Mạng và viễn thông P41

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Building, Extending and Replacing Networks It is rare to come across a telecommunications network that is not in a state of continuous evolution. At its simplest, a network could be expanding simply to cope with increased demand. In addition the network may be expected at the same time to provide increasingly sophisticated telecommunications services.

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  1. 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) PART 6 SETTING UP NETWORKS
  2. 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) 41 Building, Extending and Replacing Networks It is rare to come across a telecommunications network that is not in a state of continuous evolution. At its simplest, a network could be expanding simply to cope with increased demand. In addition the network may be expected at the same time to provide increasingly sophisticated telecommunications services. For example,sincethemid-l980s,many of the world’s public telecommunications operators (PTOs) have responded to customer demand by the introduction of a‘freephone service’ offering the facility for callrecipients to payforthe calls. As an introduction to the subject of network evolution which the operator must be able to cope with, this chapterdescribes how networks maybe built or extended to meet capacity andservice needs. Particular attentionis paid to the design factors inherentin the equipment orderingprocess; these are crucial to the success of a network evolution plan. In addition, and because it is not always possible to achieve the evolution without the entire replacement of the network and equipment, the chapter describes various methods by which network modernization can be achieved without a major disturbance or interruption to the established service. 41.1 MATCHING NETWORKCAPACITY TO FORECAST DEMAND No matter what type of telecommunications service is provided,networkcosts are minimized by matchingcapacitytothedemand.Bothover-provisionandunder- provisionincreasecosts.Whenequipment is providedbeforeit is actuallyneeded, higher capital expenditure is incurred at an earlier date. In addition, there are higher running costs associated with equipment maintenance, accommodation, staffing, etc. What is more, when the equipmentfinally comes into use it may already be obsolescent and in need of early replacement. Under-provision, which some network operators have to put up with through lack of capital, is also expensive, because of the work needed to relieve congestion and to maintain overloaded equipment. Furthermore, higher costs are incurredin rearranging the network to incorporate exchanges, as a result of the new lack of network ‘manoeuvring room’. 743
  3. 144 BUILDING, EXTENDING AND REPLACING NETWORKS Figure 41.1 illustrates a graph of forecast demand, and showsatypicalnetwork operator’s equipment provision schedule, which in ‘steps’ of extracapacity are provided to meet the projected demand. The demand may represent the total (local and long distance) traffic originated in a given area, in which case each ‘step’ on the capacity profile could represent the provision of a local exchange extension, or of a new local exchange. Alternatively, the demand curve could correspond to a particular type of traffic. One example might be the trunk or international traffic generated by a number of localexchanges,coveringawide area.The steps of capacityshownin Figure 41.1 might then correspond to new trunk or international exchanges. Another example might be a case where the demand predicted in Figure 41.1 is the forecast demand for a new telecommunications service, for which some special new equipment will berequired.Thestepsthenrelatetothenecessaryprovisionschedule of that equipment. It is seldom possible in large public networks to match the fitted capacity exactly to the demand, because the practicalities of exchange provision or extension usually allow capacity to be added economically only in fairly large chunks. It is therefore normal to keep the ‘steps’ of the fitted capacity graph above the demand, to ensure that the new equipment is on hand before the demand seems likely to exceed capacity. This reduces the risk of under-capacity. Where a network has slipped in to under-capacity, new exchanges often become congested on the day they are opened, caused by the fact that forecasters tend to underestimate the amount suppressed traffic in congested networks. By contrast, in corporate networks and networks of smaller service providers it may be possible to match the number of ports almost exactly to the growth in customer Demand and fittedcapacity (erlangs, or customer correcttons. or equivalent measures ) Fitted v Demand 7 m Years I I I I I I I 1988 1987 1986 1989 1990 1991 1992 1993 Figure 41.1 Forecast demand and fitted capacity
  4. ORECAST CAPACITY MAND TO NETWORK MATCHING 745 demand, by quoting a lead time for connection of new customers to the network which exactly matches the delivery time of a new portcardfromthenetwork switch manufacturer. This is true just-in-time ( J Z T ) provision of the network. The forecast of demand is worked out as described in Chapter 3 1. For the purposes of exchange provision, demand the is normally quoted terms in of the highest instantaneousnetworkthroughputthat is required. In circuit switched terms, the important parameters are the traffic intensity (i.e. the busy hour traffic in Erlangs) and the number of customer connections required. In data networks, the equivalent of the traffic intensity is the percentage trunk loading and/or the volume of data in segments to be carried per hour. Forecastsformadirecttoolfordeterminingtheforwardnetworkprovisioning schedules, but to be useful the parameters to be forecast need first to be thought out carefully. Before makinganyforecast,thenetworkplanner needs to determinethe geographicalarea which the traffic forecast is intended to cover, andthe type or destination of traffic (data, voice, video, local, trunk, international) which any new switches (i.e. exchanges) will carry. In other words, he must have some idea of what type of networktopology will be employed, and howparticular services will be supported. This depends on a number of factors 0 terrain conditions 0 density of customers and/or ports orend user devices (telephones or data terminals) 0 volume of traffic generated by each customer or user device 0 volume and proportion of traffic to other local, trunk or international destinations 0 the established network (if any), its capability for extension, its suitability for the support of any new services (if required),itsstate of repair,and its degree of obsolence 0 the reliability, optimum size, and service capabilities of contemporary equipment which might be used to extend or replace the established network Designing a new network from scratch, to serve an area which has previously been ignored, has the advantage of allowing a ‘clean slate’ approach, but it is likely to be constrained by the amount of capital available. This may well limit the initial network design to choosing the optimum locations for such exchanges as can be purchased, and of deciding on each exchange’s service or ‘catchment’ area. This taskis best carried out by use of a map of the area to be served, marking it up to show the density of traffic likely to be originated and terminated in each spot. In areas that generate low volumes of traffic (e.g. rural areas or the branch offices of a corporate network it may prove economic to provide a medium size exchange to cover a very wide area or a number of corporate offices, but in extreme conditions it is usually efficient to employalarge number of smaller exchanges. The two diagrams in Figure 41.2 demonstrate the trade- off of exchange equipment against lineplant. In Figure 41.2(a) only one exchange is used, and each customer is connected to it by direct lines. Figure 41.2(6) shows an alternativenetwork using smaller exchanges and itincursgreatercost in switching equipment, but the overall lineplant requirements and costs are reduced.
