Application Layer

Application Layer

The application layer in the OSI model is the closest layer to the end user which means that the application layer and end user can interact directly with the software application. The application layer programs are based on client and servers.

It is the top most layer of OSI Model. Manipulation of data(information) in various ways is done in this layer which enables user or software to get access to the network. Some services provided by this layer includes: E-Mail, transferring files, distributing the results to user, directory services, network resources, etc.

The Application layer includes the following functions:

• Identifying communication partners: The application layer identifies the availability of communication partners for an application with data to transmit.
• Determining resource availability: The application layer determines whether sufficient network resources are available for the requested communication.
• Synchronizing communication: All the communications occur between the applications requires cooperation which is managed by an application layer.

Services of Application Layers

• Network Virtual terminal: An application layer allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user's computer talks to the software terminal, which in turn, talks to the host. The remote host thinks that it is communicating with one of its own terminals, so it allows the user to log on.
• File Transfer, Access, and Management (FTAM): An application allows a user to access files in a remote computer, to retrieve files from a computer and to manage files in a remote computer. FTAM defines a hierarchical virtual file in terms of file structure, file attributes and the kind of operations performed on the files and their attributes.
• Addressing: To obtain communication between client and server, there is a need for addressing. When a client made a request to the server, the request contains the server address and its own address. The server response to the client request, the request contains the destination address, i.e., client address. To achieve this kind of addressing, DNS is used.
• Mail Services: An application layer provides Email forwarding and storage.
• Directory Services: An application contains a distributed database that provides access for global information about various objects and services.
• Authentication: It authenticates the sender or receiver's message or both.

The Application Layer contains a variety of protocols that are commonly needed by users. Most widely-used application protocol is HTTP (Hypertext Transfer Protocol), which is the basis for the World Wide Web. When a browser wants a web page, it sends the name of the page it wants to the server using HTTP. The server then sends the page back. TELNET, FTP, DNS, SMTP etc are other protocols.