OSI Reference Model : Application Layer
ISO-OSI reference model is the standard model for data communication among different devices. The OSI reference model consists of seven layers: Application Layer, Presentation Layer, Session Layer, Network Layer, Transport Layer, Datalink Layer, and Physical Layer. Each layer is responsible for performing some task. For example: Session layer is responsible to create and manage sessions between the sender and receiver. In this OSI model, the lower layer serves the upper layer.
When a device intends to send data to another device, the data travels from one layer to another layer. At the senders end the data travels through Application layer to Physical Layer and at the receiver end the same data travels through Physical Layer to Application Layer. If the protocol is minutely analyzed, the corresponding layers at both sender and receiver end goes through peer to peer communication i.e. the application layer at the senders and communicates with the application layer at the receiver end or the network layer at the sender end communicates with the network layer at the receiver end and so on.
User is very close to the Application layer of the OSI reference model. User and Application layer of OSI reference model interact directly with each other with some software application.
Meaning of “Application” in “Application Layer”
The term “Application” in “Application layer” has a little bit different meaning. We think that “Application Layer” consists of applications. But it’s not. When we want to open some webpage we use web browser. Web browser is one application which is used to access pages from some web server. But how this application (Web Browser) will access the remote web page? For this Application Layer provides the HTTP (Hypertext Transfer Protocol) to the application (Web Browser). Similarly if we have any email application then Application layer of OSI reference model will provide the appropriate protocol like SMTP to the mail application. So Application layer provides services to different applications.
The basic functions of the Application layer of OSI reference model are: Identify the receiver with whom the communication will occur, to check the resource availability which will be communicated with the receiver and synchronize the entire communication process.
Application layer when intends to communicate with another host/device, it identifies the receiver and checks the receiver is available or not. Once the partner is identified and assured that the partner is available, application layer checks the availability of resources like the existence of network or communication channel. The cooperation between the sender and receiver is managed by Application layer.
Services Provided by Application Layer
The services provided by Application layer of OSI reference model are as follows:
- Identify the partner with whom the communication will take place.
- Application layer may authenticate either the sender or receiver or both depending on the application or type of communication.
- Application layer ensures the availability of resources which are required for the communication. For example: for data communication the application layer may check whether modem exists or not, whether channel between the sender and the receiver is available or not.
- Application Layer ensures the agreement between both the sender and the receiver. For example: both the sender and the receiver must agree on the way how the data will be transmitted, how the data will be recovered or how the data integrity will be managed. Another instance may be that both the parties must agree on the same algorithm which will be used for encoding and decoding the data.
- It is on the Application layer to decide which protocol is to be used for data communication. For web services, HTTP protocol is used and for mail services SMTP and POP protocols are used. Based on the type of communication Application layer decides on which protocol is to be used. As discussed earlier, application layer also ensures that both the sender and receiver agree on the same protocol.
Functions of Application Layer
The functions of application layer are as follows:
- Remote File access
- Resource sharing
- Inter-process communication
- Network Management
- Directory Services
- Mail Services
- Virtual Terminals.
All the functions above, more or less, use client-server architecture. In remote file access, the client tries to access the file from another host which is called server. Inter process communication or IPC is the communication between two processes. In virtual terminals also the client server model is used. Based on different applications, whether it is mail application or virtual terminal application or IPC application, the Application layer serves the application with the desired protocol.
Protocols used in Application Layer
The different protocols used in Application Layer are: HTTP, FTP, SMTP, DHCP, NFS, Telnet, SNMP, POP3, NNTP, IRC etc.
In ISO-OSI reference model, Application Layer is the only layer in the OSI stack which do not serve any higher protocol instead it serves the applications which want to use the network.