The earliest computers communicated only with their expert handlers, who programmed them by connecting circuits by hand and read their outputs on paper. Today one of a computer's essential tasks is to communicate with other computers. This would be impossible without a vast network for transmitting signals reliably and at high speeds over long distances, and such a network has evolved during the last century. A few of its aspects are described below.

Communication networks bundle slow signals together to make fast ones. In the phone system, the slowest or most basic signal is the voice signal. Early in the history of telephony it was realized that it would be more cost-effective to combine many voice signals into a few high-speed signals for long-distance transmission than to try to transmit voice signals everywhere. It was also realized that these high-speed signals might, for very long hauls...