This tutorial explains the OSI reference model step by step. Learn the seven layers of the OSI model; Application, presentation, session, transport, network, data link and physics in detail with their functions.

The OSI (Open System Interconnection) reference model is the comprehensive set of standards and rules for hardware manufacturers and software developers. By following these standards, they can create network components and software applications that operate in different environments. In 1984, the ISO (International Organization for Standardization) published this model.

The OSI model not only provides a framework for the creation and implementation of network standards, peripherals and interconnection diagrams, but also explains networking from a modular perspective, which facilitates understanding and troubleshooting .

Seven layers of the OSI model

The OSI model consists of seven different layers, divided into two groups.

Group Layer number Layer name The description
Upper layers seven Application Provide a user interface to send and receive data
6 Presentation Encrypt, format and compress data for transmission
5 Session Initiate and end the session with the remote system
Bottom layers 4 Transport Divide data flow into smaller segments and provide reliable and unreliable data delivery
3 Network Provide logical addressing
2 Data link Prepare data for transmission
1 Physical Move data between devices

Let’s understand each layer in detail.

This tutorial is the second part of the article “Networking reference models explained in detail with examples”. This article explains the following CCNA topic.

Compare and contrast OSI and TCP / IP models

Other parts of this article follow.

Explanation of the benefits and the basic purpose of the OSI model

This tutorial is the first part of the article. It briefly explains the reasons why the OSI model was created and the benefits.

Similarities and differences between the OSI and TCP / IP model

This tutorial is the third part of the article. It compares the OSI reference model with the TCP / IP model and lists the similarities and differences between the two models.

Explanation of the TCP / IP reference model

This tutorial is the fourth part of the article. It explains the five layers of the TCP / IP model in detail.

Explanation of data encapsulation and de-encapsulation

This tutorial is the fifth part of the article. It explains how data is wrapped and de-wrapped when it crosses layers.

The application layer

This is the last and the top layer of the OSI model. This layer provides an interface between application programs running in the system and the network. If an application needs to access an available resource in a remote system, it interacts with this layer. This layer then encompasses the protocols and services that the application will use to access these resources.

There are two types of application programs; network sensitive and not network sensitive. An application program is considered to support the network when it can make any kind of network request. If an application program cannot make any network requests, it is considered to be a program without a network.

Network-specific programs are divided into two categories;

  1. Programs that are primarily created to run on the local system but require it can connect to a remote system such as MS-Word, Adobe-Photoshop, VLC Player, etc.
  2. Programs mainly created to work with a remote system such as SSH, FTP, TFTP, etc.

Only programs belonging to the second type are referenced in the application layer. This does not mean that the first type of program cannot take advantage of the application layer. It just means that they are not documented in the application layer. But if necessary, they can also connect to the network via the application layer.

The top layer of the OSI model is the application layer. It provides the protocols and services required by network sensitive applications to connect to the network. FTP, TFTP, POP3, SMTP and HTTP are the few examples of standards and protocols used in this layer.

The presentation layer

The sixth layer of the OSI model is the Presentation layer. Applications running on the local system may or may not understand the format used to transmit data over the network. The presentation layer works like the translator in the OSI model. When receiving data from the application layer, it converts this data into such a format that can be sent over the network. When it receives data from the session layer, it converts this data back into a format such that the application that will use the incoming data can understand it.

Convert, compress and encrypt are the main functions that the presentation layer performs when sending the computer while when receiving the computer there are convert, decompress and decrypt. ASCII, BMP, GIF, JPEG, WAV, AVI and MPEG are the few examples of standards and protocols that work in this layer.

The session layer

The session layer is the fifth layer of the OSI model. He is responsible for the configuration, management and dismantling of the sessions between the entities of the presentation layer and the provision of dialogs between the computers.

When an application makes a network request, this layer checks whether the requested resource is available in the local system or in the remote system. If the requested resource is available in the remote system, it tests whether a network connection to access this resource is available or not. If the network connection is not available, it returns an error message to the application informing it that the connection is not available.

If a network connection is available, it establishes a session with the remote system. For each individual request, it uses a separate session. This allows multiple applications to send or receive data simultaneously. Once the data transmission is finished, it ends the session.

