Application Layer in OSI Model

This article will help you in getting a better understanding of the Application layer in the OSI model.

It is the uppermost layer in the OSI Model. The application layer establishes the interaction/communication between the end user and the software application.The application layer allows users to send and access the data and provides connectivity with the network.

As it is the last layer it depends on all the layers for the completion of a process. The application layer displays the data. It is not an application, instead, it is a component of an application that resides within the application. It is an abstraction layer that displays only the outer structure and hides the inner structure(code of application) from the user. It displays the data in a human-understandable format.

Functionalities of the Application Layer

1. Mail services: Application layer provides basic mail services like sending, receiving, and forwarding mail. It provides a storage facility for mail.
   
2. Determining resource availability: Application layer provides or determines the availability of requested resources for carrying out a specific communication. Here resources may refer to any input/output devices like printers, etc., or for access to main memory. 
   
3. Addressing: Addressing is very crucial in establishing the communication between the client and server. When clients send a request to the server the request carries both server and its address. The response of the server (to the client’s request) consists of the destination address(client address). DNS(Domain Name System) protocol is used for carrying out the addressing process.
   
4. Synchronizing communication: Synchronization is a process of adding multiple checkpoints in the communication process. If there is any interruption or any failure during the process, then the process will continue to run from the last saved checkpoint rather than repeating the process from the initial stage. All the communications between various applications are properly synchronized with the help of the Application layer.
   
5. File transfer Access and Management (FTAM): This procedure is mainly applicable for remote computers so that users can manage, and retrieve various files from a remote computer. FTAM stores file structure, file attributes, etc in a virtual hierarchical manner.
   
6. Identifying communication partners:  The application layer determines whether the communication partners are available or not for data transfer by an application.
   
7. Network Virtual terminal: A user log onto a remote host with the help of the Application Layer to create a software emulation terminal at a remote host. The user’s computer interacts with the software terminal and interacts with the host. But the remote host assumes that it is interacting with its terminal and it allows the user to get logged in.

Working of Application Layer

The application layer acts as an interface between a user and a software application. The client sends a request to the server, and the server acknowledges the client that the connection is established by setting up the Acknowledgement(ACK) Flag to 1. As soon as the client receives the acknowledgment from the server, the client can access the server and can store and retrieve data from the server.

Application Layer Protocols

Following are the various protocols used by the application layer in the Open Systems Interconnection (OSI) Model:

1. FTP (File Transfer Protocol):

FTP stands for File Transfer Protocol. This protocol is used for sending various files from one device to another device. This transfer of files is done with the help of remote computer devices. 20 port number is used for accessing the data and 21 port number is used to handle the data.

2. SMTP (Simple Mail Transfer Protocol):

SMTP stands for Simple Mail Transfer Protocol. This protocol is used for delivering mails from one device to another device. Simple Mail Transfer Protocol is mainly used by the end-users and this protocol utilizes the 25 and 587 port numbers.

3. HTTP (Hyper Text Transfer Protocol):

HTTP abbreviates Hyper Text Transfer Protocol. This protocol’s functioning is based on the client-server model. This protocol is used to transfer various types of data like photos, text, hypertext, or other hypermedia documents into the WWW(World wide web).

Hyper Text Transfer Protocol is mainly used in establishing the communication between web browsers and web servers. It acts as a basic foundation for WWW. It utilizes 80 port number.

4. DNS (Domain Name Search):

DNS abbreviates Domain Name Search. This protocol uses the Domain Name into its equivalent IP address. For example, if a user selects www.xyz.com then it converts it into its equivalent IP address i.e., 192.23.45.63(it is a random IP address that is mentioned just for understanding purposes). This protocol utilizes 53 port number.