presentation layer in osi model geeksforgeeks

  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 .

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What is the OSI Model? 7 layers explained in detail

The OSI model explained and how to easily remember its 7 layers

A tutorial on the open systems interconnection networking reference model and tips on and 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 are: Layer 1—Physical; Layer 2—Data Link; Layer 3—Network; Layer 4—Transport; Layer 5—Session; Layer 6—Presentation; Layer 7—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
• A Penguin Said That Nobody Drinks Pepsi
• A Priest Saw Two Nuns Doing Pushups

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

• Please Do Not Throw Sausage Pizza Away
• Pew! Dead Ninja Turtles Smell Particularly Awful
• People Don’t Need To See Paula Abdul
• Pete Doesn’t Need To Sell Pickles Anymore

Keith Shaw was a Network World editor and the writer of the Cool Tools column. He is now a freelance writer and editor from Worcester, Mass.

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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 Presentation Layer of OSI Model

The presentation layer (Layer 6) ensures that the message is presented to the upper layer in a standardized format. It deals with the syntax and the semantics of the messages.

The main functions of the presentation layer are as follows −

• It encodes the messages from the user dependent format to the common format and vice versa, for communication among dissimilar systems.
• It is responsible for data encryption and decryption of sensitive data before they are transmitted over common channels.
• It is also responsible for data compression. Data compression is done at the source to reduce the number of bits to be transmitted. It reduces the storage space and increases the file transfer rate. It is particularly useful for transmission of large multimedia files.

Related Articles

• The Physical Layer of OSI Model
• The Network Layer of OSI Model
• The Transport Layer of OSI Model
• The Session Layer of OSI Model
• The Application Layer of OSI Model
• The Data Link Layer of OSI Model
• Explain the functions of Presentation Layer.
• What is Presentation Layer?
• The OSI Reference Model
• What is a presentation layer?
• Advantages and Disadvantages of the OSI Model
• Computer Networks – Layers of OSI Model
• What is the OSI Reference Model?
• OSI Model in Computer Networking
• Why Does the OSI Reference Model Matter?

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The physical Layer is the bottom-most layer in the Open System Interconnection (OSI) Model which is a physical and electrical representation of the system. It consists of various network components such as power plugs, connectors, receivers, cable types, etc. The physical layer sends data bits from one device(s) (like a computer) to another device(s). The physical Layer defines the types of encoding (that is how the 0’s and 1’s are encoded in a signal). The physical Layer is responsible for the communication of the unstructured raw data streams over a physical medium.

Functions Performed by Physical Layer

The following are some important and basic functions that are performed by the Physical Layer of the OSI Model –

• The physical layer maintains the data rate (how many bits a sender can send per second).
• It performs the Synchronization of bits.
• It helps in Transmission Medium decisions (direction of data transfer).
• It helps in Physical Topology (Mesh, Star, Bus, Ring) decisions (Topology through which we can connect the devices with each other).
• It helps in providing Physical Medium and Interface decisions.
• It provides two types of configuration Point Point configuration and Multi-Point configuration.
• It provides an interface between devices (like PCs or computers) and transmission medium.
• It has a protocol data unit in bits.
• Hubs, Ethernet, etc. device is used in this layer.
• This layer comes under the category of Hardware Layers (since the hardware layer is responsible for all the physical connection establishment and processing too).
• It provides an important aspect called Modulation, which is the process of converting the data into radio waves by adding the information to an electrical or optical nerve signal.
• It also provides a Switching mechanism wherein data packets can be forwarded from one port (sender port) to the leading destination port.

Physical Topologies

Physical Topology or Network Topology is the Geographical Representation of Linking devices. Following are the four types of physical topology-

• Mesh Topology: In a mesh topology, each and every device should have a dedicated point-to-point connection with each and every other device in the network. Here there is more security of data because there is a dedicated point-to-point connection between two devices. Mesh Topology is difficult to install because it is more complex.
• Star Topology: In star topology , the device should have a dedicated point-to-point connection with a central controller or hub. Star Topology is easy to install and reconnect as compared to Mesh Topology. Star Topology doesn’t have Fault Tolerance Technique.
• Bus Topology: In a bus topology , multiple devices are connected through a single cable that is known as backbone cable with the help of tap and drop lines. It is less costly as compared to Mesh Topology and Star Topology. Re-connection and Re-installation are difficult.
• Ring Topology: In a ring topology , each device is connected with repeaters in a circle-like ring that’s why it is called Ring Topology. In Ring Topology, a device can send the data only when it has a token, without a token no device can send the data, and a token is placed by Monitor in Ring Topology.

