The Components

Cable

This exists in several forms:-

50 Ohmn Coaxial cable

There are two forms: standard and thin wire. Thin wire is cheaper and easier to lay but has a more limited range. Effective transmission rates of up to 10 MBits/sec are possible. This cable was used in our department but has been phased out.

Twisted Pair cable

Effective transmission rates of up to 100 MBits/sec or even 1 Gbits/sec for short runs).

Optical Fibre cable

Effective transmission rates of up to 1 GBits/sec.

Transceiver

This is the interface that connects a processor.

Terminal Server

Connects eight or more terminals. It contains a microprocessor and manages requests from host devices offering ``services'' and requests from terminals asking for connection ``sessions''. It contains the back off and retry logic. Both terminal and printers can be connected to terminal servers .

Repeater

This connects a number of ETHERNET cables together and boosts the signal. A signal coming in on any cable is rebroadcast on all other cables. Repeaters are only needed if the cable exceeds a certain size.

Hub

This is similar to a repeater in that it connects a number of cables together although the signal is not boosted. Internally the connection is made by a very fast backbone.

Bridge

Connects two or more branches of a network and provides filtering, only allowing certain traffic to cross from one network to the other. In particular it can:-

• Filter certain protocols.

• Allow packets to cross from one branch to another only if they are destined for a machine on that branch.

In this way a group of computers that are tightly coupled and consequently generate a lot of local traffic, can be isolated behind a bridge and yet remain connected to a larger network without loading it with unnecessary traffic.