What is Application Layer in OSI Model

What is Application Layer in OSI Model?

This article is a complete brief on What is Application Layer in OSI Model. We will explain the way in which the functions of the application layer, session layer and presentation layer operate together to provide network services to end-user applications.

WHAT IS THE APPLICATION LAYER?

The application layer is closest to the end user. As shown in the image, it is the layer that provides the interface between the applications used for communication and the underlying network in which messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. The top three layers of the OSI model (application, presentation and session) define functions of the single TCP / IP application layer.

PRESENTATION LAYER

The Presentation Layer has three main functions:

• The Format : Format the data of the source device, or present it, in a compatible way for the destination device to receive it
• Compression : Compress the data so that it can be decompressed by the target device.
• Encryption : Encrypt data for transmission and decrypt upon receipt. Common video standards include QuickTime and the Movie Expert Group (MPEG) .The common graphic image formats are: GIF, JPEG and PNG.

SESSION LAYER

As the name implies, the functions of the session layer create and maintain dialogues between the source and destination applications and also manage the exchange of information to initiate dialogues, to keep them active and to restart sessions. The so-called layer five of the OSI model, defines the connection of a user computer to a network server and from one computer to another in a network with peer-to-peer configuration.

TCP / IP APPLICATION LAYER PROTOCOLS

There are many application layer protocols, and they are constantly developing. Some of the best known application layer protocols include the hypertext transfer protocol (HTTP), the file transfer protocol (FTP), the trivial file transfer protocol (TFTP), the Internet message access protocol (IMAP) and the Domain Name System (DNS) protocol.
In summary:

• Specify the control information and format necessary for common Internet functions.
• They must be implemented on both the source and destination devices.
• The application layer protocols that are implemented on the source and destination host must be compatible to allow communication.

INTERACTION OF APPLICATION PROTOCOLS WITH APPLICATIONS FOR END USERS

Finally, we see how application protocols interact with applications for end users:

CLIENT-SERVER MODEL

• The device that requests information is called "client" and the device that responds to the request is called "server".
• Client and server processes are considered part of the application layer.
• The content of the data exchange will depend on the application in use.
• Email is an example of a client and server interaction.

POINT-TO-POINT NETWORKS (PEER NETWORK - P2P)

• Two or more computers can be connected to a P2P network to share resources without having a dedicated server.
• Any connected terminal (known as "point") can function as a server and as a client.
• Client and server functions are set by request.

A P2P application allows a device to function as a client and as a server within the same communication. In this model, each client is a server and each server is a client. P2P applications require that each terminal provide a user interface and run a background service.