Line Configuration 

• Point-to-Point configuration: In Point-to-Point configuration, there is a line (link) that is fully dedicated to carrying the data between two devices.
• Multi-Point configuration: In a Multi-Point configuration, there is a line (link) through which multiple devices are connected.

Modes of Transmission Medium

• Simplex mode: In this mode, out of two devices, only one device can transmit the data, and the other device can only receive the data. Example- Input from keyboards, monitors, TV broadcasting, Radio broadcasting, etc.
• Half Duplex mode: In this mode, out of two devices, both devices can send and receive the data but only one at a time not simultaneously. Examples- Walkie-Talkie, Railway Track, etc.
• Full-Duplex mode: In this mode, both devices can send and receive the data simultaneously. Examples- Telephone Systems, Chatting applications, etc.

Physical Layer Protocols Examples

Typically, a combination of hardware and software programming makes up the physical layer. It consists of several protocols that control data transmissions on a network. The following are some examples of Layer 1 protocols:

• Ethernet with 1000BASE-T.
• Ethernet with 1000BASE-SX.
• Ethernet at 100BaseT.
• Synchronous Digital Hierarchy/Optical Synchronisation.
• Physical-layer variations in 802.11.
• Networking for controllers.
• U.S. Serial Bus.

Frequently Asked Questions

Q.1: what is the physical layer in the osi model .

Answer:  

The Physical layer is the first layer in the OSI (Open Systems Interconnection) model, which is a conceptual framework that standardizes the functions of a communication system. The Physical layer deals with the physical transmission of data bits over a communication channel. It defines the electrical, mechanical, and procedural aspects of transmitting raw data between network devices. It primarily focuses on converting  digital data into a format suitable for transmission and ensuring reliable and efficient communication at the physical level.

Q.2: What are the primary functions of the Physical layer? 

Answer: 

The main functions of the Physical layer include: Encoding and signaling: It defines the method of encoding digital data into electrical, optical, or radio signals for transmission over the physical medium. Physical media: It specifies the characteristics of the physical media used for data transmission, such as cables, fiber optics, or wireless channels. Data transmission: It establishes the rules for transmitting raw data bits over the physical medium, including aspects like bit synchronization, line coding, and error detection. Physical topologies: It defines the physical arrangement or layout of network devices and how they are connected, such as bus, star, ring, or mesh topologies. Transmission modes: It describes the ways in which data is transmitted between devices, such as simplex (one-way), half-duplex (two-way but not simultaneously), or full-duplex (two-way simultaneously).

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Table of Contents

What is the osi model, advantages of the osi model, osi vs. tcp/ip model, understanding open systems interconnection reference model (osi).

In the complex world of computer networking, the OSI Model is a foundational framework that helps us understand how data is transmitted and received across networks. The OSI Model has become a vital tool for network engineers, administrators, and enthusiasts. In this blog, we will explore the OSI Model in-depth, delving into its seven layers, discussing its advantages, comparing it with the TCP/IP model, and concluding with its enduring significance.

The OSI or Open Systems Interconnection Model is used to standardize the functions of a telecommunication or networking system into seven distinct layers. This model defines how different network protocols and technologies should interact, allowing for interoperability between diverse hardware and software.

1. Physical Layer

It is the lowest in the OSI Model, responsible for the physical connection between devices. It transmits raw binary data over a physical medium, such as cables, optical fibers, or wireless signals. The physical layer transmits data through electrical signals, light pulses, or radio waves.

Key functions of the Physical Layer include:

• Transmission of bits: It manages the physical transmission of data, converting the digital information into electrical or optical signals that can be sent over the network.
• Data encoding: The Physical Layer also handles data encoding schemes, such as modulation, determining how data is represented as signals on the transmission medium.
• Physical connectors: This layer defines the physical connectors, pin layouts, and characteristics of cables and network devices, ensuring compatibility between different hardware components.

2. Data-Link Layer

The Data-Link Layer is responsible for creating a reliable link between two directly connected nodes, ensuring the error-free transmission of data over a shared physical medium. It is divided into the Logical Link Control and Media Access Control sublayers.