  5. 146 EXTENDING BUILDING, AND REPLACING NETWORKS 1 I I I - exchange X - customerstation C - central exchange ; also carries transit(or ‘trunk’) traffic. ( a ) One largeexchange ( b ) Nine smaller exchanges coveringlargearea covering thesame area Figure 41.2 The lineplant versus switch equipment trade-off. 0, exchange; X, customer station; C, central exchange; also carries transit (or ‘trunk’) traffic Option ( a ) will generally be appropriate for urban areas, and areas where lineplant can beprovided relatively easily and relatively cheaply. Option (b) might be more appropriate as a means of serving ‘pockets’ of remote network customers, particularly where the terrain makes the provision of lineplant difficult. An interesting feature of option (b) is the emergence of ahierarchicalnetworkstructure,wherethecentral exchange has taken on the role of transit switching between all the other exchanges. This typical occurrence in real networks was discussed more fully in Chapter 32. For both networks shown in Figure 41.2, it is necessary for the network operator to forecast the future demand from customers, not only in terms of the traffic in Erlangs (or equivalent), but also in terms of the total number demanding an exchange con- nection. Shortage of either type of capacity may lead to customer dissatisfaction and loss of business. Shortage of customer connections means new customers cannot be that connected. Shortage of busy hour throughput (i.e. Erlang)capacitymeans that the needs of the established customers are not met. Graphs may thus be drawn, similarly to Figure 41 .l, for both fitted traffic-carryingcapacityinErlangs and the number of customer connections. Such a graph enables the network planner to decide on the dates and sizes of future exchange extensions, or new exchange provisions. In this way the capacity may be matched to demand. If the rate of growth in demand is slow, then small exchange extensions may be a good way of increasing the capacity. However, cases when the growth is very rapid, it in may be more appropriate to provide completely new exchanges, with large capacities. In addition, during rapid growth the provision of entirely new exchanges presents an ideal opportunity for adjusting the network topology, perhaps splitting an exchange area into two parts,with one area to be served solely by the new exchange while the old
  6. FORECAST CAPACITY MATCHING NETWORK TO DEMAND 747 Old catchment area o exchange A f l Newcatchmentarea of exchange A Catchmentarea of new exchange B X - Customerstatlon Figure 41.3 Splitting a localexchangearea exchange continues to serve the remaining area. Figure 41.3 illustrates the splitting up of an established local exchange area into two parts. Exchange A is the established exchange. Forecast growth in connections at a new town in the east of the catchment area will exhaust the capacityof exchange A, and so it has been decided to locate a new exchange in the new town (at B), and split the exchange area accordingly. This split means that the new exchange will cater for any further growth the region around the in new town, while the offload of traffic from exchange A onto thenew exchange, will also allow for further growth in traffic in the western area. This traffic will be served by exchange A. The decision to split the area in this caseis quite straightforward, because a large quantity of new local line wiring will be needed and can be established at the new exchange B. However, in the case in which all thewiring is already centredon exchange A, the economics would probably have forcedan extension there. Going back to Figure 41.2, option (b) can be used to demonstrate a further point. A forecast of the exchange connections and busy hour Erlangs must be drawn up for each of the exchanges shown in option (b), in the same way as already described. In addition, in the caseof the central exchangeC, a forecast is also required for the transit (or trunk) traffic to be carried by this exchange. This is the traffic between different outlying small exchanges which is routed via exchange C. In practice, a forecast will also be required for the traffic which exchange C will need to carry between any of the exchangesshown and to-and-from other geographicareas, not illustratedinFigure 41.2. In effect, just as local exchanges may be extended or may have their catchment areas adjusted, so may trunk and international exchanges (like exchange C ) . Take as an example a single international exchange which is used initially to serve the international traffic needs of an entirecountry. Traffic growthmay be such as to exhaust the capacityof the exchange, necessitating the provision of a new one. One way of reconfiguring the network in these circumstances would be to offload some of the overseasroutes,corresponding tothose overseascountries which have the largest volumes of traffic. The established exchange will thereby gain growth capacity (resulting from the offload), whereas thenew exchange will serve and provide growth capacity for
