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  The GBT control system depends on the LAN to function. The network has been designed to provide a high level of performance, yet be affordable.

The choice of a network technology was not easy. There were, as usual, several competing constraints. The networks proposed in the early stages of the project were completely different than that finally chosen for implementation. This was due to many factors. The original design included two networks, one for data transfer, and one for control information. The data load on one Ethernet network would have been too great to allow proper control of the telescope over the same LAN. Therefore, a data LAN and a M&C LAN were planned. Later, the M&C group decided[3] to use a multiport RAID to store the backend data, and so the proposed data LAN was unnecessary. At the same time, technology was advancing, bringing to market full-duplex Ethernet, switching technology, and 100 Mbit Ethernet (Fast Ethernet). The LAN technologies investigated include Fibre Channel, Asynchronous Transfer Mode (ATM), Switched Fast Ethernet, and Fiber Distributed Data Interface (FDDI). Each has its merits. FDDI is (or was) the most widely used high-speed LAN. ATM shows much promise. Fast Ethernet is a speedy version of a familiar friend. In the end, Fast Ethernet won out on cost and convenience factors. Fibre Channel is by far the highest performer, with a corresponding price tag. ATM and FDDI were also determined to be more costly, without apparent benefits. Our choice seems to have been a good one, as the Fast Ethernet market has taken off, and seems assured of a long life.

John Ford
Fri Mar 13 16:06:38 EST 1998