Foundation concept: Networks.

Overview

The standard model for a voice telephone system is a circuit switching network. When two users or end-stations wish to communicate, a direct connection must be made between them, and dedicated to that call for its duration. The connection may be through copper wires, fiber-optic cables, or even radio or microwave transmissions. It may pass though any number of switching stations, staffed by human operators with plug-boards, or electronically operated. It is even possible, using frequency shifting, for many conversations to be carried by the same wire at the same time. The key concept is that a complete connection must be made, then the conversation can happen, and then the connection may be dismantled.

Circuit switching allows very simple telephony equipment built on nineteenth-century technology to operate effectively (the first telephone exchange was working in 1877), but it does not allow very efficient use of the high-cost infrastructure (long-distance cables and transmitters).

The alternative is a Packet-switching network, and requires that all communications are in digital form. In a packetswitched network, every end-station has a permanent connection to at least one switching station; each switching station has a number of permanent connections to other switching stations. The connectivity does not change during use: connections are not made before data is transmitted or broken thereafter; there is a path (perhaps a very long path, through dozens of switching stations) from any end-station to any other.