Key functions of the Data-Link Layer include:

• Framing: Data packets from this layer are divided into frames at this layer, making it easier for devices to recognize the start and end of data transmissions.
• Error detection and correction: The Data-Link Layer performs error checking to detect and sometimes correct errors in data frames, ensuring the integrity of data during transmission.
• Media access control: The MAC sublayer manages access to the physical medium to avoid data collisions in shared networks, using protocols such as Ethernet and Wi-Fi.

3. Network Layer

The network layer 's primary role involves directing and transmitting data packets between distinct networks. It employs logical addressing, such as IP addresses, to ascertain the most optimal route for data to reach its intended destination.

Key functions of the Network Layer include:

• Logical addressing: It assigns unique IP addresses to devices on a network, allowing routers to decide how to forward data packets based on these addresses.
• Routing: The Network Layer utilizes routing algorithms to ascertain the optimal route for data packets to traverse interconnected networks.
• Logical network segmentation: It allows for the creation of logical network segments, known as subnets, which can improve network organization and security.

4. Transport Layer

The Transport Layer facilitates end-to-end communication among devices on separate hosts, guaranteeing the dependable and precise transmission of data through the establishment, maintenance, and termination of communication sessions.

Key functions of the Transport Layer include:

• Segmentation and reassembly: It divides large data streams into smaller segments for transmission and reassembles them at the destination.
• Error detection and correction: Similar to the Data-Link Layer, the Transport Layer also checks errors to ensure data integrity.
• Flow control: This layer manages the data flow between sender and receiver to prevent congestion and ensure efficient data transfer.
• Multiplexing and demultiplexing: The Transport Layer uses port numbers for multiple communication sessions over a single network connection, allowing multiple applications to run simultaneously on a device.

5. Session Layer

The Session Layer is responsible for initiating, managing, and concluding communication sessions between two devices. It offers synchronization and coordination capabilities, ensuring seamless interaction among applications operating on distinct hosts.

Key functions of the Session Layer include:

• Session establishment, maintenance, and termination: It manages the process of initiating, maintaining, and closing communication sessions between devices.
• Dialog control: The Session Layer defines rules for controlling the direction of communication, such as half-duplex or full-duplex communication.
• Synchronization: It ensures that data exchange between applications is properly synchronized and that both sides know each other's state.

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6. Presentation Layer

The Presentation Layer's role encompasses translating data between the OSI Model's application layer and its lower layers. It manages tasks such as data encoding, encryption, compression, and format conversion to ensure data compatibility during presentation.

Key functions of the Presentation Layer include:

• Data translation: It converts data between different formats, such as translating between ASCII and EBCDIC character encodings.
• Data encryption and decryption: This layer can encrypt data for secure transmission and decrypt it at the receiving end.
• Data compression: Data can be compressed to reduce bandwidth usage during transmission.
• Data formatting: It ensures that data sent by one application can be properly understood and processed by another application, regardless of differences in data representation.

7. Application Layer

It is the top layer of the OSI Model and is closest to the end-users. It is responsible for providing network services directly to applications and users.

Key functions of the Application Layer include:

• Interface with user applications: It provides a user-friendly interface for applications to access network services, allowing users to interact with the network.
• Network services: The Application Layer includes various protocols and services, such as HTTP, FTP, SMTP, and DNS, that enable communication and data exchange between applications running on different devices.
• Application-specific functions: This layer supports application-specific functions, such as email, file transfer, remote access, and web browsing.

1. Clear Layered Structure

One of the primary advantages of the OSI model is its clear and organized layered structure. The model divides network communication into seven distinct layers, each responsible for specific functions. The hierarchical approach makes it easier to understand and troubleshoot network issues. It allows network professionals to focus on a particular layer when diagnosing problems, reducing the complexity of network management.

2. Interoperability

Interoperability is crucial in modern networking, as different vendors develop various networking hardware and software solutions. The OSI model's standardized layers facilitate interoperability by ensuring that devices and applications from different manufacturers can communicate effectively when they adhere to the same model. This advantage has contributed significantly to the growth and evolution of the Internet and complex enterprise networks.

3. Scalability

The OSI model provides a scalable framework for designing and expanding networks. As organizations grow and their networking needs evolve, they can add new hardware and software components at specific layers without disrupting the entire network. This scalability ensures that networks can adapt to changing requirements without major overhauls.

4. Modularity

Each OSI model layer is relatively independent of the others, promoting modularity in network design and implementation. Network professionals can develop, upgrade, or replace components within a specific layer without affecting the functionality of other layers. This modularity simplifies maintenance and upgrades, making network management more efficient.

