Overview

Up to now, we have covered telecommunications fundamentals dealing with the generation and exchange of information in electronic form over various transmission networks. The fundamentals of circuit switching, presented below, center around the need to share telecommunications resources. For example, if a separate telephone and transmission line were required to connect you with every other person you needed to talk with, you probably couldn't fit into your office. The earliest telephone systems used just such an arrangement, however, which quickly threatened to bury communities in telephone poles and wiring.

The basic notion behind circuit switching was previously illustrated in Figure 1.1, showing how it does away with the need for dedicated transmission media running between each pair of communicators. Instead, the idea of resource sharing is introduced, wherein each person can access transmission resources as necessary "just-in-time" communications, so to speak. More formally, circuit switching is a process that establishes connections on demand and permits exclusive use of those connections until they are released. In telecommunications networks, circuit switching establishes a sender/receiver voice or other information signal path in a manner analogous to an electrical switch's enabling current to flow from a power source to a light bulb.

The earliest switching arrangements revolved around telephone operators, who in the beginning were young men, often on roller skates, hurtling across central offices with plug-ended cords to connect telephone calls. These fellows tended to be rude to callers, however, and were soon replaced by better-mannered young ladies.

A major...