1. It is the topmost layer of OSI model. It provides services that directly support user application such as database access, e-mail and file transfer.
2. It allows applications to communicate with applications on other computers as though they were on the same computer.
3. The relationship of the application layer to the user and the presentation layer is shown in the figure below:

The application layer performs the following functions:

1. The application layer creates a software emulation of a terminal at the remote host (Network Virtual Terminal). The user’s computer talks to the software terminal, then the software terminal talks to the host and vice-versa. The remote host feels that it is communicating with one of its own terminal and allows you to log on.

2. The application layer provides file transfer access and management (FTAM) which allows a user to access files in a remote computer to retrieve files from a remote computer and to manage or control files in a remote computer.

3. It provides a basis for e-mail forwarding and string.

4. It provides distributed database sources and access to the worldwide information about various objects and services.

Application protocol:

• Application protocols provide services to user applications. The application protocols tend to be more visible to the user than other types of protocols.

• These protocols may be user written according to the requirement or they may be standardized applications.

• The most common application protocols in the TCP/IP protocol suite are the Telnet, File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP).

Terminal Network (TELNET):

• It is a client/ server application program. TELNET enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the remote system.

• In TELNET a user can run different application programs at a remote site and create results that can be transferred to his local computer.

• After logging on, a user can use the services available on the remote computer and transfer the results back to the local computer.

• When a user wants to access an application program utility located on a remote machine he performs remote login operation.

• As a user types at a terminal or a workstation running a terminal emulator, the keystrokes are accepted by the terminal driver.

• The terminal passes the characters to the operating system. The local operating system accepts the characters but does not interpret them.

• The characters are sent to the TELNET client, which transforms the characters to a universal set called Network Virtual Terminal (NVT) characters and delivers them to the TCP/ IP stack as shown in the below figure:

• As shown in the figure the text messages or commands in the NVT form travel through the internet and arrive at the TCP/ IP stack at the remote machine.

• From the TCP/ IP stack the characters are delivered to the operating system and passed to the TELNET server.

• The characters cannot be passed directly to the operating system because the remote operating system is not designed to receive characters from a TELNET server, but it is designed to receive characters from a terminal driver.

• To overcome this problem, software called pseudo-terminal driver is used, which pretends that the characters are coming from a terminal.

• The operating system then passes the characters to the appropriate applications program.