5. Protocol Agnosticism

The OSI model does not dictate the use of specific protocols at each layer, allowing for flexibility in protocol selection. This agnosticism enables network designers to choose the most suitable protocols for their specific needs. It also ensures that the OSI model remains adaptable to emerging technologies and protocols, keeping it relevant in a rapidly changing networking landscape.

6. Common Language

The OSI model provides a common language for network professionals, ensuring that they can communicate effectively across different teams and organizations. When discussing network issues or designs, using the OSI model as a reference point helps prevent misunderstandings and facilitates collaboration.

7. Education and Training

The OSI model is an invaluable teaching tool for students and aspiring network professionals. It provides a structured, comprehensive framework for learning about networking concepts, protocols, and technologies. Additionally, many networking certifications, such as Cisco's CCNA and CCNP , are based on the OSI model, making it essential for career advancement in the field.

8. Diagnostic Capabilities

Troubleshooting network issues is a common task for network administrators and engineers. The OSI model's layered approach simplifies the diagnostic process by allowing professionals to isolate problems to a specific layer. This targeted approach speeds up issue resolution and minimizes downtime.

9. Vendor-Neutral

The OSI model is vendor-neutral, meaning it is not tied to any specific manufacturer or technology. This neutrality promotes fair competition in the networking industry and ensures that networking solutions are not limited to a single vendor's products.

While the OSI Model provides a comprehensive framework for understanding networking, the TCP/IP model , or the Internet Protocol Suite, is another widely used reference model that predates OSI. Let's compare the two models:

• Seven layers.
• Emphasizes a clear distinction between the functions of each layer.
• Provides a detailed and theoretical framework.
• Less widely used in practical network implementations but is valuable for conceptual understanding.

TCP/IP Model

Four layers: Network Interface, Internet, Transport, and Application.

• Merges the OSI Model's physical and data link layers into the Network Interface layer.
• More closely aligned with real-world Internet technologies.
• Commonly used in actual networking implementations and forms the basis of the modern internet.

In practice, the TCP/IP model is more prevalent in the networking industry due to its direct applicability to the internet. However, the OSI Model remains valuable for its conceptual clarity and detailed breakdown of network functions.

While the OSI Model may not be the primary model used in practical network implementations, its advantages in terms of standardization, modularity, scalability, interoperability, and educational value make it an essential reference tool for network engineers and enthusiasts. With its seven layers, the OSI Model remains a crucial framework for anyone seeking to delve deeper into the world of computer networking, providing a solid foundation for understanding the complex processes that enable seamless communication across modern networks. Are you thinking of upskilling your career as an expert in Cybersecurity? The Cyber Security Expert Master’s Program will equip you with the skills needed to become an expert in this rapidly growing domain.

1. What Is the Full Form of OSI?

The full form of OSI is Open Systems Interconnection.

2. What Is the Purpose of an OSI Model?

The OSI model’s purpose is to provide a conceptual framework that standardizes and categorizes the functions of a network communication system, facilitating interoperability between different networking technologies and protocols.

3. Can the OSI Model Be Applied to Wireless Networks?

Yes, the OSI model can be applied to wireless networks. It serves as a reference model for understanding and designing the various communication layers in wireless networks, just like it does for wired networks.

4. Is the OSI Model Still Relevant in Modern Networking Environments?

Yes, the OSI model is still relevant in modern networking environments. While it may not perfectly reflect the intricacies of today's complex networks, it remains a valuable tool for understanding and troubleshooting network issues and guiding the development of new networking technologies and protocols.

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Introduction to the OSI Model

Lesson Contents

In the beginning, the development of networks was chaotic. Each vendor had its proprietary solution. The bad part was that one vendor’s solution was not compatible with another vendor’s solution. This is where the idea for the OSI model was born. Having a layered approach to networks, our hardware vendors would design hardware for the network, and others could develop software for the application layer. Using an open model which everyone agrees on means we can build networks that are compatible with each other.

To fix this problem, the International Organization for Standardization (ISO) researched different network models, and the result is the OSI-model which was released in 1984. Nowadays, most vendors build networks based on the OSI model, and hardware from different vendors is compatible….excellent!

The OSI model isn’t just a model to make networks compatible; it’s also one of the BEST ways to teach people about networks. Keep this in mind since when you are studying networking, you will see people refer a lot to the OSI model.

