tailieunhanh - Mạng và viễn thông P8

The basic line transmission theory that we have considered so far, together with theprinciples of analogue and digital signal transmission over pairs of electrical wires, is quite suitable for short range conveyance of a small number of circuits. However, it is not always practical or economic to use many multiple numbers of physical ‘pairs’ between exchanges, so we have recourse to frequency division and time division multiplexing reduce the numberof physical pairs of wires to needed to convey a large number of long haul circuits between common end-points | Networks and Telecommunications Design and Operation Second Edition. Martin P. Clark Copyright 1991 1997 John Wiley Sons Ltd ISBNs 0-471-97346-7 Hardback 0-470-84158-3 Electronic 8 Transmission Systems The basic line transmission theory that we have considered so far together with the principles of analogue and digital signal transmission over pairs of electrical wires is quite suitable for short range conveyance of a small number of circuits. However it is not always practical or economic to use many multiple numbers of physical pairs between exchanges so we have recourse to frequency division and time division multiplexing to reduce the number of physical pairs of wires needed to convey a large number of long haul circuits between common end-points. There are of course different transmission media to be considered some of them much better suited to particular applications than plain electric wire. They include coaxial cable radio satellites and optical fibre. In this chapter we discuss the main features of each of these transmission types to ease the decision about which one is best suited to any particular application. AUDIO CIRCUITS The two-wire and four-wire transmission systems described in the first chapters of this book comprising respectively one or two physical pairs of copper or aluminium wires together with amplifiers and equalizers as appropriate are correctly called audio circuits. In actual use the wires themselves are often wound or twisted around each other inside the cable hence the terms twisted pairs and twisted pair cables . The title audio or baseband circuit is given because the frequencies of the electrical signals carried by the circuit are a direct match with those of the audio signal they represent. In other words the electrical signal frequencies are in the audio range the signal has not been processed in any way neither frequency-shifted nor multiplexed. As a reminder of a typical audio circuit configuration Figure shows a .