144 BUILDING, EXTENDING AND REPLACING NETWORKS Figure 41.1 illustrates a graph of forecast demand, and shows a typical network operator’s equipment provision schedule, in which ‘steps’
Trang 1PART 6
SETTING UP NETWORKS
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)
Trang 2of a ‘freephone service’ offering the facility for call recipients to pay for the calls As a n introduction to the subject of network evolution which the operator must be able to cope with, this chapter describes how networks may be built or extended to meet capacity and service needs Particular attention is paid to the design factors inherent in the equipment ordering process; 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
No matter what type of telecommunications service is provided, network costs are minimized by matching capacity to the demand Both over-provision and under-
provision increase costs When equipment is provided before it is actually needed, 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 equipment finally 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 incurred in rearranging the network to incorporate new exchanges, as a result of the
lack of network ‘manoeuvring room’
743
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)
Trang 3144 BUILDING, EXTENDING AND REPLACING NETWORKS
Figure 41.1 illustrates a graph of forecast demand, and shows a typical network operator’s equipment provision schedule, in which ‘steps’ of extra capacity 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 local exchanges, covering a wide area The steps of capacity shown in 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 be required The steps then relate to the necessary provision schedule 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
Trang 4MATCHING NETWORK CAPACITY TO FORECAST DEMAND 745
demand, by quoting a lead time for connection of new customers to the network which exactly matches the delivery time of a new port card from the network 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, the demand is normally quoted in terms of the highest instantaneous network throughput that 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
Forecasts form a direct tool for determining the forward network provisioning
schedules, but to be useful the parameters to be forecast need first to be thought out carefully Before making any forecast, the network planner needs to determine the geographical area which the traffic forecast is intended to cover, and the 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 network topology will be employed, and how particular services will be supported This depends on a number of factors
0 terrain conditions
0 density of customers and/or ports or end 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), its state 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 task is 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 employ a large 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 alternative network using smaller exchanges and it incurs greater cost in switching equipment, but the overall lineplant requirements and costs are reduced
Trang 5146 BUILDING, EXTENDING AND REPLACING NETWORKS
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 be provided 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 a hierarchical network structure, where the central
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 that new customers cannot be connected Shortage of busy hour throughput (i.e Erlang) capacity means that the needs of the established customers are not met Graphs may thus be drawn, similarly to Figure 41 l , for both fitted traffic-carrying capacity in Erlangs 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, in cases when the growth is very rapid, it 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
Trang 6MATCHING NETWORK CAPACITY TO FORECAST DEMAND 747
Old catchment area
of exchange A
l
New catchment area of exchange A
Catchment area of new exchange B
X - Customer statlon
Figure 41.3 Splitting a local exchange area
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 capacity of 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 in the region around the new town, while the offload of traffic from exchange A onto the new 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 case is 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 the wiring is already centred on exchange A,
the economics would probably have forced an 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 case of the central exchange C, 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 exchanges shown and to-and-from other geographic areas, not illustrated in Figure 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 entire country Traffic growth may be such as to exhaust the capacity of 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 overseas routes, corresponding to those overseas countries which have the largest volumes of traffic The established exchange will thereby gain growth capacity (resulting from the offload), whereas the new exchange will serve and provide growth capacity for
Trang 7748 BUILDING, EXTENDING AND REPLACING NETWORKS
International gateway exchanges
* ( O l d ) - * Large number small traffic to each of countries,
( N e w )
Small number of countries,
o f f -Loaded from old exchange, each with Large traffic volume
Figure 41.4 Reconfiguring international exchanges
a small number of heavy traffic routes Figure 41.4 illustrates this example Similar network reconfigurations to those shown in Figures 41.3 and 41.4 might also be appropriate for trunk or other transit exchanges
41.2 OTHER FACTORS AFFECTING THE NEED 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 account when deciding the best 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 the electronic 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 ports but these cannot be used to terminate fibre lineplant unless converters are installed This reduces the useful capacity of the exchange In a similar way, older signalling system ports (for example pre-digital ones) become obsolescent and also reduce exchange capacity Finally, equipment software needs occasional upgrading to
Trang 8FACTORS IN DETERMINING AN EXCHANGE PROVISION PROGRAMME 749
provide either the capability for new services or the updating 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 their networks prior to the originally intended life-end of the equipment The modernization is carried out to reduce ongoing running costs, particularly in manpower terms brought about by 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 1980s is the introduction of intelligent networks (Chapter 11) and
sophisticated network management controls (Chapters 27 and 37)
41.3 FACTORS IN DETERMINING AN EXCHANGE
PROVISION PROGRAMME
Figure 41.5 shows a similar demand curve to that in Figure 41.1, but in this case the diagram takes account of 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 capacity comes about because of the gradual obsolescence of
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 the existing system in Figure 41.5 will diminish due to the redundancy of equipment, in itself the result of obsolescence Over this period the new 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 shows the planned ‘changeover’ replacement of exchange (1) after a seven year lifetime Diagrams similar to that shown in Figure 41.5 are an invaluable tool in determ- ining future equipment provision programmes The example shown in Figure 41.5 demands a forward provision programme as shown in Table 41.1
Trang 9750 RIJILDING EXTENDING AND REPLACING NETWORKS
Table 41.1 Exchange provision programme New exchange no Date of introduction
in Figure 41.2 no inter-exchange network is required for option (a), whereas a relatively complicated network is required to interconnect the nine exchanges of option (b))
On the other hand, the use of a larger number of small units may be the best answer in difficult terrain, or in cases where there is an established workforce, network or building already available In the end the overall exchange provision is a compromise between a number of constraints
The efficiency and reliability of a network ultimately depend on the expertise of its designer, and there is no substitute for experience in developing network evolution 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 The factors listed below set out a framework for determining and evaluating alternative network evolution schemes, and by using them the network 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, already obsolescent, 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
Trang 10DETERMINING A STRATEGY FOR NETWORK EVOLUTION 751
particular country, prior to modernization of equipment in that country) It is no good 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
The total lifetime costs of an exchange (or of a complete exchange provision programme) include not only the capital costs associated with the provision of the equipment, the net- work, the accommodation, the peripherals, the training, the documentation and the
spares, but also the ongoing running costs associated with equipment maintenance, rates, bills, electricity, staff costs, etc Although one type of equipment may be 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
The example in Table 41.2 compares the net present value of providing a single large exchange of one equipment type, 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 costing E10 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) are assumed to be 5 % per annum
of the equipment value (a typical assumption) The accommodation costs are common
to both cases, and include E1 million initial purchase and fitting out of the building, plus EO.l million per annum rent and rates, etc It is interesting to see from the full-life cost analysis shown in Table 41.2, how, despite the fact that the overall expenditure is 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 41.2 the planner could equally decide that E0.68 million was cheap insurance against unplanned demand In either case a full cost analysis would also include the cost 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 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, a rapid and large-scale network modernization may require very large