Layers and protocols of the OSI model in computer networks (2023)

What is the OSI model?

The OSI model is a logical and conceptual model that defines the network communication used by open systems to interconnect and communicate with other systems. The Open System Interconnection (OSI Model) also defines a logical network and effectively describes the transfer of computer packets using multiple layers of protocols.

In this tutorial, you will learn:

• OSI model characteristics
• Why the OSI model?
• What is the OSI model?
• History of the OSI model
• 7 layers of the OSI model
• physical head
• Data link layer
• transport layer
• network layer
• session layer
• presentation layer
• application layer
• Interaction between layers of the OSI model
• Protocols supported at various levels
• Advantages of the OSI model
• Disadvantages of the OSI model

OSI model characteristics

Here are some important features of the OSI model:

• A layer should only be created where defined levels of abstraction are needed.
• The function of each layer must be selected according to internationally standardized protocols.
• The number of layers should be large so that separate features are not placed on the same layer. At the same time, it should be small enough that the architecture doesn't get too complicated.
• In the OSI model, each layer depends on the next lower layer to perform primitive functions. Each layer must be able to provide services to the next higher layer.
• Changes made to one layer should not require changes to other sinks.

Why the OSI model?

• Helps you understand communication on a network.
• Troubleshooting is easier by separating functions into different network layers.
• Helps you understand new technologies as they are developed.
• It allows you to compare primary functional relationships at various network layers.

History of the OSI model

Here are the key milestones in the history of the OSI model:

• In the late 1970s, ISO carried out a program to develop general standards and networking methods.
• In 1973, an Experimental Packet Switching System in the United Kingdom identified the need to define higher level protocols.
• In the year 1983, the OSI model was initially intended to be a detailed specification of real interfaces.
• In 1984, the OSI architecture was formally adopted by ISO as an international standard.

7 layers of the OSI model

The OSI model is a layered server architecture system in which each layer is defined according to a specific function to be performed. The seven layers work collaboratively to pass data from one layer to another.

• the upper layers- Deals with application issues and is mostly implemented in software only. The highest is the closest to the end user of the system. In this layer, the communication from one end user to another begins through the interaction between the application layer. It will process all the way to the end user.
• the lower layers: These layers handle activities related to data transport. Physical layer and data link layers also implemented in software and hardware.

The upper and lower layers further divide the network architecture into seven different layers, as shown below.

• Inscription
• presentation
• session
• Transport
• network, data link
• physical layers

Let's study each layer in detail:

physical head

The physical layer helps define the electrical and physical specifications of the data connection. This level establishes the relationship between a device and a physical transmission medium. The physical layer does not care about the protocols or other things of the upper layer.

Examples of physical layer hardware are network adapters, ethernet, repeaters, network hubs, etc.

Data link layer:

The data link layer corrects errors that can occur in the physical layer. The layer allows you to define the protocol for establishing and terminating a connection between two connected network devices.

It is a comprehensive IP address layer, helping you define logical addressing so that any endpoint can be identified.

The layer also helps to implement the routing of packets through a network. Helps you define the best path, allowing you to get data from source to destination.

The data link layer is subdivided into two types of sublayers:

1. Media Access Control (MAC) Layer: It is responsible for controlling how the device on a network gains access to the media and allows the transmission of data.
2. Logical Link Control Layer: This layer is responsible for the identity and encapsulation of the network layer protocols and allows you to find the error.

Important functions of the data link layer:

• Frame that divides network layer data into frames.
• Allows adding header to the frame to define the physical address of the source and destination machine
• Add logical sender and recipient addresses
• It is also responsible for the provisioning process for the entire message delivery process.
• It also offers an error control system in which it detects retransmitted corruption or lost frames.
• The data link layer also provides a mechanism for transmitting data across separate networks that are interconnected.

transport layer:

The transport layer builds on the network layer to provide data transport from a process on a source machine to a process on a destination machine. It is hosted using single or multiple networks and also maintains quality of service features.

It determines how much data needs to be sent, where, and at what speed. This layer is based on the message that is received from the application layer. This helps ensure that data units are delivered without errors and in sequence.

