernet communication within local networks, wireless Wi-Fi connections, point-to-point serial communication, and protocols that ensure data integrity and efficient use of the network medium. It ensures reliable and orderly data exchange, particularly within the local network segment, enhancing network performance and ensuring data integrity.

Layer 3: Network Layer

The Network Layer is the third layer in the OSI Model and serves as a critical component of network communication. Its primary role is to manage the routing of data packets from the source to the destination across multiple networks while maintaining logical addressing and path determination.

Routing Data Packets: One of the fundamental tasks of the Network Layer is to route data packets from the sender to the receiver. It accomplishes this by determining the most efficient path for data to travel through a complex network topology. Routing algorithms and protocols, such as RIP (Routing Information Protocol), OSPF (Open Shortest Path First), and BGP (Border Gateway Protocol), are implemented at this layer to make routing decisions.

Logical Addressing: The Network Layer introduces logical addressing, where each device on a network is assigned a unique address known as an IP (Internet Protocol) address. IP addresses, both IPv4 and IPv6, are used for identifying devices and their network location. The Network Layer is responsible for managing these addresses, which are used for source and destination identification in data packets.

Packet Forwarding: As data packets traverse the network, the Network Layer handles packet forwarding. This involves inspecting the destination IP address in each packet's header and determining the next hop (next network device) on the route to the destination. The Network Layer also manages the encapsulation and decapsulation of data packets for transmission and reception.

Inter-Network Communication: The Network Layer is crucial for enabling communication between different networks. It acts as the boundary where data moves between local area networks (LANs) and wide area networks (WANs). Routers, which operate at this layer, play a central role in connecting disparate networks and ensuring data can traverse these boundaries.

Error Handling: While the primary responsibility for error handling lies with higher layers, the Network Layer may perform basic error detection and reporting. If a packet is found to be corrupt or undeliverable, the Network Layer may discard it or request retransmission.

Subnetting: The Network Layer allows for the division of large IP address spaces into smaller subnetworks or subnets. Subnetting aids in efficient IP address allocation, routing optimization, and network management.

Example Use Cases: The OSI Network Layer finds diverse applications in networking, notably in routing data packets across the global Internet, handling IP addressing and subnetting within local networks, securing communications through Virtual Private Networks (VPNs), managing Quality of Service (QoS) to prioritize critical traffic types, automating IP address assignment via DHCP, enabling efficient one-to-many communications via multicast routing, and interconnecting private networks over public or dedicated links. It serves as the linchpin for directing data to its destination efficiently, whether within local networks or across vast interconnected infrastructures, ensuring the reliability and security of data transmission.

Layer 4: Transport Layer

The Transport Layer, positioned as the fourth layer in the OSI Model, is primarily responsible for ensuring end-to-end communication and the reliable delivery of data between devices on different hosts across a network. It accomplishes this through various mechanisms, including segmentation, flow control, error detection, and multiplexing.

End-to-End Communication: The Transport Layer ensures that data reaches its intended destination reliably and in the correct sequence. To achieve this, it assigns a source port and a destination port to each data segment, creating what is known as a “socket.” Sockets—represented as a combination of IP address, source port, and destination port—allow multiple applications to simultaneously communicate on the same device while ensuring that data from different applications is correctly routed to its respective destination.

Segmentation: One of the key roles of the Transport Layer is to divide large messages into smaller segments, making them easier to transmit across the network. These segments are typically referred to as “packets” or “datagrams.” Segmenting data is especially important when the message is too large to fit into a single network packet.

Flow Control: Flow control mechanisms are implemented at the Transport Layer to manage the rate of data transmission between sender and receiver. This is crucial to prevent network congestion and ensure that the receiving device can process data at its own pace. Flow control can be achieved through techniques like windowing, where the sender adjusts its transmission rate based on acknowledgments from the receiver.

Error Detection and Correction: The Transport Layer employs error detection and correction techniques to ensure the integrity of data during transmission. Protocols like TCP (Transmission Control Protocol), a commonly used Transport Layer protocol, include checksums to detect errors in data segments. If errors are detected, TCP requests retransmission of the affected segments to guarantee data accuracy.

Multiplexing and Demultiplexing: Multiplexing is the process of combining multiple data streams into a single stream for transmission over the network. At the receiving end, demultiplexing separates the combined stream back into individual data streams using the destination port information in the Transport Layer header. This enables multiple applications to use the same network connection simultaneously.

Connection-Oriented vs. Connectionless: The Transport Layer supports both connection-oriented and connectionless communication. TCP is a connection-oriented protocol that establishes a reliable, bidirectional communication link before data transfer. UDP (User Datagram Protocol), on the other hand, is a connectionless protocol that sends data without prior connection setup, making it faster but less reliable compared to TCP.

Example Use Cases: Key Transport Layer use cases include the reliable and error-checked transmission of data across networks via protocols like TCP for applications such as web browsing and email. Additionally, the Transport Layer offers connectionless communication through UDP, making it ideal for real-time applications like voice and video conferencing. It manages data segmentation, reassembly, and flow control, ensuring that large files can be transmitted efficiently and without data loss. The Transport Layer also handles port-based addressing, allowing multiple services to run simultaneously on a single device, supporting various applications and facilitating secure data transfer over encrypted connections.

Layer 5: Session Layer

The Session Layer, positioned as the fifth layer in the OSI Model, is responsible for establishing, maintaining, and terminating communication sessions between devices. It plays a crucial role in ensuring that data exchange between applications is synchronized, organized, and reliable.

Session Maintenance: During an ongoing communication session, the Session Layer oversees its continuity. It manages synchronization between devices, ensuring that data is exchanged in an orderly and predictable manner. This layer also handles error recovery and retransmission of lost or corrupted data if needed, maintaining the integrity of the session.

Session Establishment: The Session Layer is responsible for initiating communication sessions between devices. This involves setting up parameters for communication, such as session identifiers, authentication, and synchronization points. Session establishment ensures that both sender and receiver are ready to exchange data.

Session Termination: Once a communication session has concluded or is no longer needed, the Session Layer facilitates its graceful termination. This involves releasing any resources allocated for the session, such as buffers or connections, and ensuring that both parties agree to end the session in a coordinated manner.

Dialog Control: The Session Layer is responsible for managing the dialog between devices, which involves determining which device can transmit and when. It helps prevent conflicts and ensures that data is exchanged in an organized and coherent manner.

