s of switching centers. We understand how a person's speech gets from one place to another using the phone system. Before we move on, I want to discuss what signals are sent from one switch- ing center to another. Chapter Two A telephone call starts out with the phone in your hand. That call goes to a Class 5 switching center. If the call is local, it goes from that Class 5 switching center right to the person you're talking to. If the call is not local, the Class 5 switching center puts the call together with other long-distance calls, and sends them together to a Class 4 switching center. Let's look at the signal created at a Class 5 switching center that is sent to a Class 4 switching center. Class 5 to Class 4 The Class 5 switching center gets the call you place. It also gets a lot of other calls from your neighborhood. When the Class 5 switching center realizes there are a bunch of calls that are not in its area (and instead are long-distance calls), it puts these signals together and sends them out. Specifically, it puts the signals together as shown in Figure 2.12: Class 5 Switching Center Your call T=1/8000=0.125ms 8000 samples/sec Switch To class 4 Forces each amplitude to the nearest one out of 256 permitted output amplitudes Piece from line #2 Piece from Piece from line #1 line #24 0.125ms Figure 2.12 The signal created at a Class 5 switching center (headed to Class 4) Telecommunication Networks 1. First, at the line marked #1, is your call. But that is only one thing coming into the Class 5 switching center. The center takes your call and at the same time takes 23 others, for a total of 24 calls. Those calls incoming to the Class 5 switch- ing center are the lines marked #1 to #24. 2. The switching center puts these 24 calls on a single line using TDM. That idea is explained in Section 2.1.3, but here are some more details explaining exactly what the Class 5 switching center does. 2a. Each voice call is sampled. Specifically, samples of each voice call are taken at a rate of 8000 samples/second (i.e.,