ION AND SWITCHING: CORNERSTONES OF A NETWORK calling subscriber To called subscriber Bank 1. Bank 2. Bank 3. (Hundreds digit) (Tens digit) (Units digit) Figure 4.10 Conceptual operation of a step-by-step switch (exchange). 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. 4.3.5.3 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)¹ 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. Figure 4.11 The crossbar concept. 11 SPC, stored program control, simply means that the switch or exchange is computer-controlled. Of course, all modern digital switches are computer-controlled. 12 "Busy-out" means that a line or connection is taken out of the pool because it is busy, it is being used, and is not available for others to utilize. 4.3 INTRODUCTION TO SWITCHING 4.3.6 Common Control (Hard-Wired) First, we must distinguish common control from direct progressive control des