Synchronization Points: In some applications, the Session Layer establishes synchronization points within the data stream, allowing devices to resume communication from specific points in case of interruptions. This is particularly useful in scenarios where large files are being transferred, and resuming from the last synchronized point reduces data loss.

Checkpointing: The Session Layer may implement checkpointing, where it periodically saves the current state of the communication session. This enables recovery from unexpected failures by allowing the session to resume from the last checkpointed state.

Example Use Cases: The Session Layer is particularly relevant in scenarios such as file transfer protocols (e.g., FTP), remote desktop connections (e.g., RDP), and online gaming, where maintaining a synchronized and reliable session is critical for a seamless user experience.

Layer 6: Presentation Layer

The Presentation Layer, positioned as the sixth layer in the OSI Model, is responsible for managing data translation, encryption, compression, and ensuring that data exchanged between applications is in a format that can be understood by both sender and receiver.

Data Translation: One of the primary functions of the Presentation Layer is to handle data translation and format conversion. This includes tasks like character encoding, where it ensures that characters from different character sets (e.g., ASCII, Unicode) are correctly represented and understood by both communicating devices. It also deals with issues like byte order, making sure that multi-byte data is arranged consistently for devices with different architectures.

Encryption and Decryption: The Presentation Layer plays a vital role in data security by providing encryption and decryption services. It can encrypt data before transmission, ensuring that sensitive information remains confidential during its journey across the network. Upon receipt, it decrypts the data, allowing the recipient to access the original content.

Data Formatting: The Presentation Layer also handles data formatting and representation, ensuring that data is structured in a way that can be easily interpreted by the receiving application. For example, it may convert data into a standard format for presentation on a web browser or a word processing application.

Compression: In scenarios where bandwidth efficiency is crucial, the Presentation Layer can compress data before transmission and decompress it at the receiving end. Data compression reduces the amount of data transmitted over the network, improving transfer speeds and reducing network congestion.

Translation Between Data Formats: When data is exchanged between applications running on different systems, the Presentation Layer can translate the data between different formats or representations to ensure compatibility. This is particularly important in heterogeneous environments with a variety of software and hardware platforms.

Example Use Cases: The Presentation Layer is particularly relevant in applications like web browsers, where it translates and renders web content (HTML, images) for display. It's also essential in secure communication, where it handles encryption and decryption of data for confidentiality.

Layer 7: Application Layer

The Application Layer, positioned as the seventh and highest layer in the OSI Model, is the interface between network services and end-user applications. It is the layer closest to the end-users and encompasses a wide range of functions that facilitate communication and interaction between applications and network services.

Application Protocols: This layer defines numerous application protocols that govern specific types of communication between applications and services. These protocols dictate how data is formatted, transmitted, and interpreted. Common application layer protocols include HTTP (for web browsing), SMTP and POP3/ IMAP (for email), and FTP (for file transfer).

User Interface: The Application Layer provides the user interface that allows individuals or software applications to interact with network services. This includes applications like web browsers, email clients, instant messaging programs, and more. The user interface ensures that users can input commands, request data, and view responses.

Data Exchange: The Application Layer is responsible for the exchange of data between applications running on different devices over a network. It ensures that data sent from one application can be correctly received and understood by another application, regardless of their operating systems or hardware platforms.

Data Presentation: Building on the work of the Presentation Layer, the Application Layer may also handle data presentation and rendering. This includes tasks such as displaying web content, rendering multimedia, or formatting documents for printing.

Security and Authentication: Security mechanisms, including user authentication and data encryption, are often implemented at the Application Layer to safeguard data and ensure that only authorized users can access certain services or resources.

Example Use Cases: The Application Layer encompasses a wide array of applications and services, ranging from web browsers, email clients, and remote desktop software to online gaming platforms, file sharing tools, and more. Essentially, any software that relies on network communication falls within the domain of the Application Layer.

The OSI Model's relevance in networking lies in its ability to serve as a universal reference framework. Here's why it's crucial:

Layered Approach: The model breaks down network communication into discrete layers, making it easier to design, develop, troubleshoot, and maintain network protocols and systems.

Standardization: It offers a standardized way to discuss and understand network communication. This standardization simplifies communication between different vendors and technologies.

Interoperability: By defining clear boundaries and responsibilities for each layer, the OSI Model promotes interoperability. Devices and software developed independently can communicate effectively if they adhere to the model's specifications.

Troubleshooting: When network issues arise, the model helps in pinpointing the layer at which the problem exists. This aids network administrators and engineers in diagnosing and resolving issues efficiently.

The OSI Model and the TCP/IP Model are two fundamental frameworks used to conceptualize and understand network architecture and communication. While both models serve as guides for designing and troubleshooting networks, they differ significantly in their approach.

The OSI Model, with its seven-layer structure, offers a comprehensive and highly structured view of networking concepts, promoting interoperability and clear separation of concerns. In contrast, the TCP/IP Model, with its four-layer structure, mirrors the practical implementation of the Internet, simplifying the model for real-world network communication. This simplification, however, can come at the cost of some of the comprehensive structure found in the OSI Model, making each model suitable for different networking contexts.

Pros and Cons of the OSI Model compared with the TCP/IP Model:

Radware's solutions seamlessly integrate with various OSI model layers to provide enhanced network and application security to protect against threats and vulnerabilities. Below are a few examples—in the context of the OSI Model—of how Radware solutions help enhance network and application security:

Transport Layer Protection:

Scenario: An e-commerce platform experiences slow performance due to a high volume of bot-generated traffic on its payment gateway.

Radware Solution: Radware Bot Manager, operating at Layer 4, employs device fingerprinting to distinguish between legitimate users and malicious bots. By identifying and blocking bot-generated requests at the transport layer, it safeguards the payment gateway from fraudulent activities.

Network Layer Protection:

Scenario: A company's network faces repeated DDoS attacks targeting the routing infrastructure, causing network disruptions.

Radware Solution: Radware's DDoS protection solutions, operating at Layer 3, use advanced traffic analysis techniques to detect and mitigate DDoS attacks in real time. By identifying and diverting malicious traffic away from critical network resources, these solutions ensure uninterrupted network availability.

Application Layer Protection:

Scenario: A healthcare provider's web application handles sensitive patient data, making it a target for cyberattacks.

Radware Solution: Radware's Application Security solutions, operating at Layer 7, provide comprehensive protection against application-layer attacks such as SQL injection and cross-site scripting (XSS). By examining application traffic patterns and using device fingerprinting, these solutions identify and block malicious activities, ensuring the security and privacy of patient data.

