modulate or change the analog waveform at the rate of the binary data they receive from the terminal. Instead they encode a set of bits into a digital symbol that may get many more values than just two. Each multilevel symbol corresponds to a set of bits and it is sent as one analog waveform to the line. When receiving a certain analog signal on the other end, the receiver detects a set of bits defined to cor- respond to that signal. Use of more than two signals increases the data rate through the speech channel compared with the binary principle, in which only two different signals are used. Speech channels have quite a narrow bandwidth, but a good S/N, which allows use of many different signals, as we will explain in Chapter 4. When a digital network is used to transmit digital messages, signals are in digital form from end to end. Instead of a modem, a network terminal is needed at the subscriber's premises to encode binary signals into digital pulses suitable for cable transmission to an exchange site; see the ISDN example in Figure 3.5. Signals Carried over the Network 3.5 Analog Signals over Digital Networks In this section we look at how analog signals are handled before transmission through a digital network. In the next section we concentrate on the pulse code modulation, which is performed in the network on our voice during a telephone call, and in Section 3.7 we present a brief review of other voice- coding schemes. If a digital signal is to be transmitted through an analog network, it has to be converted into an analog signal suitable for the frequency band of the channel, as we saw in Figure 3.5. Digital networks provide communication only with a set of discrete symbols (in the binary case these symbols are called bits) at a certain data rate and the analog signal has to be converted into a series of these symbols for digital communication. The data rate of a digital network corresponds to the channel bandwidth of an analog network. The higher the data rate, the wider the required bandwidth and vice 