Presentation layer and Session layer of the OSI model

OSI and TCP/IP are two popular networking models. Presentation and session layers are available in both models differently. In OSI, they exist separately. In TCP/IP, they are part of the application layer. No matter how they are available, they provide the same features and functions in both models. This tutorial outlines the basic concepts and fundamentals of these layers.

The Presentation Layer

The presentation layer is the sixth layer in the OSI Reference Model. It defines how data and information are transmitted and presented to the user. It translates data and formats code for the application layer.

It identifies the syntaxes used by different applications and formats data accordingly. For example, a web browser receives a web page from a web server in HTML language. The HTML language includes many tags and markup that have no meaning for the end user but do have special meaning for the web browser. The web browser uses the presentation layer's logic to parse those syntaxes and format the data as the web server wants it presented to the user.

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

Examples of the presentation layer

Example standards for representing graphical information: JPEG, GIF, and TIFF.
Example standards for representing audio information: WAV, MIDI, MP3.
Example standards for representing video information: WMV, MOV, MP4, MPEG.
Example standards for representing text information: doc, xls, txt, and pdf.

Functions of the presentation layer

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

Session layer

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

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

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

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

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

Functions of the session layer

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

Examples of the session layer

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

Conclusion

The presentation and session layers of the OSI model play crucial roles in ensuring effective data communication and user experience. The presentation layer is responsible for data formatting, encryption, compression, and translation, enabling different systems to communicate seamlessly. The session layer manages the establishment, maintenance, and termination of sessions, supporting various communication modes and reliable data exchange. Understanding these layers helps in troubleshooting network issues and designing robust networking solutions.