Mạng và viễn thông P2

Introduction to Signal Transmission and the Basic Line CircuitTo make it suitable for carriage over most telecommunications networks, information must first be encoded in an electrical manner, as anelectrical signal. Only such signalscan be conveyed over the wires and exchanges that comprise ‘transport the mechanism’ of telecommunications networks.Overtheyears,avarietyofdifferentmethodshave been developed forencoding different types of information.
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Mạng và viễn thông P2 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) Introduction to Signal Transmission and the Basic Line Circuit To make it suitable for carriage over most telecommunications networks, information must first be encoded in an electrical manner, as anelectrical signal. Only such signalscan be conveyed over the wires and exchanges that comprise ‘transport the mechanism’ of telecommunications networks.Overtheyears,avarietyofdifferentmethodshave been developed forencoding different types of information. This chaptercategorizes all methods of information transmission intoone oftwobasictypes,analoguetransmissionanddigitaltransmission,describingthe principles of both types but concentrating on the technique of analogue transmission. Chapter3 continues the subject of analogue transmission, explaining its use for information conveyance over long distances, and Chapters 4 and 5 expand the details of digital transmission.2.1 ANALOGUE AND DIGITAL TRANSMISSION The simplest type of physical medium (transport mechanism) used in telecommunica- tions networks is a pair of electrical wires. These allow the conveyance of an electrical current or signal from a transmitter at one end of the wires to the receiver at the other end. There are two basic methods for information encoding which may be applied by the transmitteranddecoded by the receiver. These areanalogue signal encoding and transmission, and digital encoding and transmission. As the name suggests, analogue signal encodinginvolves the creation of an electrical signal waveform which is analogous to the waveform of the original information (e.g. a speech wave pattern). Inthis way the electrical signal is similar in shape to thespeech or other information waveform that it represents. Analogue transmission lines are used to convey analogue encoded signals and, until the early 1980s, predominated as the links of the world’s telecommunications networks. Analogue excllunges have provided the nodalinterconnections between these links. Digital encoding and transmission of information (a technique in which information is conveyed as a string of digits, or more 1718 INTRODUCTION SIGNAL TO TRANSITION AND THE BASIC CIRCUIT LINE Electrical I ( aT y p i c a l n a l o g u s i g n a l ) a e Electrical current I ( b ) T y p i c ad i g i t as i g n a l l l Figure 2.1 Typical analogue and digital systems precisely, pulses of ‘on’ and ‘off electrical current) is rapidly taking over as the main method of telecommunications transmission. This is because of the significant benefits that digital transmission offers, in terms of performance and cost. Figure2.1 illustrates typical analogue and digital signal waveforms. As for the transmission line systems themselves, Table 2.1 presents a short summary of some of the physical mediawhich may be used,and compares theirrelative merits. In general any physical medium may be designed to work in either analogue or digital mode, and each of the different transmission media listed in Table 2.1 has been used as the basis of both analogue anddigital transmission systems, though digital systems tend today to be the system of choice. Choosing exactly which medium and encoding system should be used is a decision for the network planner, who needs to consider the relative costs and the ease with which any new type of line system could be incorporated into the existing network. We shall discuss the relative benefits of different transmission media in more detail in Chapter 8. Switchingequipment and exchanges arealso classified into analogue and digital types. A switch, is after all, only a specialized and very flexible form of transmission equipment. To develop our understanding of information encoding, we now discuss the various techniques in chronological order.TELEGRAPHY 19 Table 2.1 Comparison of common transmission media Transmission medium usedComments of line assystem basisCopper wire Ordinary wire. The gauge (or thickness) will be carefully selected for the(or aluminium wire) particular use. The pairsof wires may be either ‘parallel’ or ‘twisted’ within the ‘cable’, which usually comprises many pairs. Copper and aluminium is the wire predominant system used for the ‘local loop network’.Coaxial cable Centre conductor plus cylindrical conducting sheath. Used on lon ...