The transport layer helps control the reliability of a link through flow control, error control, and segmentation or non-segmentation.

The transport layer also provides confirmation of successful data transmission and sends the next data if no errors occur. TCP is the best known example of the transport layer.

Important functions of the transport layers:

• It divides the message received from the session layer into segments and numbers them to make a sequence.
• The transport layer ensures that the message is delivered to the correct process on the destination machine.
• It also ensures that the entire message arrives without errors, otherwise it must be retransmitted.

network layer:

The network layer provides the functional and procedural means to transfer data streams of variable length from one node to another connected on "different networks".

Network layer message delivery does not provide any reliable network layer protocol.

The layer management protocols that belong to the network layer are:

1. routing protocols
2. multicast group management
3. Network layer address assignment.

session layer

The session layer controls the dialogues between computers. It helps you to establish start and end connections between local and remote application.

This layer requests a logical connection that must be established based on the end user's requirements. This layer handles all important login or password validation.

The session layer offers services as a dialog discipline, which can be full or half duplex. It is mainly implemented in application environments that use remote procedure calls.

Important function of the session layer:

• Establishes, maintains, and terminates a session.
• The session layer allows two systems to enter into dialogue.
• It also allows a process to add a checkpoint to the data flow.

presentation layer

The presentation layer allows to define the way in which data should be exchanged between the two communicating entities. It also helps you deal with data compression and data encryption.

This layer transforms the data into the form that the application accepts. It also formats and encrypts data that needs to be sent across all networks. This layer is also known assyntax layer.

The function of presentation layers:

• ASCII to EBCDIC character code translation.
• Data Compression – Allows you to reduce the number of bits that must be transmitted over the network.
• Data Encryption – Helps you encrypt data for security purposes, for example, password encryption.
• It provides a user interface and support for services like email and file transfer.

application layer

The application layer interacts with an application program, which is the highest level of the OSI model. The application layer is the OSI layer, which is closest to the end user. This means that the OSI application layer allows users to interact with other software applications.

The application layer interacts with software applications to implement a communication component. The interpretation of the data by the application program is always outside the scope of the OSI model.

An example of an application layer is an application such as file transfer, email, remote login, etc.

The function of the Application Layers are:

• The application layer helps you identify communication partners, determine resource availability, and synchronize communication.
• Allows users to log in to a remote host
• This layer provides various email services.
• This application offers distributed database sources and access to global information about various objects and services.

Interaction between layers of the OSI model

Information sent from one computer application to another must pass through each of the OSI layers.

This is explained in the following example:

• Each layer within an OSI model communicates with the other two layers below it and with its peer layer in some other networked computing system.
• In the following diagram, you can see that the data link layer of the first system communicates with two layers, the network layer and the physical layer of the system. It also helps you to communicate with the data link layer of the second system.

Protocols supported at various levels

Differences between OSI and TCP/IP

Here are some important differences between the OSI and TCP/IP models:

Advantages of the OSI model

These are the main benefits/pros of using the OSI model:

• Helps you standardize router, switch, motherboard and other hardware
• Reduce complexity and standardize interfaces
• Facilitates modular engineering
• Helps you ensure interoperable technology
• It helps you speed up evolution.
• Protocols can be replaced with new protocols when technology changes.
• Provide support for connection-oriented services as well as connectionless services.
• It is a standard model in computer networks.
• Supports connectionless and connection-oriented services.
• Offers flexibility to adapt to various types of protocols

Disadvantages of the OSI model

Here are some cons/disadvantages of using the OSI model:

• Adapting the protocols is a tedious task.
• You can only use it as a reference model.
• It does not define any specific protocol.
• In the OSI network layer model, some services are replicated across many layers, such as the transport and data link layers.
• The layers cannot work in parallel as each layer has to wait to get data from the previous layer.

Summary

• The OSI model is a logical and conceptual model that defines the network communication that open systems use to interconnect and communicate with other systems.
• In the OSI model, the layer should only be created where defined levels of abstraction are needed.
• The OSI layer helps you understand communication on a network
• In 1984, the OSI architecture was formally adopted by ISO as an international standard.