  7. 748 EXTENDING BUILDING, AND REPLACING NETWORKS International gateway exchanges * (Old) - * Largenumber smalltraffic to of countries, each Small number of countries, (New) o f f -Loadedfrom old exchange, eachwith Large trafficvolume Figure 41.4 Reconfiguring internationalexchanges a small number of heavy traffic routes. Figure 41.4 illustrates this example. Similar networkreconfigurationstothoseshowninFigures41.3and 41.4 mightalso be appropriate for trunk or other transit exchanges. 41.2 OTHER FACTORS AFFECTING THENEED FOR NEW EXCHANGES Apart from a straightforward increase in demand, there are a number of other factors which a network operating company may take into accountwhen deciding thebest time to replace exchanges. These are 0 the age and lifetime of the existing equipment (hardware and software) 0 the obsolescence (or continuing usefulness) of the existing equipment 0 the comparative running costs of existing and replacement equipment 0 the new service (and retention of old services) demands of customers Historically, electromechanical telephone exchanges were planned and operated on the basis of a 20 or 25 year lifetime. Over this period, the basic technology did not change very much, and the eventual need for replacement was governed by the increasing wear, and the consequent increase in running costs associated with maintenance, spares and labour. Modern computer-controlled equipment has a much shorter lifetime, perhaps as little as 3-7 years. The problem is not the reliability and wear of theelectronic components, but the increasing rate of change of technology, and customer service expectations. Equipment lifetimes are becoming shorter, as the result of more rapid equipment obsolescence. An exchange might have plenty of copper wire termination portsbut these cannot be used to terminate fibre lineplant unless converters are installed. This reducesthe useful capacity of theexchange. In asimilar way, older signalling system ports (forexamplepre-digitalones)becomeobsolescent and also reduce exchange capacity. Finally, equipment software needs occasional upgrading to
  8. FACTORS IN DETERMINING AN EXCHANGE PROVISION PROGRAMME 749 provideeither capability the for new services or updating the of maintenance capabilities. Without upgrade, certain categories of traffic may need to be diverted to newer exchanges for processing. With older exchanges the running costs alone may render them uneconomic. Many of the world’s public telecommunications operators have been modernizing networks their prior to the originally intended life-end of the equipment. The modernizationis carried out to reduce ongoing running costs, particularly manpower terms brought about by in the use of digital (as opposed to analogue) transmission and stored program control (SPC) exchanges. The new wave of modernisation following the digitalization which took place in the 1980sis the introduction of intelligent networks (Chapter 11) and sophisticated network management controls (Chapters 27 and 37). 41.3 FACTORS IN DETERMINING EXCHANGE AN PROVISIONPROGRAMME Figure 41.5 shows a similar demand curve to that in Figure 41.1, but in this case the diagram takes accountof not only the need to match capacity to the forecast growth in demand, but also the decay in the effective capacity of the existing equipment. The decline in effective capacitycomesabout becauseofthe gradual obsolescenceof equipment and the eventual need for the replacement of life-expired exchanges. Over the period up until 1990 the forecast is that the effective capacity of theexisting system in Figure 41.5 will diminish due to the redundancy of equipment, in itself the result of obsolescence. Over this period thenew exchanges, (l), (2) and (3), are required to handle the forecast growth in traffic, and to make good the shortfall which would otherwise arise as a result of the reducing effective capacity of existing equipment. The diagram showsthe planned ‘changeover’ replacement of exchange(1) after a seven year lifetime. Diagrams similar to that shown Figure 41.5 are invaluable toolin determ- in an ining futureequipmentprovisionprogrammes.Theexampleshown in Figure 41.5 demands a forward provision programme as shown in Table 41.1. Demand A Existing capacity (fall-off on / effective capacity, due to obsolescence) 1986 1987 1988 1989 1990 1991 1992 1993 Figure 41.5 An exchange provision programme
  9. 750 EXTENDING RIJILDING. AND REPLACING NETWORKS Table 41.1 Exchange provision programme New exchange no. Date of introduction 1986 1987 1988 1990 1991 1993 Any provision programme is critically dependent on the closure dates chosen for the older units, and also on the installed size of the new units. Counteracting some of the disadvantages of over-provision (discussed earlier in the chapter), the provision of larger units in advance of demand reduces the frequency of exchange extensions and new exchange provisions, and it brings benefits in the way of reducing and simplifying the installation work programme. Theuse of larger exchanges also tends to simplify the job of inter-exchange network and traffic design (for example in Figure 41.2 no inter-exchange network is required for option (a), whereas a relatively complicatednetwork is required tointerconnectthenineexchanges of option (b)). On the other hand, theuse of a larger number of small units may be the best answer in difficult terrain, orin cases where there an established workforce, network or building is already available. In the end the overall exchange provision a compromise between a is number of constraints. 41.4 DETERMINING A STRATEGYFORNETWORK EVOLUTION The efficiency and reliability of a network ultimately depend on the expertise of its designer, and there is no substitute for experienceindevelopingnetworkevolution schemes tailored to particular circumstances. Nonetheless, it is helpful to have a system for selecting the best available evolution scheme when there is more than one to choose from. A systematic approach also guards against the accidental oversight of crucial considerations.Thefactorslistedbelowset out aframeworkfordeterminingand evaluatingalternativenetworkevolutionschemes,and by usingthemthenetwork planner can devise a forward strategy for evolution and development. Capacity exhaustion date This provides the ‘backstop’ date, by which new capacity must have been provided. Some evolution schemes may be precluded if they do not provide extra capacity early enough. Short-term network constraints It may be impossible to avoid the provision of further, alreadyobsolescent, equipment to to meet a short term peak in requirements (for example, a new international exchange may need to include a few lines using an archaic signalling system, to meet growth to a
