Understand the OSI model in five minutes

The Open Systems Interconnection (OSI) model is a standard that defines how computers, servers, and users communicate within a system. It was the first standard model for network communication and was adopted by all major computer and communications companies in the early 1980s.

The OSI model provides a common language for describing networks in terms of discrete blocks or layers.

This model describes the seven layers at which computer systems communicate over a network.

7 Application Layer

6 Presentation Layer

5 Session Layer

4 Transport Layer

3 Network Layer

2 Data Link Layer

1 Physical Layer

Each layer has its own way of working and a set of protocols that are different from the other layers. This article will analyze these layers one by one.

Application Layer

The application layer is implemented in software. It is the layer that interacts with applications.

Let's take sending a message as an example. The program that sends the message interacts with the application layer and sends the message. The application layer then sends the message to the next layer of the OSI model, the presentation layer.

Presentation Layer

The data from the application layer is forwarded to the presentation layer. The presentation layer receives data in the form of text, characters, letters, numbers, etc. and converts them into machine-readable binary format data. This process is called compilation.

At this stage, ASCII (American Standard Code for Information Interchange) characters are converted to Extended Binary Coded Decimal Interchange Code (EBCDIC). The converted data is also encoded and encrypted before it is transmitted further, using the SSL protocol for encryption and decryption.

The presentation layer is an abstraction that assumes that the layers below it will process the data they receive. It is also responsible for compressing the data. The compression of data can be lossy or lossless, depending on many factors that are beyond the scope of this article.

Session Layer

The role of the session layer is to establish and manage connections. The main work of this layer is to establish sessions. For example, when you log in to an online store, a session is established between your machine and the server.

The role of the session layer is to send and receive data, and once completed, the connected session is terminated. Before a session is established, authentication is performed. Similar to the previous layer, the session layer also assumes that after its work is completed, the layers below will also process the data accurately.

Transport Layer

The role of the transport layer is to manage the transmission of data and its own protocols for how data is transmitted. Data coming here from the session layer is divided into smaller units of data called fragments. This process is called "segmentation". Each fragment contains a source port number, a destination port number, and a sequence number. The port number is used to identify the application sending the data. Note that data is transmitted in blocks. The sequence number is used to put the fragments in the correct order.

The transport layer is responsible for controlling the flow rate, or the amount of data that is transmitted in a given time. It is also responsible for error control, such as when data is lost or corrupted. It uses an error detection value, usually called a checksum. The transport layer adds a checksum to each piece of data so that it can check whether the data being sent is received correctly. The data is then transferred to the network layer.

Network Layer

The role of the network layer is to communicate with other networks. It transmits the data fragments received from one machine to another machine located on a different network. Routers work at the network layer.

The function of the network layer is logical addressing (that is, determining the IP address). It assigns IP addresses to senders and receivers, and the data packets are transmitted to the correct destination machine with this address. The network layer then routes the data packets. Load balancing is also performed at the network layer to ensure that overload does not occur. Next, the data is transmitted to the data link layer.

Data Link Layer

The data link layer supports direct communication with other devices.

It receives packets from the network layer containing the sender and receiver IP addresses, performs physical addressing, and then assigns the sender and receiver MAC addresses to the packets to form frames.

Physical Layer

The physical layer consists of all the hardware and physical devices of the system (including network cables, navigation systems, adapters, etc.). Here, the data received from the previous layers are in the form of 0s and 1s. The physical layer converts this data and transmits it to the local medium through various means (wires, electrical signals, optical signals in the case of fiber optic cables; radio signals in the case of WIFI).

Note that the physical layer acts on the receiving end, passing the received signal to the data link layer in the form of frames (converting it back to binary data). The frames are then passed to the upper layers, and finally the application layer (application software) receives the required data.

Conclusion

Knowledge of the OSI model will be helpful when you need to describe a network architecture or troubleshoot a network problem. I hope this article has given you a clear understanding of all aspects of this model.