ansmissions ranges from 300 Hz to 3,400 Hz, and voice trans- mitted using this range is perfectly understandable by the recipient. The main advantage of analog transmission is the absence of delay. Because switches in the transmission path do not add delays to the signal, transmission speed is effectively the speed of the electromagnetic wave in the medium. This feature suits interactive communications-where the parties involved are active senders and receivers-exceptionally well. Digital Transmission By now you've probably noted a pattern developing whereby advances in the user plane prompt advances in the signalling plane to take advantage of them, in turn re-enabling the user plane for growth. The next step in tele- phony evolution was accordingly the introduction of digital transmission. Digital transmission changed the way voice was transported and, in SO doing, made signalling protocols evolve dramatically. Just how did this work? We explained earlier that analog signals take on continuous values. Conversely, digital signals take on discrete values. A good example of a digital signal is text. The text is made of letters, and since every letter is chosen from a set of letters-the alphabet-so, the discrete values that any letter can take range from "a" to "Z." Just two values are used in voice digital transmissions: 0 and 1. There- fore, the information exchanged between systems consists of a stream of 0s and 1s. Several ways exist for encoding this kind of stream on the medium being used. One way, among many, is to assign a constant voltage level to 1 and a different voltage level to 0. The receiver can recover the original dig- ital stream by analyzing the voltage levels received. Chapter 1 However, voice is not a digital signal. It is an analog acoustic signal that takes on continuous values in the time domain and requires translation before it can be transported. In order to transport an analog signal over a digital link, a coder-decoder is needed. A coder receives an analog signal as input and produces a