TELNET is an abbreviation for TErminaL NETwork.

Timesharing Environment

• TELNET was designed at a time when most operating systems, such as UNIX, were operating in a timesharing environment. In such an environment, a large computer supports multiple users.

• The interaction between a user and the computer occurs through a terminal, which is usually a combination of keyboard, monitor, and mouse.

• Even a microcomputer can simulate a terminal with a terminal emulator

Logging

• In a timesharing environment, users are part of the system with some right to access resources. Each authorized user has an identification and probablY,a password.

• The user identification defines the user as part of the system. To access the system, the user logs into the system with a user id or log-in name. The system also includes password checking to prevent an unauthorized user from accessing the resources. Figure shows the logging process.

• When a user logs into a local timesharing system, it is called local log-in. As a user types at a terminal or at a workstation running a terminal emulator, the keystrokes are accepted by the terminal driver.

• The terminal driver passes the characters to the operating system. The operating system, in turn, interprets the combination of characters and invokes the desired application program or utility.

• When a user wants to access an application program or utility located on a remote machine, she performs remote log-in. Here the TELNET client and server programs come into use.

• The user sends the keystrokes to the terminal driver, where 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 character set called network virtual terminal (NVT) characters and delivers them to the local TCP/IP protocol stack.

• The commands or text, in NVT form, travel through the Internet and arrive at the TCP/IP stack at the remote machine. Here the characters are delivered to the operating system and passed to the TELNET server, which changes the characters to the corresponding characters understandable by the remote computer. However, 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: It is designed to receive characters from a terminal driver.

• The solution is to add a piece of software called a pseudoterminal driver which pretends that the characters are coming from a terminal.

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