Secure Remote Access - Across OSI Layers:

Scenario: A financial institution needs to provide secure remote access for employees working from various locations.

Radware Solution: Radware's remote access solutions, operating across OSI layers, use device fingerprinting to ensure that only authorized and secure devices can access the institution's network. By examining device attributes and behavior, these solutions enhance security during remote access, regardless of the OSI layer.

Client Reputation - Across OSI Layers:

Scenario: An online gaming platform faces challenges with user accounts engaging in suspicious or cheating behavior.

Radware Solution: Radware's Client Reputation solution, spanning OSI layers, assesses the reputation of incoming clients based on their behavior and device characteristics. This multi-layer approach, including device fingerprinting, helps in identifying and taking action against users with a history of cheating or fraudulent activities.

6 Common Cloud Vulnerabilities That Can Lead To A Data Breach

This guide outlines 6 common cloud computing vulnerabilities that can lead to a data breach and how automation can stop it.

Radware 360° Application Protection Guide

For many organizations, the greatest concern they have about migrating their application environment to the cloud is what it may mean to their attack surface.

The State Of Web Application And API Protection

This report uses survey data to examine organizations’ application and API security awareness, visibility, best practices and security strategies and looks at how different roles within a company view app security.

Best Practices For Frictionless Application Protection

This white paper outlines the challenges organizations face when security becomes a roadblock to innovation and agility, and provides 6 best practices to follow to ensure cybersecurity remains frictionless

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Presentation layer and Session layer of the OSI model

There are two popular networking models: the OSI layers model and the TCP/IP layers model. The presentation layer and session layer exist only in the OSI layers models. The TCP/IP layers model merges them into the application layer.

The Presentation Layer

The presentation layer is the sixth layer of the OSI Reference model. It defines how data and information is transmitted and presented to the user. It translates data and format code in such a way that it is correctly used by the application layer.

It identifies the syntaxes that different applications use and formats data using those syntaxes. For example, a web browser receives a web page from a web server in the HTML language. HTML language includes many tags and markup that have no meaning for the end user but they have special meaning for the web browser. the web browser uses the presentation layer's logic to read those syntaxes and format data in such a way the web server wants it to be present to the user.

On the sender device, it encapsulates and compresses data before sending it to the network to increase the speed and security of the network. On the receiver device, it de-encapsulates and decompresses data before presenting it to the user.

Examples of the presentation layer

Example standards for representing graphical information: JPEG, GIF, JPEG, and TIFF.

Example standards for representing audio information: WAV, MIDI, MP3.

Example standards for representing video information: WMV, MOV, MP4, MPEG.

Example standards for representing text information: doc, xls, txt, pdf.

Functions of the presentation layer

• It formats and presents data and information.
• It encrypts and compresses data before giving it to the session layer.
• It de-encrypts and decompresses the encrypted and compressed data it receives from the session layer.

Session layer

The session layer is the fifth layer of the OSI layers model. It is responsible for initiating, establishing, managing, and terminating sessions between the local application and the remote applications.

It defines standards for three modes of communication: full duplex, half-duplex, and simplex.

In the full duplex mode, both devices can send and receive data simultaneously. The internet connection is an example of the full duplex mode.

In the half duplex mode, only one device can send data at a time. A telephone conversation is an example of the half-duplex mode.

In the simplex mode, only one device can send data. A radio broadcast is an example of the simplex mode.

Functions of the session layer

• It is responsible for terminating sessions, creating checkpoints, and recovering data when sessions are interrupted.
• It opens and maintains logical communication channels between network applications running on the local host and network applications running on the remote host.
• If a network application uses an authentication mechanism before it opens a logical communication channel (session) with the remote host, it handles the authentication process.

Examples of the session layer

Structure Query Language (SQL), Remote Procedure Call (RPC), and Network File System (NFS) are examples of the session layer.

By ComputerNetworkingNotes Updated on 2023-04-25

ComputerNetworkingNotes CCNA Study Guide Presentation layer and Session layer of the OSI model

• EtherChannel Load Distribution Explained
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• EtherChannel Manual Configuration
• EtherChannel Basic Concepts Explained
• STP, RSTP, PVST, RPVST, and MSTP
• Similarities and Differences between STP and RSTP
• RSTP / RPVST Explained with Examples
• PVST/RPVST and EtherChannel Explained
• STP/RSTP Timers Explained

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Network Basics: The 7 Layers of the OSI Model

In the 1980s, networking was in its infancy. Engineers needed a way to visualize the different elements of a networking system. The computing world urgently required a standard language to communicate across companies, business sectors and even cultures. The OSI model filled the gap, providing a functional way to describe and analyze network structures.

The 7-layer OSI model is now common knowledge across the world. Despite decades of IT development and the emergence of the internet, it remains relevant. This article will explain how the model works, why it is still useful -- when used carefully.

What is the OSI model (Open Systems Interconnection)?

The OSI (Open Systems Interconnection) model was first published in 1984 by the International Organization for Standardization (IOS). IOS sought to create a standardized language for network analysis and communication. The OSI model provided this language, enabling different devices and networks to transmit data smoothly.

The OSI model divides networking into 7 separate "layers". Each OSI model layer is part of a seven-stage stack. Information descends and ascends the stack as data flows through networks. In theory, the stacks represent critical processes in data transmission. These stages could include encryption, packet creation, flow management, and presentation.

How does the OSI model work?

The OSI reference model generally flows downwards from Level 7 (the Application Layer) to Level 1 (the Physical Layer).

Every stack in the model describes a stage in the journey of an idealized data packet through a communication system. In a typical transmission, data flows from Layer 7 downwards to Layer 1, and then back upwards to Layer 7 where it can be used by recipients.

The model layers communicate with each other. Each layer deals with levels directly above and below, creating a neat chain of activity.

This stacked construction makes sense from a troubleshooting perspective. Engineers can isolate problems at the network or application layer. Or they might look at physical medium issues such as cabling.

By focusing on specific network issues, engineers can reduce workloads and diagnose problems more effectively.

Importance of OSI Model

The OSI model was important because it represented the first systematic attempt to standardize networking language. The fact that the model became used worldwide shows that the creators succeeded. And forty years later the OSI concept still has many uses.

