Introductory Switching Concepts

All local telephone switches have, as a minimum, three functional elements: concentration, distribution, and expansion. Concentration and expansion were discussed in Section 4.3.3. Viewing a switch another way, we can say that it has originating line appearances and terminating line appearances. These are illustrated in a simplified conceptual drawing in Figure 4.8, which shows three different call possibilities of a typical local exchange:

1. A call originated by a subscriber who is served by the exchange and bound for a subscriber who is served by the same exchange (route A-B-C-D-E).

2. A call originated by a subscriber who is served by the exchange and bound for a subscriber who is served by another exchange (route A-B-F).

3. A call originated by a subscriber who is served by another exchange and bound for a subscriber served by the exchange in question (route G-D-E).

Call concentration takes place in B and call expansion at D. Figure 4.9 is simply a redrawing of Figure 4.8 to show the concept of distribution. The distribution stage in switching serves to connect by switching the concentration stage to the expansion stage.

Early Automatic Switching Systems


We summarize several earlier, space division switching systems because of the concepts involved. Once the reader grasps these concepts, the ideas and notions of digital switching will be much easier to understand. First, the operation of the original step-by-step switch is described. This is followed by a discussion of the crossbar switch.

The Step-by-Step Switch

The step-by-step (SXS) switch use was widespread in the United States prior to 1950, when the crossbar switch tended to replace it. Its application was nearly universal in the United Kingdom, where it was called the Strowger switch.

The step-by-step switch has a curious history. Its inventor was Almon B. Strowger, an undertaker in Kansas City. Strowger suspected that he was losing business because the town’s telephone operator was directing all requests for funeral services to a competitor, which some say was a boyfriend, others say was a relative. We do not know how talented Strowger was as a mortician, but he certainly goes down in history for his electromechanical talents for the invention of the automatic telephone switch. The first “step” switch was installed in Indiana in 1892. They were popular with independent telephone companies, but installation in AT&T’s Bell System did not start until 1911. The step-by-step switch is conveniently based on a stepping relay of 10 levels. In its simplest form, which uses direct progressive control, dial pulses from a subscriber’s telephone activate the switch. For example, if a subscriber dials a 3, three pulses from the subscriber subset are transmitted to the switch. The switch then steps to level 3 in the first relay bank. The second relay bank is now connected waiting for the second dialed digit. It accepts the second

digit from the subscriber and steps to its equivalent position and connects to the third relay bank and so on for four or seven dialed digits. Assume that a certain exchange only serves three-digit numbers. A dialed number happens to be 375 and will be stepped through three sets of banks of 10 steps each. This is conceptually illustrated in Figure 4.10.

The Crossbar Switch

Crossbar switching dates back to 1938 and reached a peak of installed lines in 1983. Its life had been extended by using stored program control (SPC)11 rather than hard-wire control in the more conventional crossbar configuration. The crossbar is actually a matrix switch used to establish the speech path. An electrical contact is made by actuating a horizontal and vertical relay. Consider the switching mechanism illustrated in Figure 4.11. To make contact at point B4 on the matrix, horizontal relay B and vertical relay 4 must close to establish the connection. Such closing is usually momentary, but sufficient to cause latching. Two forms of latching are found in crossbar practice: mechanical and electrical. The latch keeps the speech path connection until an “on-hook” condition occurs. Once the latching occurs, connection B4 is “busied out,”12 and the horizontal and vertical relays are freed-up to make other connections for other calls.

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