ansion 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. 4.3.5 Early Automatic Switching Systems 4.3.5.1 Objective. 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. 4.3.5.2 The Step-by-Step Switch. The step-by-step (SXS) switch use was wide- spread 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 electromechani- cal 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 trans- mitted 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 TRANSMISS