  10. DETERMINING A STRATEGY EVOLUTION NETWORKFOR 751 particular country, prior to modernization of equipment in that country). It no good is buying such modern equipment that it will not interwork with the existing network. Some evolution schemes or particular types of equipment may therefore be precluded. Objective long-term network topology Over the lifetime of any exchange, the network will constantly be evolving towards the goal of the long term strategy. A good network planner will optimize the procurement date and design of the exchange to maximize the value of the exchange throughout its life, and ensure its compatibility with the long term objectives. Evaluation o overall costs f The totallifetime costs of an exchange (or of complete exchange provision programme) a include not only the capital costs associated with the provision equipment, the of the net- work, the accommodation, the peripherals, the training, the documentation and the spares, but alsotheongoingrunningcostsassociated with equipment maintenance, rates, bills, electricity, staff costs, etc. Although onetype of equipment maybe cheaper to buy initially, it may be significantly more expensive in running costs. A discounted cash flow ( D C F ) analysis, to determine the net present value ( N P V ) (also called the present worth) costs of the exchange, or of the entire provision programmeis often carried out. The example in Table 41.2 compares the net present value of providing a single large exchange of one equipmenttype, against an alternative strategy of purchasing a different manufacturer’s exchange equipment, which allows some of the capital outlay to be deferred by phasing the overall provision capacity. The equipment capital costs shown in Table 41.2 compare the provision of a large unit costingE10 million in one go, with the provision of an initially smaller unit which is extended twice, and costs E12 million in total. The running costs (maintenance and manpower)assumed to be 5 % per annum are of the equipment value (a typical assumption). The accommodation costs are common of to both cases, and include E1 million initial purchase and fitting out the building, plus EO.l million per annum rent and rates, etc. It is interesting to see from the full-life cost analysis shownin Table 41.2, how, despite the fact that theoverall expenditureis cheaper for option 1 than for option 2 (515.2 million as against E16.7 million), it is nonetheless cheaper in present value terms to adopt option 2 (E13.45 million compared with i14.13 million). The reduced present value results from the deferred outlay. In the early years, more money is therefore ‘left in the bank’ (we may think of it as earning interest). From Table 41.2theplannercouldequallydecidethat E0.68 million was cheap insurance against unplanned demand. In either case a full costanalysis would also includethecost of transmission plant provisions and rearrangements; these can be much greater than the switching costs. In a private network, where line capacity is leased from the public telecommunica- tions network operator, it is easy to match lineplant to demand: pay as you need. The projile o installation work f The profile of installation work may be an important factor, because it may either be impossible for the chosen manufacturer to deliver equipment at a very fast rate (for example,rapid a and large-scale networkmodernization require may very large
  11. 752 BUILDING, EXTENDING AND REPLACING NETWORKS Table 41.2 Discountedcash flow analysis Year costscapital Equipment 10 - - - - - - 10 Running costs 0.5 0.5 0.5 0.5 0.5 3.5 0.5 0.5 Other costs (accommodation etc.) 1.1 0.1 0.1 0.1 0.1 0.1 0.1 1.7 Total cost in year 0.6 0.6 0.6 11.6 0.6 0.6 0.6 15.2 Present value of cost discount) (10% 11.6 0.54 0.350.54 0.44 0.49 0.39 14.13 Year capital Equipment costs 4 - 4 - 4 - - 12 0.2 costs Running 0.2 0.4 0.4 0.6 0.6 0.6 3 (accommodation costs etc.) Other 0.1 0.1 1.1 0.1 0.1 0.1 0.1 1.7 year Total cost in 0.3 5.3 16.7 0.7 0.5 4.5 0.7 4.7 Present value of cost (10% discount) 5.3 0.27 0.36 3.65 3.080.37 0.41 13.45 resources beyond the means of some suppliers). It may also prove impossible for the installation workforce to match a stop-start programme. It is much better, once an installation team has been established, to maintain as steady a workload as possible. Recruitment o maintenance s t a f f Consideration should be given to the maintenance of both old and new exchanges. Transferring and retraining staff from old units onto new ones may help to reduce recruitment difficulties, but the ability to do so will depend on the relative geographical locations of the two units and the skills match of the staff involved. Associated network changes The introductionof a new exchange to support a new service, or as part a technology of modernization strategy, may have to be coordinated with other network rearrange- ments, with a corporate reorganization, with public announcements, or with changes in the law. Ongoing network administration One provision strategy may lead to much easier ongoing network administration than another.Perhapsonestrategy allows centralized operations (all maintenance staff