The session layer is responsible for establishing, managing and terminating communications between two computers. RPC and NFS are examples of the session layer.

The transport layer

The transport layer is the fourth layer of the OSI model. It provides the following features: –

Segmentation

When sending the computer, it divides the data stream into small pieces before transmission. Each element is called a segment and the process of breaking down data into smaller elements is called segmentation. Upon receipt of the computer, it joins all segments in the data flow. The upper layers therefore receive the data in the format in which they were sent.

Data transport

This layer establishes a logical connection between the sending system and the receiving system and uses this connection to provide end-to-end data transport. For data transport, it mainly uses two protocols; TCP and UDP.

The TCP protocol is used for reliable data transport. TCP is a connection oriented protocol. The UDP protocol is used for the transport of unreliable data. UDP is a connectionless protocol.

The main difference between a connectionless protocol and a connection oriented protocol is that a connection oriented protocol provides reliable data delivery. For reliable data delivery, it uses several mechanisms such as the three-way handshake process, acknowledgments, sequencing and flow control.

multiplexing

Through the use of port numbers, this layer also provides connection multiplexing. Connection multiplexing allows multiple applications to send and receive data simultaneously.

The main features of the transport layer are segmentation, data transport and connection multiplexing. For data transport, it uses the TCP and UDP protocols. TCP is a connection oriented protocol. It provides reliable data delivery.

The network layer

The third layer of the OSI model is the network layer. This layer takes the data segment from the transport layer and adds a logical address to it. A logical address has two components; network partition and host partition. The network partition is used to group network components while the host partition is used to uniquely identify a system on a network. The logical address is called the IP address. Once the logical address and other related information is added to the segment, it becomes packets.

This layer decides whether the packet is intended for the local system or the remote system. It also specifies the standards and protocols used to move data packets over networks.

To move the data packet between two different networks, a device called a router is used. The router uses a logical address to make the routing decision. Routing is a process of transmitting data packets to its destination.

Defining logical addresses and finding the best path to reach the destination are the main functions of this layer. The router works in this layer. Routing also takes place in this layer. IP, IPX and AppleTalk are examples of this layer.

The data link layer

The data link layer is the second layer of the OSI model. This layer defines how network components access media and the transmission methods they use. This layer has two sublayers; MAC and LLC.

MAC (Media Access Control)

This sublayer defines how data packets are placed in the media. It also provides physical addressing. The physical address is called the MAC address. Unlike the logical addresses that must be configured, the physical addresses are preconfigured in the network card. The MAC address is used to uniquely identify a host in the local network.

LLC (Logical Link Control)

This sublayer identifies the network layer protocol. When the computer is sent, it encapsulates the network layer protocol information in the LLC header from which the data link layer receives the data packet. Upon receipt from the computer, it checks the LLC header for information about the network layer protocol. In this way, a data packet is always delivered to the same network layer protocol from which it was sent.

Defining physical addresses, finding a host on the local network, specifying standards and methods for accessing media are the main functions of this layer. The switching takes place in this layer. Switches and bridges operate in this layer. HDLC, PPP and Frame Relay are examples of this layer.

The physical layer

The physical layer is the first layer of the OSI model. This layer specifies the standards for the devices, media and technologies that are used to move data across the network, such as: –

  • Type of cable used to connect the devices
  • Pin models used on both sides of the cable
  • Type of interface card used in the network device
  • Type of connector used to connect the cable to the network interface
  • Encoding of digital signals received from the data link layer according to the type of medium attached, such as electrical for copper, light for fiber or radio waves for wireless.

When the computer is sent, it converts the digital signals received from the data link layer into analog signals and loads them into physical media. Upon reception from the computer, it selects analog media signals, converts them to digital signals, and transfers them to the data link layer for further processing.

The physical layer primarily defines standards for the media and devices used to move data over the network. 10BaseT, 10Base100, CSU / DSU, DCE and DTE are some examples of the standards used in this layer.

That’s it for this tutorial. In the next part of this article, I will compare the OSI model to the TCP / IP model and explain the similarities and differences between the two models. If you like this tutorial, don’t forget to share it with your friends.

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