mplistically, we can say that transmission is responsible for the transport of the signal from user X to user Y. In the old days of telephony, these disciplines were separate with strong demarcation between one and the other. Not SO today. The demarcation line is fast disappearing. For example, under normal circumstances in the PSTN, a switch provides network timing that is vital for digital transmission. What we have been dealing with SO far is baseband transmission. This is the transmis- sion of a raw electrical signal described in Section 2.3.2. This type of baseband signal is very similar to the 1s and Os transmitted electrically from a PC. Another type of base- band signal is the alternating current derived from the mouthpiece of a telephone handset (subset). Here the alternating current is an electrical facsimile of the voice sound wave impinging on the telephone microphone. Baseband transmission can have severe distance limitations. We will find that the signal can only be transmitted SO far before being corrupted one way or another. For example, a voice signal transmitted from a standard telephone set over a fairly heavy copper wire pair (19 gauge) may reach a distant subset earpiece some 30 km or less distant before losing all intelligibility. This is because the signal strength is SO very low that it becomes inaudible. To overcome this distance limitation, we may turn to carrier or radio transmission. Both transmission types involve the generation and conditioning of a radio signal. Carrier transmission usually implies (not always) the use of a conductive medium such as wire pair, coaxial cable, or fiber-optic cable to carry a radio or light-derived signal. Radio transmission always implies radiation of the signal in the form of an electromagnetic wave. We listen to the radio or watch television. These are received and displayed or heard as the result the reception of radio signals. 2.4.2 Modulation At the transmitting side of a telecommunication link a radio carrier is generated. The carrier is charact