• Troubleshooting -- The OSI hierarchy is a good shorthand for detecting network flaws. Technicians can use the model to detect network-wide problems, application issues, or faults in physical equipment. OSI provides a clear way to break problems down into manageable chunks.
• Marketing – The OSI layer model allows software and hardware vendors to describe the functions of products. Marketers can clearly explain to buyers where their products fit into the OSI hierarchy. Buyers can understand how those products will fit into network architecture.
• Software Development – The OSI model helps developers during planning and coding phases. Developers can model how applications will function at specific layers. The layer model provides a guide to how apps will interact with other network components.
• Security awareness – The OSI system allows security teams to identify security vulnerabilities. Security teams can classify risks according to OSI layers. They can identify where data rests in the network hierarchy, and assign protective controls to ensure data security. OSI layers also help to stage secure cloud migrations.

The OSI hierarchy is just a model. But it is a very useful way to conceptualize network structures and connections between communication partners. The model makes it easier to compare applications, protocols, hardware profiles, and much more.

The OSI model provides a language for experts to use when discussing IT architecture. So while newer models have appeared, we still rely on the OSI template to understand networking.

Advantages of the OSI model

Despite being published in 1984, the OSI concept has many advantages. Put simply, the OSI model:

• Helps with sourcing hardware and software to build network architecture
• Assists IT teams in understanding how network components communicate
• Allows experts to troubleshoot network problems
• Makes it easier to develop tools that can communicate with those from other vendors
• Provides a way to communicate how software and hardware operate in networks. Allows technicians to talk to outsiders in a simple manner, with a reasonable degree of accuracy.

Disadvantages of the OSI model

The advantages above are significant but need to be qualified with a few important drawbacks:

• Most networking experts argue that the OSI system is outdated. The division of network structures into 7 different layers no longer makes sense in the age of the internet and cloud computing. The internet generally suits the TCP/IP model more closely than OSI.
• The 7-layer model may also feature redundant elements. For instance, the Session Layer and Presentation Layer may not have practical relevance in real-world networks.
• Some network functions reach across OSI layers, creating unnecessary confusion.

7 layers of the OSI model and their functions

The 7 layers of the OSI model are usually viewed from 7 downwards. So it makes sense to explain each one as data descends the hierarchy.

Layer 7 – The Application Layer

The application layer is where users interact with data. This does not include all applications at the edge of the network. For instance, email clients or video conferencing apps would not be included. Instead, the application layer includes the software used to allow network-facing apps to function.

Application layer functions include the operation of protocols and data formatting tools. Common layer 7 protocols include SMTP and HTTP. The function of the application layer is to accept data for software to use, or to carry out preparations before sending data down the OSI chain.

Layer 6 – The Presentation Layer

The presentation layer manipulates data before the application layer can use it. This layer "presents" raw data. The presentation layer turns it from a bitstream into something that applications can decode and use.

The presentation layer is important in a security context. This is the stage where data is encrypted and compressed (or decrypted and decompressed). Data encryption allows secure transmission. Compression allows networks to transmit more data at higher speeds.

Layer 5 – The Session Layer

When data is transferred in computer networking, two devices agree to create a "session." The session layer applies agreed rules about how data will be transmitted and authenticated. It expires when the transmission is complete.

The Session Layer is responsible for commencing communication between devices. It determines how long sessions last and checks that data is transmitted accurately. This generally involves the use of data checkpoints. Checkpoints break down data into smaller segments. Each segment is checked for fidelity before the session closes.

The Session Layer has a security function. Sessions must close quickly and include authentication systems to identify data sources and recipients. But the main function of the session layer is ensuring efficient data transfer with minimal resource use.

Layer 4 – The Transport Layer

The Transport Layer involves setting up direct communication between connected devices. This layer may also break down data, an operation that reaches across OSI layers. But the overall function of the Transport Layer is ensuring that data leaves and arrives in the same condition.

The Transport Layer controls the flow of data in end-to-end communication. Tools decide the correct speed for data transmission. This may vary depending on the connection speeds involved. Devices with faster connections can flood those with slower speeds, creating performance issues.

The transport layer also carries out error control. Error control tools assess data packets at the receiving device. If data arrives in poor quality, Transport Layer tools will request a repeat transmission.

Well-known Transport Layer protocols include the Transmission Control Protocol (TCP). This protocol functions alongside Internet Protocol (IP) information, forming the TCP/IP standard.

Layer 3 – The Network Layer

The Network Layer is where data is actually sent between connected devices. This makes the network layer a common area of focus for network engineers, and one of the most important nodes in the OSI chain.

The role of the Network Layer is to create and maintain stable network connections. Data is divided into packets that are ready for network transmission. These packets are then put back together at the receiving end of the transmission, reconstituting the original data.

Hardware and software tools at the network layer are also responsible for routing data. Routers decide an optimal route for a data transfer. At Layer 3, routing generally involves communication between different networks. Layer 2 tends to deal with local data routing.

Layer 2 – The Data Link Layer

The Data Link layer is closely related to the Network Layer but usually refers to communication between locally-connected devices. For instance, the data link layer might model connections between on-premises workstations and routers.

At the data link layer, data is accepted and broken down into frames. Frames are suited to local transmission, and interact with two sub-layers of the data link layer:

• Media Access Control (MAC) layer – The media access control layer connects related local devices and manages flow rates across the network.
• Logical Link Control (LLC) layer – Sets up the logical basis for local data transmission.

The data link layer regulates flows between local devices in a similar way to the network layer. The two layers are therefore often analyzed together when assessing network problems.

Layer 1 – The Physical Layer

The Physical Layer covers all of the physical infrastructure and equipment needed to transfer data. The physical layer includes network cables and switches, as well as radio frequency links, voltage regulators, and routing devices.

Data is converted into a digital bitstream formed from 1s and 0s at the physical layer. The form of this bitstream is agreed by two devices before transmission. This makes it possible to reconstruct data at the receiving end.

The Physical Layer is often the first place to look when troubleshooting networks. Cable connections and faulty power supplies are common problems with relatively simple solutions.

Cross-layer functions

Many applications or services bridge different layers in the OSI hierarchy. These services are called cross-layer functions. Cross-layer functions include critical services that affect multiple parts of the data transmission process. Examples could include:

• Security management tools to configure and monitor communications between network devices.
• Multi-protocol label switching (MPLS) services to carry data frames between networks.
• Protocols that translate IP addresses into MAC addresses and work across the data link layer and the network layer.
• Domain Name System (DNS) lookup services.
• General security architecture recommended by ITU's x.800 standard.

Cross-layer functions tend to deliver security, availability or reliability. They work across network layers to regulate and monitor traffic, ensuring data security and resolving problems as they arise. Because of this, cross-layer services are a core part of network security planning.