Here’s what the OSI model looks like:

  “ A ll P eople S eem T o N eed D ata P rocessing”

  This is the OSI model, which has seven layers; we work our way from the bottom to the top. Let’s start at the physical layer:

• Physical Layer: This layer describes stuff like voltage levels, timing, physical data rates, physical connectors, and so on. Everything you can “touch” since it’s physical.
• Data Link: This layer makes sure data is formatted the correct way, takes care of error detection, and makes sure data is delivered reliably. This might sound a bit vague, but for now, remember that this is where “Ethernet” lives. MAC Addresses and Ethernet frames are on the Data Link layer.
• Network: This layer takes care of connectivity and path selection (routing). This is where IPv4 and IPv6 live. Every network device needs a unique address on the network.
• TCP lives here; it’s a protocol that sends data in a reliable way.
• UDP lives here; it’s a protocol that sends data in an unreliable way.

I’m taking a short break here, these four layers that I just described are important for networking , and the upper three layers are about applications .

• Session: The session layer takes care of establishing, managing, and terminating sessions between two hosts. When you are browsing a website on the internet, you are probably not the only user of the web server hosting that website. This web server needs to keep track of all the different “sessions.”
• Presentation: This one will make sure that information is readable for the application layer by formatting and structuring the data. Most computers use the ASCII table for characters. If another computer would use another character like EBCDIC, then the presentation layer needs to “reformat” the data, so both computers agree on the same characters.
• Application: Here are your applications. E-mail, browsing the web (HTTP), FTP, and many more.

  “ P eople D o N eed T o S ee P amela A nderson”

This one normally gives me more smiles when I’m teaching CCNA in class, and it’s another way to remember the OSI-Model.

P = Physical D = Data Link N = Network T = Transport S = Session P = Presentation A = Application

Remember that you can’t skip any layers in the OSI model. It’s impossible to jump from the Application layer directly to the Network layer. You must always go through all the layers to send data over the network.

Let’s take a look at a real-life example of data transmission:

• You are sitting behind your computer and want to download some files from a local webserver. You start up your web browser and type in the URL of your favorite website. Your computer will send a message to the web server requesting a certain web page. You now use the HTTP protocol, which lives on the application layer.
• The presentation layer will structure the information of the application in a certain format.
• The session layer will make sure to separate all the different sessions.
• Depending on the application, you want a reliable (TCP) or unreliable (UDP) protocol to transfer data to the web server. In this case, it’ll choose TCP since you want to ensure the webpage makes it to your computer. We’ll discuss TCP and UDP later.
• Your computer has a unique IP address (for example, 192.168.1.1), and it will build an IP packet. This IP packet will contain all the data of the application, presentation, and session layer. It also specifies which transport protocol it’s using (TCP in this case) and the source IP address (your computer 192.168.1.1), and the destination (the web server’s IP address).
• The IP packet will be put into an Ethernet Frame. The Ethernet frame has a source MAC address (your computer) and the destination MAC address (webserver). More about Ethernet and MAC addresses later.
• Finally, everything is converted into bits and sent down the cable using electric signals.

Once again, you cannot “skip” any layers of the OSI model. You always have to work your way through ALL layers. If you want a real-life story converted to networking land, just think about the postal service:

• First, you write a letter.
• You put the letter in an envelope.
• You write your name and the name of the receiver on the envelope.
• You put the envelope in the mailbox.
• The content of the mailbox will go to the central processing office of the postal service.
• Your envelope will be delivered to the receiver.
• They open the envelope and read its contents.

If you put your letter directly in the mailbox, it won’t be delivered. Unless someone at the postal office is friendly enough to deliver it anyway, in network land it doesn’t work this way! Going from the application layer all the way down to the physical layer is what we call encapsulation . Going from the physical layer and working your way up to the application layer is called de-encapsulation .

Now you know about the OSI model, the different layers, and the function of each layer. During peer-to-peer communication, each layer has “packets of information.” We call these protocol data units (PDU). Now every unit has a different name on the different layers:

• Transport layer: Segments; For example, we talk about TCP segments .
• Network layer: Packets; For example, we talk about IP packets here.
• Data link layer: Frames; For example, we talk about Ethernet frames here.

This is just terminology, so don’t mix up talking about IP frames and Ethernet packets…

OSI Model in Action

All this talk about layers is nice and all, but what about some action? We can see the different layers of the OSI model in action if we capture our network traffic on our computer.