  12. NETWORKSTRATEGY A DETERMINING FOR EVOLUTION 753 performing remote maintenance from a single central location). One scheme may ease the task of monitoring ongoing network congestion and quality performance. Another might ease the task of network design, reducing the planning manpower that is needed. Finally, one strategy might leave the traffic less prone to network failure, so that less traffic would be lost in the event of a single exchange or transmission link failure. Available exchange size Exchange costs are often ‘front-loaded’.(By this expression we refer to the fact that you need to pay for a building, a maintenance staff and a central processor for an SPC exchange, no matter what its size.) Per unit of capacity,larger exchanges therefore tend to becheaper.Thediscountsavailablefromsuppliersforbiggerordersarealso generally more favourable. Commercial conditions for equipment provision Open tenders for equipment, sent to a number of potential suppliers, will often attract lower prices, becausecompetingequipmentprovidersareeager to gainmaximum business. To allow open tendering,the requirements for the equipment should kept as be open as possible. Non-competitive tenders, for the extension of existing equipment by theoriginal supplier,should avoided be where possible. Where extension an is anticipated, it is better to make this part of the initial tender and contract rather than negotiate it after the first installation has been made. When one supplier is ruled out, either by the stringency of the requirements or because of equipment being extended, there is little incentive forthis supplier to offer competitive prices. Somenetwork operating companiesdeliberately buy equipment of two compatible makes. Even though in the very short term this is clearly not the cheapest option, it protects them from becoming wholly dependent on either of the individual manufacturers. (We discuss the commercialconsiderationsforcontractplacement in Chapter 42 on ‘selecting and procuring equipment’.) Equipment provision lead times There is a finite period of time required to develop new software for exchanges, to manufacture the hardware, and to plan, install and test the equipment. This time is called the equipment lead-time, and must be taken into account when determining the exchange (or other equipment) provision programme. Indeed, some manufacturers may rule themselves out of possible consideration for a contract because the lead-time that they are able to tender is not short enough. Insufficient respect for the lead-time, or over-optimistic estimation of development timescales leads to delayed opening dates, and almost inevitably to network congestion or some other problem. Suppressed demand Afactor sometimes worthy consideration, of when injecting new capacity into congested networks, is the quantity and likely effect of any suppressed traffic demand. In particular,will an unexpected heavy load present any problems to the exchange, new and have sufficient tests been planned?
  13. 754 BUILDING, EXTENDING AND REPLACING NETWORKS 41.5 COMPARISON OF STRATEGY OPTIONS When planning the evolution of networks, it is usually best to compare a number of differentprovisionstrategies,evaluatingthedifferences(asoutlined in theprevious section) between perhaps 0 a strategy relying on the extension of existing exchanges (i.e. switches) 0 a strategy adding new exchanges 0 astrategyreplacingallexchanges 0 a long term strategy for a small number of large exchanges 0 a long term strategy for a large number of small, geographically dispersed exchanges The options used in the evaluation should be restricted to a small number of practical options (say three or four) and compared on as many factors as possible. A complete long term provision programme should always be worked out, even though it may be necessary, in the short term, only to commit expenditure to a single new exchange or transmission line system. This ensures that consideration has been to how the new given item of equipment operate in the prevailing network will throughout itslife. The analysis will finally lead to a decision about the exchange size, location, in-service date, service requirements, and possibly which manufacturer is to be asked to provide it. A similar analysis can be used to determine the required-by date and capacity for a new line system. 41.6 EXCHANGE DESIGNAND SPECIFICATION Having decided on the size, location and in-service date for anew exchange (i.e. switch) or exchange extension, next comes the taskof mapping out the design of the exchange itself.This is usually conducted in two phases. First an outline design sets out the functions and overall requirements, leading to ultimate more an and detailed specification. The specification must list the exact functions or services to be provided, defining the required software capabilities and performance requirements. It must also include signalling the systems to be provided, routing number the and analysis capabilitiesrequired, billing charging the and featuresneeded, the and expected maintenance and network management features, etc. These are all features without which the exchange could not operate, either in isolation or within the network. In addition, the specification needs to list any other interfaces to be provided so that the exchange can be interconnected to, and interwork with, any peripheral equipment. Such peripheral equipment might include echo suppressors, circuit multiplication equipment, statistics post-processing computers (for the processing of accounting, traffic recording, and billing records), recorded announcement machines, network management systems, network configuration databases, etc. Each interface needs to be clearly and unambiguously defined, so that the exchange correctly interworks with the peripheral equipment. The likelihood of incompatibility is increased when the two pieces of equipment are provided by different manufacturers,
