There are two popular networking models: the OSI layers model and the TCP/IP layers model. The presentation layer and session layer exist only in the OSI layers models. The TCP/IP layers model merges them into the application layer.
The Presentation Layer
The presentation layer is the sixth layer of the OSI Reference model. It defines how data and information is transmitted and presented to the user. It translates data and format code in such a way that it is correctly used by the application layer.
It identifies the syntaxes that different applications use and formats data using those syntaxes. For example, a web browser receives a web page from a web server in the HTML language. HTML language includes many tags and markup that have no meaning for the end user but they have special meaning for the web browser. the web browser uses the presentation layer's logic to read those syntaxes and format data in such a way the web server wants it to be present to the user.
On the sender device, it encapsulates and compresses data before sending it to the network to increase the speed and security of the network. 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, JPEG, 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, 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 de-encrypts and decompresses the encrypted and compressed 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 modes of communication: full duplex, half-duplex, and simplex.
In the full duplex mode, both devices can send and receive data simultaneously. The internet connection is an example of the full duplex mode.
In the half duplex mode, only one device can send data at a time. A telephone conversation is an example of the half-duplex mode.
In the 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 it opens a logical communication channel (session) with the remote host, it handles the authentication process.
Examples of the session layer
Structure Query Language (SQL), Remote Procedure Call (RPC), and Network File System (NFS) are examples of the session layer.