To do this, we will download Wireshark .

Wireshark is a network capture tool that allows us to capture all packets we receive/transmit on our computer, and we can look at them.

Once you have downloaded and installed Wireshark, select the “Options” in the Capture menu:

You will now see an overview of all your network cards:

In my case, it’s the Ethernet interface that I want to capture. Hit Start, and it will capture all packets entering and exiting this interface. It will look like this:

You will see a lot of stuff, don’t worry about what you see here. As you learn more about networking, you will also learn more about the different networking protocols and their packets/frames. We will capture one single frame and take a closer look at it. To do this, we will use a filter so that Wireshark only shows this traffic:

In the green bar on the top left, enter the following filter:

Now open your web browser and open http://cisco.com.

Once the website has loaded, take a look at Wireshark:

A single packet will show up with the request from our browser to fetch the Cisco.com website. At the bottom half of the screen, we can take a look at the contents of this frame. Let me break it down for you:

Wireshark has added the first piece of information. It tells us that we received an Ethernet frame that is 908 bytes. It also shows the arrival time. Here’s the second part:

Above we see layer two of the OSI model. This is the Ethernet frame, and it shows the source and destination MAC addresses. It also tells us the type. In this case, our Ethernet frame contains an IPv4 packet. Let’s check it out:

Above, we see the IP packet. This is layer three of the OSI model. Don’t worry about all the different fields here, we will cover it later. Two things you can recognize at the top are the source and destination IP addresses. Let’s continue:

Above we see layer four of the OSI model. We are using TCP as the transport protocol here (which we will discuss later in detail). Last but not least, the last layer of the OSI model:

Above, you see layer seven, the application layer. Note that you don’t see a separate session and/or presentation layer here. You can see some information about the HTTP protocol here. We used a GET request to fetch cisco.com; the user agent I used was Mozilla (Firefox).

Want to take a look at this yourself? You can download my capture file:

Wireshark Capture HTTP Cisco.com

You have now learned about the OSI model and its different layers. You have also seen how this applies to the real world with a packet capture in Wireshark. In other lessons, you will see that we use Wireshark quite often to look at different networking protocols and their inner workings.

Forum Replies

thank you for helpful lesson. i have a question about osi layers,are layers 5,6,7 in our browser(firefox,chrom…)? what about the rest of layers?for example is layer 2 only started to working when frame gets to switch or all these happen in pc in advance? it is a big question for me and mixed me up. thanks

thank you Rene

Its really easy and simple to understand and refer

Thanks again!

thanks for ur info

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Presentation Layer in OSI choose

Prerequisite : OSI Model

Introduction : Presentation Layer is the 6th shifts in of Get System Interconnection (OSI) model. This layer is see renown like Translation layer, as this layer aids as a data translator for the web. The dating which those layered receives from the Software Layer is extracted and manipulated her as per the requirement format to move over the network. The main task of this layer is to provide or define the data format and encryption. The video layer is also called while Syntax layer since it is responsible since maintaining the proper structure of the data the it either receives oder transmitted to other layer(s).

Advanced of Lecture Layer :

The presentation layer, being the 6th layer in the OSI model, performs several types of functions, which are described below-

• Powerpoint layer format and encrypts data to be sent across the network.
• This layer takes care that the data is sent in such a way that the reception will understand the information (data) additionally will will able to use that data efficiently and effectively. How Layer include OSI model - GeeksforGeeks
• This plane guides the abstract data forms and can high-level data structures (example- banking records), which are to be defined or exchanged.
• This layer carries out the encryption during the transmitter additionally recoding at the beneficiary.
• This layer carries out file compression to lessen to bandwidth of one data to may transmitted (the primary goal of data compression is until diminish the number out bits which is to be transmitted). Explore how data compression works, why it's critical, different processes and how it compares into deduplication.
• This layer is responsible for interoperability (ability of calculators to exchange and makes getting of information) between enable methods as differents computers use different coding typical. Introduction to Data Print - GeeksforGeeks
• This shift basically deals with the presentation part of an data.
• Presentation layer, carries going the data compression (number of bits reduction during transmission), which at return improves to data throughput.
• This layer also deals with of issues are string representation.
• The presentation layer is also responsible for integrated all the formats into a standardized format to effectual and effective communication.
• This layer enciphers the message from the user-dependent sheet to the gemeinhin format and vice-versa for communication between dissimilar solutions.
• This layer deals with the syntax and semantics of the messages.
• This shift also ensures this the messages which are to be presented to the upper as well as the lower layer should breathe standardized as well as in an accurate format too.
• Presentation layer belongs also responsible for translation, formatting, and delivery of information to processing or display.
• This coating also performs serialization (process of translating a data texture otherwise an object include an format that able be stored otherwise transmitted easily).