  14. EXCHANGE DESIGN AND SPECIFICATION 755 and for this reason standard interfaces should be used as far as possible. The use of standard interfaces also tends to reduce the development costs incurred by the manu- facturer, because these interfaces are likely to have been developed already. A lower equipment price therefore pertains (as compared with equivalent exchange for which an special development has to be undertaken). Figure 41.6 illustrates a possible exchange configuration for a modern day, digital and stored program control (SPC) exchange. In Figure 41.6 the exchange is shown surrounded by a number of different types of equipment, each with its own particular interface. There are computer systems, one per- forming the network management functions of network status monitoring and network control (as already described in Chapter 37); the other is a database, with up-to-date an record of thenetworkconfigurationsurroundingtheexchange.Bothsystems are connected electronically to the exchange to be kept automatically up to date with any changes in the network. Two 2 5 datalinks are employed for the interconnection. These X carrydatainformationacrossaQ3-interfaceaccordingtothespecificationsofthe telecommunications management network (TMN, Chapter 27). The exchange in Figure 41.6 is shown with magnetic tape interfaces for downloaded accounting, billing, and traffic statistics, and uploading software and exchange data. The important interface in this instance is the format and informational content of the data on thetape. The peripheral computerneeds sufficient information to carry out any necessary accounting, billing or traffic recording functions, and it needs to understand the format in which this information is passed out by the exchange, so that the data may be correctly interpreted. Another couple of interfaces shown in Figure 41.6 are the control mechanisms for circuitmultiplicationequipment andfor echosuppressors.Thesecan be relatively simple, ‘hard-wired’ electrical leads, indicating simple on/off controls, or they may be the more sophisticated interface defined in ITU-T recommendation Q.50. (Used for monitoring and controlling management (Used for planning) congestion ) database X25 datalink X 2 5 datalink Maintenance Alarms (warning of failures) workstations Control mechanism for output, Magnetic tape Exchange circuit multiplication postprocessed far equipment ( CME 1 accounting. and billing t- traffic recording - Control mechanism for statistics. echo suppressors Magnetic tape Circuitstootherexchange exchange software (signallingsystem interface) and data upgrades (digital or analogue Linesystem) Recorded announcement device Figure 41.6 Some typical exchange interfaces and peripherals
  15. 756 EXTENDING BUILDING, AND REPLACING NETWORKS The final interfacesshown inFigure41.6arethoseintendedformaintenance workstations, for recorded announcement devices, and for reporting equipment alarms. Inthecaseinwhichtheperipheral devices required are provided by theexchange manufacturer as part of the overall contract for the exchange, it may not be important for the network operator to specify the interface in great technical detail. The exchange manufacturer is then free to use a proprietary interface, probably specially designed for the purpose. Thebenefit to the network operator might be simpler working practices at the maintenance workstation, or perhaps informative more alarms or recorded announcements. Figure 41.6 is not meant to illustrate a comprehensive or exclusive set of interfaces; there are other interfaces which are appropriate, depending on the type and intended use of the exchange. Just likeexchanges,lineterminatingequipment,cross-connectframes and multi- plexorsrequiremaintenance andalarms,andalso likeexchangestheyareunder progressive development to be capable of being remotely maintained or reconfigured using datalink interfaces. For these reasons, large portions of the specifications may be the same or similar. Exchange specifications are often produced in two stages. An initial outline design lays out the network topology and describes the overall functional requirements of the exchange, and it leads to the lengthier detailed specification. The next chapter covers in somedetailthepoints to be consideredwhenpreparingafullspecification.The remainder of this chapter concentrates on the important considerations and tactics appropriate to the outline design stage. 41.7 OUTLINE CIRCUIT-SWITCHED DESIGN: CIRCUIT NUMBERSAND TRAFFICBALANCE In designing an exchange for a circuit-switched network it is crucial to order the right number of circuits and the correct ‘mix’ of different signalling system termination types. If therearenotenoughterminations of aparticularkind, or too few outgoing or incoming circuits, then the exchange cannot befullyloaded and its effective traffic- carrying capacity is reduced. Figure 41.7 shows two scenarios in which inappropriate circuit and signalling mixes have led to premature exchange exhaustion. In Figure 41.7(a), with the circuits that have been ordered, the exchange cannot be suppliedwith enough incoming traffic, because incoming the circuits been have exhausted. spare The outgoing circuitterminations wasted, the are and effective exchangecapacity is correspondinglyreduced.Asimilarwastagecouldalsohave resulted from a shortage of outgoing circuits and an excess of incoming ones. In this case, incoming circuits would need be left spare,to prevent the exchange runninginto to congestion. The best way to alleviate either of these conditions is to carry out a small exchange extension (if possible) of incoming or outgoing circuits as appropriate. In Figure 41.7(b) a slightly less obvious traffic imbalance has crept in. Illustrated an is international exchange, drawingincoming traffic a from national network, and connecting it onward to both continental and inter-continental overseas destinations. For continental destinations, R2 signalling is used; for inter-continental destinations,
