A terminal server is a network computer used to get a remote login on the administration console of a computer (a terminal server is also called a console server). It has an ethernet interface connected to a network and a lot of console interfaces connected to computers. It runs remote access services such as telnet and SSH. These provide different types of user interface to manage a computer.
Cyclades, Cisco and Blackbox manufacture terminal servers. A terminal server can also be built using a PC with a lot of serial ports, the Linux OS and a console service.
Many types of application computer include a special serial interface for adding a terminal. This terminal is intended to be used by administrators for controlling the behaviour of the computer, rather than by customers for running applications. The terminal is called an administration console. Sun and Cisco computers have these interfaces built into them but PCs don't.
The serial interfaces on the computers are connected to a terminal server. A terminal server is a computer that allows access to a lot of computers. It is a box with a bunch of sockets in the back which looks like a switch or a router. A terminal server has a quantity of sockets that seems vaguely binary, such as 8, 24 or 48.
Every socket is an RJ-45 type, the same type that a million billion PCs use to connect them to an ethernet network. The serial cable that connects the computer to the terminal server is not the same as a cable between a PC and an ethernet network computer: it is more like a crossover cable or null modem cable. Every computer plugged into a socket is a terminal: the end of the line for a data's journey.
The ethernet socket on the terminal server is connected to a network. An administrator does not like to hang around in a computer room so she uses a terminal server to let her login remotely. The administrator can work from the comfort of her plush designer penthouse, pretending to be logged into the adminstration console attached to a computer.
The terminal server waits for IP packets to arrive from the network, translates these packets into a serial conversation and passes it onto one of the computers connected to it. It figures out which computer to forward a conversation to by looking at the port number the packets are destined for. Data coming out of the computer is split up and shoved into IP packets which are addressed to the remote user and sent on their way.
The network connection that data takes from her computer to the terminal server is not the same one used by all the other data. Terminal servers are used for OOB (Out of Band) Management. OOB is an acronym from old telegraph days meaning signals coming from somewhere other than the normal waveband. Signals for customers were sent back and forth within a waveband and signals for engineers were sent outside that waveband. In a network OOB means an alternative route used only by support people, safe from network problems that screw up normal connections.
Only one person at a time can use an administration console. An SSH server lets lots of people work on a computer at the same time so administration is usually done via this server (see open a CLI on a host), not via a terminal server.
Gaining access via the console port rather than via a network port is useful for some work. If you want to reboot a computer, use the console to see startup messages. If the host fails to reboot properly, these messages are crucial for determining what went wrong. Sometimes the only administrative access to a computer is via its console. Access to other interfaces on these hosts may be blocked by a firewall or not correctly networked. There may be a problem such as the SSH server not running.
Some old terminal servers run SSH servers, but only old versions. Older Cisco terminal servers use old stream ciphers like DES. These servers do not understand 3DES. PuTTY objects to these old stream ciphers so you may want to (you don't have to) change your PuTTY settings to accept weak ciphers.
Graphical. A terminal in UNIX land harks back to an electric typewriter. It only understands characters.
A KVM (Keyboard Video Mouse) terminal server. A KVM is used with PCs to make up for the lack of console port.
Terminal servers are connected to every Sun and Cisco computer.