terminal

Definition of terminal in The Network Encyclopedia.

What is Terminal (in computer networking)?

Generally, any device that terminates a communication channel. In computer terminology, a terminal is an input/output (I/O) device, usually consisting of a keyboard and monitor, that acts as a front end for a mainframe, terminal server, or other back-end processing device.

The earliest terminals were called teletypes (abbreviated TTY), which were essentially electric typewriters in which users would enter commands and data for a mainframe, and on which the mainframe would type the output returned to the user.

A terminal that supports only text output is sometimes called an ASCII terminal.

How It Works

Terminals generally have little or no inherent data-processing power and rely entirely on the back-end system to do the processing. The terminal is responsible only for processing and queuing input from the keyboard (or other input device such as the mouse), transmitting this in a recognized format to the back-end host (mainframe or terminal server), and receiving output from the host and presenting it on the screen in suitable format for the user (ASCII text in older systems, graphical desktop environment in newer systems). This explains the origin of the term “dumb terminal,” which means that a terminal by itself is generally useless without connecting to the back-end system. However, there are also “smart” or “intelligent” terminals that have various degrees of inherent processing capability. The information the user enters on the keyboard is typically transmitted to a mainframe over an RS-232 or RS-423 asynchronous serial connection, but it is sometimes transmitted over an Ethernet or a Token Ring local area network (LAN) connection. The mainframe processes the input and returns the output to the terminal, which displays the output on the monitor. In other words, the application runs in one location (the mainframe), while the user interface is in a different location (the terminal).

Terminals originated in the mainframe environment, and a number of standards (terminal protocols) have evolved that govern their use. The VT-100 terminal originated by Digital Equipment Corporation was a popular ASCII-text-based terminal standard that is still used in places such as library online catalog systems, which remote users typically access by running a telnet client over a dial-up connection. IBM’s 3270 terminal protocol is still widely used in IBM mainframe environments, while 5250 is popular in AS/400 mid-range computing environments. Other terminal standards include ANSI, VT52, and VT220.

Terminals can be one of the following:

  • Local terminals that are directly connected to the back-end host via a serial or LAN connection
  • Remote terminals that are typically connected via a phone line with a modem at both ends

The popularity of terminals declined in the late 1980s and early 1990s with the advent of distributed client/server environments and the eclipse of mainframe computing environments. In a client/server environment, data processing is shared between the front-end client computer, usually a full-featured PC with a graphical user interface (GUI) such as Microsoft Windows, and the back-end server, which can be a Windows NT–based server, a Novell NetWare server, an AS/400, or some other system.

In the late 1990s, the pendulum started to swing back toward terminals with the rising popularity of PC-based terminal emulators and terminal servers.

A terminal emulator is hardware and/or software that runs on a stripped-down PC with no operating system and causes the PC to function as a terminal, while a terminal server is a back-end server that generates and delivers the user desktop environment to the terminals and performs all the processing.

This arrangement allows for low-cost “thin clients” to be used and centralizes system administration at the back end, reducing deployment and management costs associated with a distributed client/server systems environment.

See also