  16. OUTLINE CIRCUIT-SWITCHED DESIGN: CIRCUIT NUMBERS AND TRAFFIC BALANCE 757 CCITTRZ Overseas Route A (continental) Overseos (intercontinental ( a ) Outgoing traffic carrying ( b ) Early exhoustion of 0 copacity exceeds incoming porticulor signolling type Figure 41.7 Poor circuit mix causing premature exchange capacity exhaustion the signalling system is CCITTS (this signalling type is rapidly becoming obsolete, but nonetheless is used here to illustrate the point). The diagram illustrates the exhaustion of CCITTS signalling terminations.Althoughoverallthereare still incoming and outgoing circuits available (incoming from national, and outgoing in R2), any further loading of the exchange will lead to congestion on the CCITTS routes. This premature exhaustion comes about because, when further incoming circuits are connected from the national network, they feed a mix of further traffic to both the inter-continental (CCITTS) andcontinental (R2) destinations.However,nofurtherCCITTScircuit termination can be made available. The imbalance is therefore in the relative quantities of CCITTS and R2 signalling terminations, and the actual ratio of signalling termination types should match the balance of intercontinental to continental traffic. Two methods are possible for alleviating the type of premature exhaustion shown in Figure 41.7(b): one is to order an exchange extension to correct the CCITTS shortfall, and the other is to pre-sort thetraffic (at a prior exchange within the national network), so that adisproportionateamount of continental traffic is sent to thisparticular international exchange. This allows the spare incoming and outgoing R2 terminations to be used, without adding traffic to cause congestion on the inter-continental (CCITTS) routes. Of course, in this circumstance, some other international exchange will need to carry a disproportionately large share of intercontinental (CCITTS) traffic. The following procedure will help to determine an appropriate exchange termination mix, and to avoid such traffic imbalances. It should be applied with care, and due consideration should also be given to studying the whole range of expected network configurationsin which the exchange will be used throughoutits life. For each configuration, do the following. (i) Calculate circuit the number, signalling system types, and traffic direction requirements to meet the busy hour requirements of each individualroute connected to the exchange. (ii) Add up the total incoming and outgoing traffic inErlangs, and compare these totals with the total incoming and outgoing circuit numbers. (Bothway circuits, if
  17. 758 EXTENDING BUILDING, AND REPLACING NETWORKS any,should be considered as halfincoming,halfoutgoing).Ensure that the numbers are balanced, or within about 10%of one another, but do not arbitrarily re-adjust the numbers togive a closer match. If an imbalance exists, go back and check it. Oneeffect which can lead to apparent imbalance that of non-coincident is route busy hours. Figure 41.8 shows an example in which the optimum circuit numbers may at first glance appear misaligned, but in fact are not. Outgoing Route A carries mainly business traffic, predominantly during the day time. The busy hour itself is at 9 a.m., when the traffic is 5 Erlangs (requiring 1 1 circuits on 1% tables). Route B by contrast carries mainly residentialtraffic, with a busy hour traffic load of 4 Erlangs at 8 p.m. (requiring 10 circuits). Route C is the incoming route, serving both outgoing routes; its busy hour traffic is 7 Erlangs, at 7p.m. (requiring 14 circuits).Summing uptheroute busy hour traffic values, and incoming/outgoing circuits, there appearsto be an imbalance, as demonstrated by Table 41.3. However, as the analysis above shows, the imbalance of circuits is relatively extreme (being 50% extra on the outgoing side). Large real exchanges rarely show an imbalance greater than about 10%. (iii) Other traffic balancing studies may be appropriate. A useful one to perform on international exchanges is of national ‘side’ traffic to international ‘side’ traffic. Similar balancing of local ‘side’ to trunk ‘side’ traffic could be made for trunk (toll) exchanges. Route A to local town, (carryingbusiness/shop traffic) Exchange Incoming route C Route B to other suburbexchanges (carrying eveningsocial calls Traffic quantity Route C (total) (erlangst Route A Route B (residential t ( business t I I I I I I l , Time of day 6 9 1 2 3 6 9 1 2 3 6 Pm am am Pm am Figure 41.8 Traffic balance and non-coincident route busy hours