General of Presentation Laying in the OSI model: Presentation layer, creature aforementioned 6th layer in the OSI model, plays a vital role while communication is taking put between two devices the a network.

List of features which are provided by the presentation ply are:

• Presentation layer could apply certain sophisticated compression techniques, so lessons bytes of data are required to represent the information when this is sent over the network.
• Are two or view devices are communicating over on scramble connection, then this presentation layer is responsibility for adding security on the sender’s end as well as the decoding the enrollment on the receiver’s end so the she sack represent to application layer with unencrypted, readable data. evidence compression technique - Download since a PDF or view online for free
• This layer sizes and encrypts data to be sent over a network, providing freedom from compatibility problems.
• All presentation location also negotiates the Transfer Syntax.
• To presentation layer is plus responsible for compressing data computers receives from the application level previously delivering it to the attend layer (which is the 5th strata in the OSI model) and thus improves the speed as fine as the energy of communication by minimizing the qty of the datas to be transferred.

Working of Presentation Layer in the OSI model : Presentation layer in the OSI model, as a english, converts the data sent by and login layer of who transmitting node into an acceptable and compatible data format based on an applicable network protocol plus architecture.  Upon arrival during and receiving computer, the presentation layer translates data into einen adequate format usable in the application layer. Basically, stylish others words, this layer takes care of any issues occurring when transmitted data must be viewed includes a format different from the original format. Being the functional section of to OSI operation, of present layer performing a multitude (large number of) data conversion calculating and character translation responsibilities. Mainly, this lay is responsible for administering two network characteristics: protocol (set in rules) and architecture.

Presentation Strata Reporting : Presentation layer being the 6th layer, instead the majority important layer in the OSI model performs several types of operating, which builds sure that data which are being transferred or received should be accurate or clear to all the devices which are there in a closed network. Presentation Layer, for performing translations with other specified functions, needs to use certain pact that are defined slide –

• Sphere Filing Protocol (AFP): Apple Registering Protocol is that protective network protocol (communications protocol) the offers services to macOS or the classic macOS. This is basically an network file control protocol specifically designed used Mac-based sources.
• Lighter Presentation Protocol (LPP): Low Presentation Reporting is the protocol which is used to provisioning OIST presentation services on the top of TCP/IP based protocol bin.
• NetWare Core Protocol (NCP): NetWare Nuclear Protocol is the network protocol which is used at access file, printer, directory, clock synchronization, messaging, remote command execution and different network services functions.
• Network Data Representation (NDR): Network Data Show is principles the implementation of the presentation level for the OSI model, which provides or defines various primitive data types, constructed data types and also several gender of data representations.
• External Data Representation (XDR): External Data Representation (XDR) will the standard for the feature and encoding of info. Items is effective for carrying data between computer architectures and has been used to communicate data between highly diverse machines. Switch from local representation to XDR has called encoding, though converting XDR into local representation is called decoding.
• Secure Socket Layer (SSL): The Safe Socket Layer protocol allows security to the data that is being transferred among the webs browser and the server. SSL encrypts the linking between a web your and a web, which ensures the all data passed between them corpse private and free from attacks.

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What is OSI Model: Essential Foundation to Networking

Updated: Jan 30

No technology that is connected to internet is un-hackable. It's only a matter of time.

Introduction to OSI Model

What is the OSI Model? The OSI, short for (the Open Systems Interconnection model) is a conceptual framework for understanding how network communication works. It was the first standard model adopted by all major computer and telecommunication companies in the early 1980s.

The OSI Model (aka ISO-OSI, i.e., International Organization of Standardization – Open System Interconnection) divides the communication process between two devices into seven layers. It provides a standard reference model that allows different networking technologies and protocols to interoperate and communicate.

Imagine you have two servers that need to share information. The message doesn't just magically teleport from an application on the first machine to the application on the other. Instead, it transits down the layers and eventually reaches the transmission line. Once it jumps across the gap to the other device, it has to repeat the process in reverse by ascending layers until it reaches the receiving application.

