Mạng và viễn thông P38

Network Economy MeasuresPeople who plan telecommunications networks are always searching for equipment economy a network for a given information carrying capacity; or, measures: reducing cost the of conversely, increasing the information throughput of a fixed network resource. To achieve their aims, can they either maximize electrical the bandwidth available from a given physical transmission path,or (if it is the other kindof economy they want)they can reduce the amount of electrical bandwidth required to carry individual messages or connections. ...
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Mạng và viễn thông P38 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) 38 Network Economy Measures People who plan telecommunications networks are always searching for equipment economy measures: reducing cost the of a networkfor a given information carryingcapacity; or, conversely, increasing the information throughput of a fixed network resource. To achieve their aims, can they either maximize electrical the bandwidthavailable from a given physical transmission path,or (if it is the other kindof economy they want)they can reduce the amount of electrical bandwidth required to carry individual messages or connections. This chapterdescribes a few of the practical economy measures open to them.38.1 COST MINIMIZATION Reducing the cost of equipmentrequiredfora given informationthroughput is important for public and privatenetwork operators alike; both will be keen to toreduce the quantity and the cost of lineplant and switch gear. If a given resource, say a transmission link, already laid on then is there is not much to be gained by applying economy measures which have the sole effect of making some of the available capacity redundant. In such circumstances it may be advantageous to ‘squeeze’ extra capacity from the line, especiallyit is nearing its limit. This can be valu- if able for one three reasons; first, it enables expenditure morecapacity to be delayed; of on second, it may be the only practicable means; or third, the cost of duplication may be prohibitive. The first reason might postpone the need for a private networkoperator to lease more capacity from the PTO (public telecommunication operator). The second case might arise because of a need to make more telephone channels available from a limited radio bandwidth. The third might reflect a lack of resources to finance the pro- hibitive cost of a transatlantic undersea cable. Earlierchapters this in book havecovered important one means of lineplant economy, that of bandwidth multiplexing, by either the (analogue) frequency division multiplex ( F D M ) method, or the (digital) time division multiplex ( T O M ) method. This chapter briefly recapitulates these two methods, and goes on to describe some other 695696 NETWORK ECONOMY MEASURES important techniques including circuit multiplication equipment ( C M E ) , statistical multiplexing (used in data networks), speech interpolation, low rate encoding (LRE) and differential or adaptive differential PCM (DPCM and ADPCM).38.2 FREQUENCY DIVISION MULTIPLEXING (FDM) Frequency division multiplexing (FDM) provides a means of carrying more than one telecommunications channel over a single physical analogue bearer circuit,as Chapter 3 records. FDM relies on the carriage of a large electrical bandwidth over the circuit. Bandwidthforindividual telecommunicationschannels is made available by sub- division of the overall bandwidth, much as some main roads are marked out into a number of lanes. Standard large bandwidths are employed over the physical circuit. These are called groups, supergroups, hypergroups, etc. They are normally exact integer multiples of a base unit of 4 kHz, which is the nominal bandwidth required for a single telephone circuit. As we may recall from the example of Figure 38.1, a single four-wire circuit and a pair of channel translating equipments (CTE) enable us to derive 12 telephone channels between the end points A and B. This compares with the 12 individually wired tele- phone circuits which might otherwise be required. In much the same way as 4 kHz bandwidths (individual telephone channels) can be multiplexed by CTE to form an FDM group, so FDM groups can be multiplexed by group translating equipment (GTE) formsupergroups, and supergroups can be formed to into hypergroups by STE. As we alsorecall(from Chapter 33), lineplantsavingsarepossible by making connections out of any number of segments, each composed of a channel or circuit drawn from a number of different cables. This can save the need to lay a new dire ...