  18. DESIGN OUTLINE OF OTHER TYPES OF NETWORK 759 Table 41.3 Apparent trafficandcircuitimbalance Total incoming Total outgoing Route busy-hour traffic in Erlangs 7 =9 5+4 Appropriate number of circuits 14 11 + 10 = 21 Finally, it is worth calculating the peak cross-exchangetraffic and comparing this with the sum of the individual route busy hours. The maximum traffic across the exchange (the cross-exchange traffic) willbe less than the sum of all the route busy hours, unless all the route busy hours are coincident in time. This gives a plausibility check on the exchange traffic design. The nominal value of the cross- exchange traffic is also invaluable to the exchange manufacturer, because it is the maximum call throughputrate whichtheexchangecommonequipment(for example, the central processor) must be capable of handling. Should any of the traffic balances highlight unexplainable imbalances, then the original design should be checked. 41.8 OUTLINE DESIGN OF OTHER TYPES OF NETWORK Traffic balanceconsiderationssimilartothose in the preceding sectionshould be applied when designing networks other than circuit-switched ones. For example, in a traffic balance of a packet-switched exchange the overall packet volume ratecarried and by each route and the exchange as a whole would be taken into account. 41.9 THE EFFECT OF LOWCIRCUITINFILL ON EXCHANGE AND LINEPLANT PLANNING One final trap for the unwary exchange or lineplant planner, is the effect of low circuit injill. The problem arises when circuits are multiplexed together within the exchange, either on a higher order analogue FDM system, or as a 2 Mbit/s (or 1.5 Mbits) higher bit rate digital line system. In such a case, the whole analogue or digital group must be provided between the two end points,even if the circuit demand is not sufficient to need all the circuits in the group (12 circuits for a normal analogue group, 30 circuits for a 2 Mbit/s digital line system, 24 circuits for 1.5 Mbit/s). Theeffect is known as low circuit injill. In cases where the circuit inzll is lower than loo%, extra lineplant and exchange termination capacity may be required. Figure 41.9 shows an example in which a total of 20 circuits is required between exchange A, and two other exchanges, B and C. The terminations available on all the
  19. 760 BUILDING, EXTENDING AND REPLACING NETWORKS 1 circuits required but 0 l x Z M b i t / sm u s t be provided Exchange B Exchange A 10 circuitsrequiredbut Exchange Figure 41.9 T h e effect of poor circuit infill exchanges are standard 2 Mbit/s (European) digital terminations, and the only lineplant is also 2Mbit/s. In result, two 2Mbit/s digital line systems must be used, together with two 2 Mbit/s exchange terminations at exchange A. In other words, unless instead the planner reverts to transit routes to access the destinations, the equivalent of 60 direct circuitsmust be provided,with an effective circuit infill of 10 circuitswithineach 2Mbit/s line system, i.e. 33% infill. Poor infills can alsooccur in higher order linesystems. For instance,within an analogue supergroup it may be that only four FDM groups are used. (To provide the circuits using a supergroup may still be cheaper than providing separate lines for four individual analogue groups), A similar example in digital practice might be the use of only three 2 Mbit/s tributaries of an 8 Mbit/s digital line system, or the use of only two 34 Mbit/s tributaries within a 140 Mbit/s digital line system, etc. So, as with exchange terminations, extra lineplant capacity must be provided on all occasions when poor circuit infills are encountered. 41.10 FUNCTIONAL REQUIREMENTS OF EXCHANGES OR LINE SYSTEMS We have now discussed in some detail the traffic flow design of exchanges, but what other functional characteristics will be required? The answer depends on the expected performance of theswitch. The next chapter,onequipmentprocurement,at least provides a rough checklist of items which may be appropriate to the final specification. 41.11 METHODS OF NETWORK OR EXCHANGE MODERNIZATION Finally, before going on in the next chapter to discuss the production of a detailed equipment specification, it is worth considering various tactical methods for network or exchange modernization. The earlier part of this chapter considered how to determine the provision programme forexchanges or new line systems, and talked about the need
  20. methods of modernization network or exchange 761 for new equipment to meet growth or extension in demand, for equipment upgrade or replacement of life-expired equipment. What was not covered in any detail was the tactical methods that might be employed for network modernisation (or replacement). Basically, two methods are available for either exchange or lineplant modernization; they are the integration method and theoverlay method. Such methods were used in the 1980s modernization from analogue to digital networks, and will be needed again in the upgrade from ISDN to B-ISDN. 41.11.1 Integration A new exchange or line system is provided in a manner fully integrated with theexisting network. A typical example might be a one-for-one or one-for-many replacement of olderequipment.Figure41.10(a)showsaone-for-manyreplacement of four small analogue local exchanges with single larger modern exchange. Figure 41.10(b) shows a a one-for-one transfer of an analogue line system onto a digital replacement. The integration method of replacement is generally used where the old and new technologies are expected to co-exist for a long time, as partof a gradual modernization regime. The changeover of individual exchanges or line systems may be carried out slowly, circuit by circuit, or by a special rapid wiring ‘changeover’ device. 41.11.2 Network Overlay Whennetworkoperatorsareembarkingonmore radicalmodernization,perhaps undertaking a heavy capital investment programme to replace the network very quickly withmodernequipment, an overlaJ) network approach may be themosteconomic method. Network overlay is also a good method of introducing networks for new or Fouroldanalogue local exchanges Replacementdigital exchange ( a ) Exchangereplacement ( b ) One-for-onereplacement of an (one-formany) analogue line by a digital one Figure 41.10 Methods of equipment replacement
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