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Introduction 9

readers may still remember the rotary telephone sets, in which the impulses were created mechanically by the spin of the rotor. The situation changed com-pletely with the entry of Computer technology into telecommunications. The microchip utilized by telecommunications opens today, at the end of the twentieth century, a multitude of new signaling functionality, which were unthinkable even 20 years ago. Computers are the backbone of modern telecommunications systems.

This new technology makes mobile Communications possible in the first place. The signaling reąuirements of modern mobile systems are so vast that the former technology would not have been able to manage them. Computeriza-tion, however, did not change much of the principle. As in the old days, electri-cal or optical signals are sent, over an appropriate medium (typically serially) and interpreted by the receiver. What did change is the speed of the transmis-sion. The progress in this area has been exponential.

The smallest unit of a signal is called a bit and can, for example, be repre-sented by an electric voltage, which a receiver can measure during a specified period of time. If the receiver measures the voltage as “Iow” over the specified time period, it interprets the value as a 0. If the voltage is “high,” the receiver interprets the value as a 1. It does not matter which level represents which value, so long as both the sender and the receiver agree on which is which.

A seąuence of bits allows the coding and sending of complex messages, which, in turn, allows a process to be controlled or information to be conveyed. The result is a bit stream, as shown in Figurę 1.3.

Pulse codę modulation (PCM) is the worldwide process for transmission of digital signals. PCM is used to transmit both signaling data and payload. PCM is categorized into hierarchies, depending on the transmission ratę. The PCM link of 2 megabits per second (Mbps) (one that is referred to freąuently in this book) is only one variant of many. By utilizing a time-division

. bit value = 1 U(high) > U(low) . bit value = 0 U(low) < U(high)

Figurę 1.3 Decoding of a bit stream.