OSI Model vs TCP IP Model

The Transfer Control Protocol/Internet Protocol (TCP/IP) model is the major alternative to the OSI reference model.

TCP/IP actually pre-dates OSI, and was created by the US Department of Defense in the 1970s. Many argue that the emergence of the internet as the dominant form of telecommunication has made TCP/IP more useful as a way of describing network environments.

The main difference between the TCP/IP and OSI models is the number of layers they include. OSI includes 7 layers. TCP/IP removes OSI layers 5-7 and blends them into a single application layer. OSI layers 1 and 2 are also combined in a Network Interface Layer.

The TCP/IP model tends to be a good fit for networks extended across the public internet. It also accurately models the operation of internet communication protocols. OSI is a much more general model. It does not refer to any specific protocols. Instead, the OSI reference model describes network communication as a whole.

TCP/IP is more focused on practical operations. All of the layers are used by relevant applications. In the OSI model, applications may only use a few of the layers. Layers 1-3 are the only essential elements in transmitting data.

In practice, security architects can learn from both models. OSI remains valuable in comparing products and troubleshooting networks. Both the TCP/IP model and the OSI model have roles to play in the way we visualize network security.

Published: 11 June 2024 Contributors: Chrystal R. China, Michael Goodwin

The Open Systems Interconnection (OSI) model—also called the OSI reference model—is a conceptual model that divides network communication and interoperability into seven abstract layers. It provides a standardized model that enables different applications, computer systems and networks to communicate.

The OSI model emerged as a solution to communication incompatibilities between the diverse array of networking protocols in use around the turn of the century. The layers of OSI gave developers and engineers a framework for building interoperable hardware and software across networks by providing a categorical approach to networking .

At each layer of the stack—typically shown in reverse order to illustrate how data moves through a network—the OSI model provides guidelines and criteria for network components and their unique computing functions.

The layers are:

• Layer 7: The application layer initiates communication with the network, including the protocols and data manipulation processes that convert computer-readable network data into user-readable responses.
• Layer 6: The presentation layer prepares data for the application layer, including data translation, compression and encryption .
• Layer 5: The session layer initiates and terminates connections between two devices interacting on the network, making sure that resources are neither overused nor underutilized.
• Layer 4: The transport layer transmits end-to-end data between two devices interacting on the network, making sure that data isn’t lost, misconfigured or corrupted.
• Layer 3: The network layer handles data addressing, routing and forwarding processes for devices interacting across different networks. If the devices are on the same network, they don’t need the network layer to interact.
• Layer 2: Unlike the network layer, the data link layer manages data routing between two interacting devices on the same network.
• Layer 1: The physical layer comprises the physical assets, like routers and USB cables, that convert data into strings of 1s and 0s for transmission to higher layers.

The OSI model focuses on providing a list of tasks for engineers to complete in building each layer of a network architecture, instead of specifying protocols for communication between layers. Its theoretical approach enables developers to visualize and build highly complex computing networks, even without prior knowledge of the networking system itself. It also helps teams better understand how data traverses a network and tailor network functions with layer-specific coding.

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Although the OSI model isn’t the direct basis for modern computer networking technologies, it’s had a profound impact on computing standards development, helping shape contemporary understandings of network architecture.

In the late 1970s and early 1980s, computer systems were becoming increasingly interconnected, but manufacturers often developed their own networking solutions, creating a patchwork of proprietary and non-interoperable systems. 

Several early networking efforts attempted to address compatibility issues with the ARPANET (which laid the groundwork for the modern internet) and the TCP/IP protocol suite (commissioned by the Department of Defense). Both represented significant advancements, but they also highlighted the need for a more comprehensive and universally accepted approach. 

Recognizing the growing importance of networking and the need for a universal framework, the International Organization for Standardization (ISO) and the International Telegraph and Telephone Consultative Committee (CCITT) initiated the development of a standardized networking model.

The ISO formally published the OSI model, a seminal framework for developing interoperable network solutions, in 1984. Unlike previous standardization attempts, the layered configuration of OSI enables disparate systems to communicate despite differences in their underlying architectures and protocols.

The OSI model remains integral to understanding network architecture even as technologies evolve and new models emerge. Whether a team is designing a simple local area network (LAN) or managing a complex global network , the principles of the OSI model provide a clear, structured approach to networking.

The OSI model includes seven distinct layers. The application layer (layer 7), the presentation layer (layer 6) and the session layer (layer 5) comprise the software layers of an OSI, where all transmissions to and from software apps (including operating systems and utilities, such as web browsers and email clients) occur.

The transport layer (layer 4) is the “heart of OSI,” handling all data communication between networks and systems. Finally, the network layer (layer 3), the data layer (layer 2) and the physical layer (layer 1) comprise the hardware layers of OSI, where data moves through the physical components of the network as it’s processed.

Data moves bi-directionally through the OSI model; each layer communicates with the layers below and above it in the stack. Furthermore, both the sending and receiving devices transmit data through the data layers; and senders and receivers often switch roles in the process.

For example, if a user wants to send an email to another person, the user would first write the email and send it. When the user presses “send,” their email goes to the application layer, which will choose the correct protocol (typically SMTP) and send the email to the presentation layer. The presentation layer then compresses the message data and sends it to the session layer, which initiates a communication session and sends the data to the transport layer for segmentation.

Since the email is going to another network, the email data must go to the network layer, where it’s divided into packets and then to the data link layer where it’s further broken down into frames. Those frames are subsequently transmitted through the physical layer (the recipient’s wifi), at which point the recipient’s device receives the bit stream and the email data traverses the same layers in reverse. At the end of the process, the email data lands in the application layer of the recipient’s device where it’s delivered, in human-readable form, to the recipient’s inbox.

The OSI model is foundational to protocol development, with each layer of the framework managing specific network processes.

The application layer is the OSI layer closest to the end user. It provides network services directly to user applications and facilitates communication between  API  endpoints and lower layers of the OSI model. In other words, software applications use the application layer to initiate communication with the network and send data to the presentation layer.

Applications themselves are not part of this layer. Rather, the application layer provides the protocols (HTTP, FTP,  DNS  and SMTP, for instance) that enable software to send and receive data. It’s responsible for processes such as:

• File transfer.  The application layer takes human-readable data files from the user’s device and transmits them to the presentation layer.
• Communication and authentication.  The application layer makes sure that the receiving device can accept the data and that the communications interfaces needed for the transfer exist. It can also be used to  authenticate the devices involved  in the transfer.
• Remote access.  The application layer enables users to access web browsers, email clients and other services from various geographical locations. It also enables users to access and manage files on a remote computer.

