e following section we introduce the basic func- tions that are needed in all networks no matter what services they provide. The three technologies needed for communication through the net- work are (1) transmission, (2), switching, and (3) signaling. Each of these technologies requires specialists for their engineering, operation, and maintenance. 2.1.1 Transmission Transmission is the process of transporting information between end points of a system or a network. Transmission systems use four basic media for information transfer from one point to another: 1. Copper cables, such as those used in LANs and telephone sub- scriber lines; 2. Optical fiber cables, such as high-data-rate transmission in telecom- munications networks; 3. Radio waves, such as cellular telephones and satellite transmission; 4. Free-space optics, such as infrared remote controllers. In a telecommunications network, the transmission systems intercon- nect exchanges and, taken together, these transmission systems are called the transmission or transport network. Note that the number of speech channels (which is one measure of transmission capacity) needed between exchanges is much smaller than the number of subscribers because only a small fraction of them have calls connected at the same time. We discuss transmission in more detail in Chapter 4. 2.1.2 Switching In principle, all telephones could still be connected to each other by cables as they were in the very beginning of the history of telephony. However, as the number of telephones grew, operators soon noticed that it was necessary to switch signals from one wire to another. Then only a few cable connections were needed between exchanges because the number of simultaneously ongo- ing calls is much smaller than the number of telephones (Figure 2.1). The The Telecommunications Network: An Overview Connections to other areas Exchange (Switch) Exchange (Switch) Exchange Transmission (Switch) network Exchange (Switch) Figure 2.1 A basic telecommunications network. first switches were not a