 at 8000 Hz) That means that each sample taken from a voice call lasts a total time of T = 1/8000 = 0.125 ms. 2b. Each sampled voice signal meets "the big switch." The big switch makes contact with each digital voice signal briefly once every 0.125 ms. As a result, the signal that ends up on the wire following "the big switch" is a collection of all 24 voice signals. This is shown in Figure 2.12 better than my words can explain. 2c. On the wire following the big switch, we have all 24 voice samples smooshed together in the time T = 0.125 ms. The number of samples in each second is now 24 X 8,000 samples/second = 192,000 samples/second 3. The 192,000 samples/second on our wire now enter through a device called a quantizer. The quantizer simply changes the amplitude of the incoming samples. Each incoming sample, which has some amplitude A, is mapped to an outgoing sample whose amplitude is one of 256 possible values. 4. Each sample, with an amplitude that is now one of 256 levels, can be fully represented by a set of 8 bits. (This is because with 8 bits we can represent all integers between 1 and 256.) A device called a symbol-to-bit mapper takes each sample with one of 256 possible amplitudes, and represents it with 8 bits. While before we had 24 samples in each 0.125 ms, we now have 24 X 8 = 192 bits in each 0.125 ms. 5. To tell people where each set of 192 bits begins and ends, an extra bit (a 0) is squeezed in SO that we now have 193 bits in each 0.125 ms. That means we have a bit rate of 193 bits/0.125 ms = 1.544 Mb/s. These bits, with a bit rate of 1.544 Mb/s, are sent from the Class 5 switching center to the Class 4 switching center. The signal sent between the Class 5 and Class 4 switching centers is named DS-1. The wire which connects the Class 5 to the Class 4 switching center is called a trunk line, specifically a T-1 trunk line. Chapter Two Other Signals between Switching Centers Figure 2.13 shows the different signals sent between different switching centers. Coming into a Class 4 center is a DS-1 sign