Directory services.  The application layer provides directory services—a shared database of information about network devices and users—to facilitate network resource management.

The presentation layer transforms data into a format that the application layer can accept for transmission across the network (from an EBCDIC-coded text file to an ASCII-coded file, for instance). Due to its role in converting data and graphics into a displayable format for the application layer, it is sometimes referred to as the syntax layer.

It supports secure sockets layer/transport layer security (SSL/TLS) protocols, JPEG protocols (for image compression) and MPEG protocols (for video The presentation layer transforms data into a format that the application layer can accept for transmission across the network (from an EBCDIC-coded text file to an ASCII-coded file, for instance). Due to its role in converting data and graphics into a displayable format for the application layer, it is sometimes referred to as the syntax layer.

It supports secure sockets layer/transport layer security (SSL/TLS) protocols, JPEG protocols (for image compression) and MPEG protocols (for video compression). The presentation layer is responsible for:

Data translation.  The presentation layer converts data into the correct format (specified by the application layer) during the encapsulation process, as outgoing messages move down the protocol stack from sender to receiver.

• Data encryption and decryption.  The presentation layer encrypts data for secure transmission and decrypts it upon delivery.

Data compression.  The presentation layer reduces the size of a data stream for transmissions and decompresses it for use.

Sometimes formatting and translation are reversed during the de-encapsulation process, as incoming messages move up the protocol stack. In those instances, outgoing messages are converted into the specified format during encapsulation, while incoming messages undergo a reverse conversion during de-encapsulation.

The session layer is responsible for session management, the process of establishing, managing and terminating connections—called "sessions"—between two or more computers. It initiates the connections between local and remote applications, keeping the session open long enough to transmit the necessary data and closing them when complete to preserve network resources.

Key functions of the session layer include:

Session interactions.  The session layer manages user logon (establishment) and user logoff (termination), including any authentication protocols integrated into client software.

• Synchronization.  The session layer helps ensure that data streams are properly synchronized and handles recovery points (checkpoints that allow devices to resume a session from a specific point, if interrupted).

Session recovery.  The session layer manages session failures and re-establishes connections if there are network problems.

It also establishes protocols for connecting and disconnecting sessions between related data streams, such as audio and video in web conferencing. Therefore, the session layer is often explicitly implemented in network environments that utilize remote procedure calls.

The transport layer uses protocols like transmission control protocol (TCP) and the user datagram protocol (UDP) to manage the end-to-end delivery of complete messages. It takes messages from the session layer and breaks them into smaller units (called “segments”), each with an associated header. At the destination, the transport layer reassembles the segments in the correct order to reconstruct the original message.

The transport layer also handles:

Service point addressing.  The transport layer helps ensure that messages are delivered to the correct process by attaching a transport layer header (including a service point or port address).

• Flow control.  The transport layer prevents data overflow and manages the rate of data transmission between two devices interacting on the network, making sure that sending devices transmit data to receiving devices (and vice versa) at the appropriate speed.

Multiplexing.  The transport layer allows multiple network applications to use the same connection simultaneously.

At the sender's end, the transport layer receives formatted data from the upper layers, performs segmentation and implements flow and error control to ensure accurate data transmission. It adds source and destination port numbers to the header and then forwards the segmented data to the network layer.

At the receiver's end, the transport layer reads the port number from the header and forwards the received data to the appropriate application. It also handles the sequencing and reassembly of the segmented data and retransmits data if errors are detected.

The transport layer provides two types of service. 

With  connection-oriented service , a three-part process including connection establishment, data transfer and termination (or disconnection), the data receiver sends an acknowledgment of receipt back to the sender when the data packet is delivered.  Connectionless service , however, only involves data transfer. The receiver does not confirm receipt, which accelerates communication but can be less reliable than connection-oriented service.

The network layer of the OSI model is responsible for facilitating data transfer from one node to another across different networks. The network layer determines the best path (routing) for data to travel between nodes. If segments are too large, the network layer breaks them up into smaller “packets” for transport and reassembles them on the receiving end.

A network serves as a medium where multiple nodes (each with a unique address) can connect. The network layer allows nodes to send messages to nodes on other networks by providing the message content and the destination address, leaving the network to  determine the optimal delivery path  (which may involve routing through intermediate nodes).

The network layer primarily uses the Internet Protocol v4 (IPv4) and IPv6 and is responsible for:

Packet fragmentation and reassembly.  The network layer splits large packets (those that exceed the size limits of the data link layer) into smaller ones for transmission and reassembles them at the destination.

• Traffic control.  The network layer manages network traffic to prevent congestion and safeguard efficient data flow. 

Reliability isn’t guaranteed in the network layer; while many network layer protocols offer reliable message delivery, some do not. Furthermore, error reporting isn’t mandatory at this layer of OSI, so data senders may or may not receive confirmation of delivery.

The data link layer’s primary function is to manage error-free data transfer between multiple devices interacting on the same network.

The DLL is divided into two sublayers.

The  logical link control (LLC) layer —which serves as an interface between the media access control (MAC) layer and the network layer—handles flow control, synchronization and multiplexing (where two or more data streams share a single connection to the host). The  MAC layer  controls how devices access network mediums and transmit data.

When the DLL receives a packet from the network layer, it divides the packet into data “frames”—according to the frame size of the network interface card (NIC)— and transmits it to the host using its MAC address.

DLL functions include:

Framing.  The DLL allows the sender to transmit a set of bits (data) that are meaningful to the receiver by attaching special bit patterns to the beginning and end of the frame.

• Physical addressing.  The DLL uses the Address Resolution Protocol (ARP) to convert IP addresses to MAC addresses and then adds the MAC addresses of the sender and receiver to the header of each frame after framing is complete.

Error control.  The DLL detects damaged or lost frames and manages retransmission (if necessary) to ensure data integrity.

• Flow control.  To prevent corruption, the DLL dictates how much data a sender can send before receiving an acknowledgment of delivery, keeping the data rate consistent on both sides.

Access control.  When multiple devices share a single communication channel, the MAC sublayer determines which device has control over the channel at a given moment.

The physical layer comprises the physical network components responsible for transmitting raw data—in the form of “bits,” or strings of 1s and 0s—between devices (connectors, routers, repeaters and fiber optic cables, for instance) and a physical medium (like wi-fi).

The physical layer is responsible for:

Bit rate control.  The physical layer defines the data transmission rates, often in bits per second.