Core Definition of OSI Model

For any starting number N representing a layer that transmits a message, the OSI model can be used to explain the transmission few key concepts:

Protocol Data Units (PDUs) are abstracted messages that include payloads, headers, and footers.

Service Data Units (SDUs) are equivalent to the payloads.

At each subsequent transition from some layer N to some layer N-1, a layer-N PDU becomes a new N-1 SDU. This payload gets wrapped up in a layer N-1 PDU with the relevant headers and footers. On the opposite end, the data passes up the chain, unwrapping at each relevant stage until it's just a payload that the corresponding layer-N device can consume.

The 7 Layers of OSI

We'll describe OSI layers "top-down" from the application layer that directly serves the end user to the physical layer.

7. The Application Layer

The application layer is the highest layer of the OSI Model and is responsible for providing the interface between the network and the end user's application.

Standard network services such as file transfer, email, and web browsing are provided at the application layer. Protocols such as HTTPS (Hypertext Transfer Protocol Secure) and, FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol) operate at this layer, allowing users to access and transfer files and other resources over the network.

The application layer also provides the interface for user authentication and authorization. Protocols such as LDAP (Lightweight Directory Access Protocol) and Kerberos are used to verify the identity of users and grant them access to specific resources or services on the network.

6. The Presentation Layer

The presentation layer is responsible for formatting and encoding data in a standardized way independent of the application or system being used. It includes protocols like SSL (Secure Sockets Layer) that provide secure communication.

It deals with issues such as data compression and encryption.

An example of a presentation service would be converting an extended binary-coded decimal interchange code text computer file to an ASCII-coded file. The presentation layer could translate between multiple data formats using a standard format if necessary.

5. The Session Layer

The session layer establishes, maintains, and terminates connections between devices. Some standard protocols that operate at the session layer include Remote Procedure Call (RPC), NetBIOS (Network Basic Input Output System), and Windows Internet Name Service (WINS).

Some standard functions of the session layer include :

Setting up and tearing down communication sessions between devices.

Synchronizing the flow of data between devices.

Resuming communication after a temporary interruption or fault.

Negotiating the options and parameters for a communication session.

Managing access to shared resources during a communication session.

4. The Transport Layer

The transport layer provides end-to-end communication services and error recovery for the application layer. It includes protocols like TCP (Transmission Control Protocol) and (UDP) User Datagram Protocol that provides error correction, flow control, and data segmentation and reassembly.

Every protocol uses a unique decimal number to ensure that the data is sent and received on the intended application as it passes through the network or Internet.

TCP is a connection-oriented protocol that guarantees the delivery of the message, while UDP is a connectionless protocol that sends the data without error correction. Under the TCP and UDP are port numbers used to distinguish the specific type of application.

3. The Network Layer

The network layer is responsible for routing data between different networks. It includes protocols like (IP) Internet Protocol, (IPX) Internetwork Packet Exchange, and AppleTalk. These protocols provide the necessary functions for routing data across a network and ensuring it reaches its destination.

It is responsible for determining the best path for data as it travels from its source to its destination. The network layer also assigns logical addresses to devices on the network, which are used to identify the devices and route data to them.

The network layer is often considered the "heart" of the OSI model because it plays a central role in the operation of a network. It is a critical component of modern computer networks and is essential for allowing devices to communicate with each other and exchange information.

2. The Data Link Layer

The data link layer links two devices on the same physical network, such as a local area network (LAN). It ensures that data is transmitted correctly and without errors.

It includes protocols like (SDLC) Synchronous Data Link Protocol, (HDLC) High-Level Data Link Protocol, (SLIP)Serial Line Interface Protocol, (PPP)Point - to - Point Protocol, (LCP) Link Control Protocol, and (NCP) Network Control Protocol.

This layer comprises two parts—Logical Link Control (LLC), which identifies network protocols, performs error checking, and synchronizes frames. Media Access Control (MAC) uses MAC addresses to connect devices and define permissions to transmit and receive data.

Overall, the data link layer is crucial in ensuring data's reliable and efficient transmission over a network.

1. The Physical Layer

The physical layer is responsible for transmitting raw data over a communication channel, including the hardware, cables, and other components that make up the network.

It defines the physical characteristics of the communication channel, including the signaling used, the frequency range, and the data rate.

The physical layer ensures that data is transmitted accurately and reliably from one device to another.

References:

https://www.geeksforgeeks.org/open-systems-interconnection-model-osi/

https://en.wikipedia.org/wiki/OSI_model

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