A terminal server is a device that accepts incoming data calls and routes request for services to the appropriate source. Examples of termial servers are the Ascend MAX 4000, the U.S. Robotics Total Control Hub, the Shiva LAN Rover, and the Computone Intelliserver.
Terminal servers generally support incoming analog as well as ISDN calls. Some terminal servers can support analog calls which are delivered over a T1 line, called a Digital Supertrunk. A channelized-T1 line provides 24 inbound data calls, one per DS0 channel. Usually one phone number is assigned for customers to call in on and software at the Telco's CO switch provides a "hunt-group" which directs the next call to the first available modem. Each of the 24 lines has a unique phone number, but 23 of these are only seen by the telco switch. This is a convienience to both ISPs and customers.
Older terminal servers like the Computone Intelliserver require discreet phone lines be connected to individual discreet analog modems. This is a less than satisfactory solution since all those modems take up enormous amounts of space in an ISP's Network Operations Center.
A terminal server also incorporates a router, which is responsible for sending requests out to the proper server, and for receiving replies from servers and sending them back out to the appropriate modem line.
Some newer terminal servers, like the Ascend MAX and the U.S. Robotics Total Control Hub accept both analog and ISDN calls. They use special devices which can handle both types of calls. They can do this because an analog phone call is actually a digital call at every point in the phone network except the local loop (the wire from your house to the Telco's Central Office (CO) switch. Modem stands for MOdulator/DEModulator. An Analog signal is modulated to become digital, sent out over the phone company network, and then demodulated (returned to an analog signal) at the other end. If the "other end" can handle digital data, then why remodulate the signal to analog? Why not just deal with it as a digital signal? That's exactly what the new terminal servers do. They can recognize the difference between an ISDN call and a digitized analog call by the signaling information sent over the D channel (in the case of an ISDN call) or the absence of it (in the case of a digitized analog call). Once they recognize the type of call, they switch it to the appropriate internal circuitry for decoding and routing.
Terminal servers also handle login authentication. Most use a RADIUS (Remote Access Dial-In User System) database directly to check that a dial-in connection has a valid login name and password before they will accept the connection.
Terminal servers are expensive devices, costing anywhere from $19,000 to $40,000.
Most vendors now refer to their terminal server products as Access Switches.
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