• Bit synchronization.  The physical layer imposes a clock on bit streams, ensuring that the sender and receiver are synchronized at the bit level. 

Transmission mode.  The physical layer defines how data will flow between connected devices (as simplex, half duplex or full duplex transmission).

• Physical topologies.  The physical level specifies how network devices and nodes are situated (in  bus, star or mesh topologies , for example). Standards like USB, Bluetooth and Ethernet include physical layer specifications. 

The physical layer also defines how encoding occurs over a physical signal (using electrical voltage, radio or light pulses, for example). 

The OSI reference model provides a theoretical underpinning that helps engineers and developers understand the intricacies of network communication. However, it’s sometimes compared with another networking model: the transmission control protocol/internet protocol (TCP/IP) model.

Unlike the OSI model, the TCP/IP model is based on standardized protocols that are widely and directly implemented and in real-world networks. It consists of four layers—instead of seven—but each layer corresponds to one or more layers of the OSI model.

Network access layer. Also called the data link layer or the physical layer, the network access layer of a TCP/IP network includes both the hardware and software components necessary for interfacing with the network medium, combining the OSI model’s physical and data link layers. It handles physical data transmission—by using Ethernet (for LANs) and ARP protocols—between devices on the same network.

• Internet layer. Similar to the OSI model's network layer, the internet layer is responsible for logical addressing, routing and packet forwarding. It primarily relies on the IP protocol and the Internet Control Message Protocol, which manages addressing and routing of packets across different networks.

Transport layer. The TCP/IP transport layer serves the same function as the OSI model's transport layer; it enables reliable data transfer between upper and lower layers. Using TCP and UDP protocols, it also provides mechanisms for error checking and flow control.

• Application layer. TCP/IP’s application layer encompasses the OSI model’s session, presentation and application layers. It uses HTTP, FTP, Post Office Protocol 3 (POP3), SMTP, DNS and SSH protocols to provide network services directly to applications and manages all the protocols that support user applications. 

The OSI model's primary value lies in its educational utility and its role as a conceptual framework for designing new protocols, making sure that they can interoperate with existing systems and technologies.

However, the TCP/IP model's practical focus and real-world applicability have made it the backbone of modern networking. Its robust, scalable design and horizontal layering approach has driven the explosive growth of the internet, accommodating billions of devices and massive amounts of data traffic.

Its abstracted, vertically layered approach to networking enables modular protocol design, where each layer can be developed and updated independently.

The modularity of the OSI model encourages faster innovation in protocol development, since software engineers can integrate new technologies without overhauling the entire network stack.

It also enables developers to abstract away the lower layers of the model to simplify development.

Software engineers can separate the operating layers of each network component and organize them according to their primary roles in the network. This decomposability makes it easier for developers to conceptualize a network and share simplified models between development teams.

When a device on the network fails or an app loses connection, the OSI model allows teams to pinpoint and isolate the problematic layer to address any security issues or networking vulnerabilities without disrupting the entire framework.

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Modern network infrastructures built for digital transformation require solutions that can be just as dynamic, flexible, and scalable as the new environments. IBM SevOne provides application-centric, network observability to help NetOps spot, address and prevent network performance issues in hybrid environments. 

With IBM Cloud load balancers, you can load balance traffic among your servers to help improve uptime. You can also easily scale your applications by adding or removing servers, with minimal disruption to your traffic flows. 

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Three-tier architecture is a well-established software application architecture that organizes applications into three logical and physical computing tiers.

5G, or fifth-generation mobile technology, is the new standard for telecommunications networks launched by cell phone companies in 2019.

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Data integration refers to the process of combining and harmonizing data from multiple sources into a unified, coherent format that can be put to use for various analytical, operational and decision-making purposes.

Cloud computing is the on-demand access of computing resources—physical servers or virtual servers, data storage, networking capabilities, application development tools, software, AI-powered analytic tools and more—over the internet with pay-per-use pricing.

IBM NS1 Connect provides fast, secure connections to users anywhere in the world with premium DNS and advanced, customizable traffic steering. NS1 Connect’s always-on, API-first architecture enables your IT teams to more efficiently monitor networks, deploy changes and conduct routine maintenance.

• Network infrastructure

OSI model (Open Systems Interconnection)

• Andrew Froehlich, West Gate Networks
• Linda Rosencrance
• Kara Gattine, Director of Editorial Operations

What is OSI model (Open Systems Interconnection)?

OSI (Open Systems Interconnection) is a reference model for how applications communicate over a network. This model focuses on providing a visual design of how each communications layer is built on top of the other, starting with the physical cabling, all the way to the application that's trying to communicate with other devices on a network.

A reference model is a conceptual framework for understanding relationships. The purpose of the OSI reference model is to guide technology vendors and developers so the digital communications products and software programs they create can interoperate and to promote a clear framework that describes the functions of a networking or telecommunications system that's in use.

Most vendors involved in telecommunications try to describe their products and services in relation to the OSI model. This helps them differentiate among the various transport protocols, addressing schemes and communications packaging methods. And, although it's useful for guiding discussion and evaluation, the OSI model is theoretical in nature and should be used only as a general guide. That's because few network products or standard tools keep related functions together in well-defined layers, as is the case in the OSI model. The Transmission Control Protocol/Internet Protocol ( TCP/IP ) suite, for example, is the most widely used network protocol, but even it doesn't map cleanly to the OSI model.

History of the OSI model

In the 1970s, technology researchers began examining how computer systems could best communicate with each other. Over the next few years, several competing models were created and published to the community. However, it wasn't until 1984 when the International Organization for Standardization (ISO) took the best parts of competing networking reference models to propose OSI as a way to finally create a framework that technology companies around the world could use as the basis of their networking technologies .

From ISO's perspective, the easiest way to create a conceptual model was to organize the models into different abstraction layers required to organize and send data between computing systems. Looking inside each abstracted layer to see the details shows one part of this network communication process. Each layer can be thought of as a separate communication module or piece of the puzzle. But, to actually accomplish the goal of sending data from one device to another, each module must work together.

How the OSI model works

Information technology (IT) networking professionals use OSI to model or conceptualize how data is sent or received over a network. Understanding this is a foundational part of most IT networking certifications, including the Cisco Certified Network Associate (CCNA) and CompTIA Network+ certification programs. As mentioned, the model is designed to break down data transmission standards, processes and protocols over a series of seven layers, each of which is responsible for performing specific tasks concerning sending and receiving data.

The main concept of OSI is that the process of communication between two endpoints in a network can be divided into seven distinct groups of related functions, or layers. Each communicating user or program is on a device that can provide those seven layers of function.

