Terminal server

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A terminal server is a specialized computer which aggregates multiple communication channels together. Because these channels are bidirectional, two models emerge: Multiple entities connecting to a single resource, and a single entity connecting to multiple resources. Both of these models are widely used. Both physical and virtual resources can be provided through a terminal server: centralized computing can provide multiple users access to a remote virtual operating system. Access Providers often use terminal servers to terminate physical connections to their customers, for example customers who get Internet through some form of modem.

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[edit] History

Historically, a terminal server was a device that attaches to serial RS-232 devices, such as "green screen" text terminals or serial printers, and transports this traffic via TCP/IP TELNET, SSH or other vendor-specific network protocol (e.g. LAT) via an Ethernet connection.

Digital Equipment Corporation's DECserver 100 (1985), 200 (1986) and 300 (1991) are early examples of this technology. (An earlier version of this product, known as the DECSA Terminal Server was actually a test-bed or proof-of-concept for using the proprietary LAT protocol in commercial production networks.) With the introduction of inexpensive Flash memory components, Digital's later DECserver 700 (1991) and 900 (1995) no longer shared with their earlier units the need to download their software from a 'load host' (usually a Digital VAX or Alpha) using Digital's proprietary MOP protocol. In fact, these later terminal server products now also included much larger Flash memory and full support for the TELNET part of the TCP/IP protocol suite.

Many other companies, such as Spider Systems, entered the terminal server market with terminal servers pre-loaded with software fully compatible with LAT and TELNET. Some manufacturers also stated specifically that they had emulated Digital's command set for terminal server management. Besides retaining the ability of the older terminal servers to obtain their run-time code from a load host, most were able to bootstrap from on-board flash memory or from a floppy disc held in a drive in the terminal server. Some Xyplex terminal servers could act as load host for each other; one would hold the code on a PCMCIA Flash card and serve it to another.

Starting in the mid-1990s, several manufacturers such as U.S. Robotics produced "modem terminal servers". Instead of having RS-232 ports, these would directly incorporate an analog modem. These devices were commonly used by Internet service providers to allow consumer dial-up. Modern versions interface to an ISDN PRI instead of having analog modem ports.

[edit] Modern usage

The term "terminal server" is used in three main ways, as of 2006:

  • Console servers (also known as serial terminal servers) are often used for connection to the console ports of Unix servers. This then allows system administrators to connect to the servers over the network. This is important for rebooting the system and for hardware debugging, where the operating system will not boot correctly.

[edit] Centralized computing

There are two contemporary models of centralized computing:

In one, the terminal server provides a Windows or Linux desktop to multiple user terminals - the modern term for these terminals is "thin client".

In the other model, an ordinary computer acts temporarily as a terminal server, providing its desktop to a remote computer over a wide area network such as the Internet, in order to enable teleworking.

A client of a terminal server is referred to as a thin client. Software clients used in the teleworking model are known as remote desktop applications; however, these remote desktop applications are also used in the thin client model as well.

For more details on this topic, see thin client.

[edit] See also