The key to being able to use run-of-the-mill Ethernet to fulfill the needs of the GBT is to break the Ethernet up into segments, and use high-speed switches to connect the segments.
In a conventional Ethernet, each machine shares a portion of the 10 Mbit/s bandwidth. In a switched topology, the network is broken up into a set of smaller segments, with a much greater aggregate bandwidth. The packets from one segment are switched only to the segment where the target interface resides, and not to the entire network. If the switch does not know which segment the packet should be sent to, it sends it to all the ports, but the switch learns which devices are on each port, so this happens only when the switch is reset, or a device is moved from one port to another.
Figures 1 and 2 show the design of the telescope's networks.
Figure 1: GBT Alidade Network Design
Several things are apparent from these figures. Many separate physical networks are defined, each with a local traffic pattern. The Antenna Control Computer talks to the servo system over a network free of other traffic. The Laser System also has a segment of the network with which to work. The Pointing UltraSPARC is connected to the network with 100 Mbit/s Fast Ethernet. In the equipment and control rooms, the computers all communicate via the Fast Ethernet. The legacy machines can still communicate with them, due to the ability of the switches to translate packets from 10 Mbit/s Ethernet to 100 Mbit/s Ethernet. As we retire the old machines, we can migrate the new ones to Fast Ethernet.
