Communication is one of the most important functions performed by computers. It is easy to understand that a computer must calculate, compare, and store data. It is also easy to see that the computer must input and output data used to communicate with the input and output devices. Also, within the computer, data must be transferred from one location—for example, from read only memory to random access memory (RAM), or from permanent storage such as a compact disc to temporary memory, etc. Transferring data, therefore, is just another way of referring to communication.

Communicating from one device to another within the computer is usually done through a bus, which is essentially a set of printed circuit board traces or wires within the computer. Buses are usually local to the device. Very special wire arrangements, called transmission lines, must be used when the required...