In this architecture, each layer serves the layer above it and, in turn, is served by the layer below it. So, in a given message between users, there will be a flow of data down through the layers in the source computer, across the network and then up through the layers in the receiving computer. Only the application layer at the top of the stack doesn't provide services to a higher-level layer.

The seven layers of function are provided by a combination of applications, operating systems (OSes), network card device drivers, networking hardware and protocols that enable a system to transmit a signal over a network through various physical mediums, including twisted-pair copper, fiber optics, Wi-Fi or Long-Term Evolution (LTE) with 5G .

7 layers of the OSI model

What is the function of each layer of the OSI model? The seven Open Systems Interconnection layers are the following.

Layer 7. The application layer

The application layer enables the user -- human or software -- to interact with the application or network whenever the user elects to read messages, transfer files or perform other network-related tasks. Web browsers and other internet-connected apps, such as Outlook and Skype, use Layer 7 application protocols.

Layer 6. The presentation layer

The presentation layer translates or formats data for the application layer based on the semantics or syntax the application accepts. This layer also handles the encryption and decryption that the application layer requires.

Layer 5. The session layer

The session layer sets up, coordinates and terminates conversations between applications. Its services include authentication and reconnection after an interruption. This layer determines how long a system will wait for another application to respond. Examples of session layer protocols include X.225 and Zone Information Protocol (ZIP).

Layer 4. The transport layer

The transport layer is responsible for transferring data across a network and provides error-checking mechanisms and data flow controls. It determines how much data to send, where it gets sent and at what rate. TCP within the TCP/IP suite is the best-known example of the transport layer. This is where the communications select TCP port numbers to categorize and organize data transmissions across a network.

Layer 3. The network layer

The primary function of the network layer is to move data into and through other networks. Network layer protocols accomplish this by packaging data with correct network address information, selecting the appropriate network routes and forwarding the packaged data up the stack to the transport layer. From a TCP/IP perspective, this is where IP addresses are applied for routing purposes.

Layer 2. The data-link layer

The data-link , or protocol layer, in a program handles moving data into and out of a physical link in a network. This layer handles problems that occur as a result of bit transmission errors. It ensures that the pace of the data flow doesn't overwhelm the sending and receiving devices. This layer also permits the transmission of data to Layer 3, the network layer, where it's addressed and routed.

The data-link layer can be further divided into two sublayers. The higher layer, which is called logical link control (LLC), is responsible for multiplexing, flow control, acknowledgement and notifying upper layers if transmit/receive (TX/RX) errors occur.

The media access control sublayer is responsible for tracking data frames using MAC addresses of the sending and receiving hardware. It's also responsible for organizing each frame, marking the starting and ending bits and organizing timing regarding when each frame can be sent along the physical layer medium.

Layer 1. The physical layer

The physical layer transports data using electrical, mechanical or procedural interfaces. This layer is responsible for sending computer bits from one device to another along the network. It determines how physical connections to the network are set up and how bits are represented into predictable signals as they're transmitted either electrically, optically or via radio waves.

Cross-layer functions

Cross-layer functions, or services that may affect more than one layer, include the following:

• security service telecommunication as defined by the International Telecommunication Union Standardization Sector (ITU-T) X.800 recommendation;
• management functions that enable the configuration, instantiation, monitoring and terminating of the communications of two or more entities;
• Multiprotocol Label Switching ( MPLS ), which operates at an OSI model layer that lies between the Layer 2 data-link layer and the Layer 3 network layer -- MPLS can carry a variety of traffic, including Ethernet frames and IP packets;
• Address Resolution Protocol (ARP) translates IPv4 addresses (OSI Layer 3) into Ethernet MAC addresses (OSI Layer 2); and
• domain name system (DNS), which is an application layer service that's used to look up the IP address of a domain name.

Pros and cons of the OSI model

The OSI model has a number of advantages, including the following:

• It's considered a standard model in computer networking.
• The model supports connectionless , as well as connection-oriented, services. Users can take advantage of connectionless services when they need faster data transmissions over the internet and the connection-oriented model when they're looking for reliability.
• It has the flexibility to adapt to many protocols.
• The model is more adaptable and secure than having all services bundled in one layer.

The disadvantages of the OSI model include the following:

• It doesn't define any particular protocol.
• The session layer, which is used for session management, and the presentation layer, which deals with user interaction, aren't as useful as other layers in the OSI model.
• Some services are duplicated at various layers, such as the transport and data-link layers.
• Layers can't work in parallel; each layer must wait to receive data from the previous layer.

OSI model vs. TCP/IP model

The OSI reference model describes the functions of a telecommunication or networking system, while TCP/IP is a suite of communication protocols used to interconnect network devices on the internet. TCP/IP and OSI are the most broadly used networking models for communication.

The OSI and TCP/IP models have similarities and differences. The main similarity is in their construction, as both use layers, although the OSI model consists of seven layers, while TCP/IP consists of just four layers.

Another similarity is that the upper layer for each model is the application layer, which performs the same tasks in each model but may vary according to the information each receives.

The functions performed in each model are also similar because each uses a network and transport layer to operate. The OSI and TCP/IP model are mostly used to transmit data packets, although they each use different means and paths to reach their destinations.

Additional similarities between the OSI and TCP/IP models include the following:

• Both are logical models.
• Both define standards for networking.
• They each divide the network communication process in layers.
• Both provide frameworks for creating and implementing networking standards and devices.
• They enable one manufacturer to make devices and network components that can coexist and work with the devices and components made by other manufacturers.
• Both divide complex functions into simpler components.

Differences between the OSI and TCP/IP models include the following:

• OSI uses three layers -- application, presentation and session -- to define the functionality of upper layers, while TCP/IP uses only the application layer.
• OSI uses two separate layers -- physical and data-link -- to define the functionality of the bottom layers, while TCP/IP uses only the link layer.
• OSI uses the network layer to define the routing standards and protocols, while TCP/IP uses the internet layer.

Next: Explore 12 common network protocols all network engineers should know here .

Continue Reading About OSI model (Open Systems Interconnection)

• What is the difference between TCP/IP model vs. OSI model?
• Future of networking technology relies on 5G, edge computing
• 7 TCP/IP vulnerabilities and how to prevent them
• Edge computing and 5G bring the edge to remote workers
• SANs Institute OSI model overview

Related Terms

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An explanation of TCP/IP

Transmission Control Protocol (TCP)

encapsulation (object-orientated programming)

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