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[previous article] [next article]Curt Freeland
Most people who use machines on the ECN never stop to think about how the machines are interconnected; and, to be honest, sometimes those of us who maintain the network prefer not to think of the complexity of computer lines crisscrossing the campus. We just hope that things keep running, so we can continue to connect new machines to the network! Yet every so often, curious users ask how machines communicate from one building to another.
To accomplish intramachine communication, the ECN uses a commercially available networking standard called Ethernet. It is one of many network packages available, and was chosen because of its reliability and capability to handle the hook-up of many machines at a relatively low cost per connection. The segment length limit of Ethernet cable is 500 meters and due to this constraint, ECN actually has many small Ethernet segments. When working correctly, these segments all look like one big network to the user while in actuality, these segments are tied together with special "repeaters", and "gateway" machines.
A repeater is a box that allows us to tie two segments together. The main ECN backbone network consists of three 500 meter segments tied together with repeaters. Each repeater collects packets coming in on one Ethernet segment, and forwards those packets to the next segment.
A gateway machine is a network host that functions as a repeater. Each building with computers on the network has a gateway machine to service that building's internal Ethernet. This machine collects packets addressed for machines within the building from the main network, and forwards them to the building network. Currently, there are nine building Ethernets tied into the main ECN network via gateway machines.
To allow the machines to communicate on the Ethernet, the Department of Defense commissioned the development of the Transmission Control Protocol/Internet Protocol (TCP/IP) software package. This software package is a set of protocols which define how machines communicate. TCP/IP is made to run on a broad range of computer systems. This versatility allows us to install Gould, DEC, SUN, Symbolics, ISI, IBM, and many other types of computer systems on one Ethernet with all of them maintaining the capability to communicate.
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