Mạng và viễn thông P30

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Mạng và viễn thông P30

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Teletrafic Theory Telecommunication networks, like roads. are saidcarry ‘traffic’, consisting not of vehicles but of to telephone callsor data messages. The more traffic there is, the more circuits and exchanges must be provided. On a road network the more cars and lorries, the more roads and roundabouts are needed. In any kind of network, if traffic exceeds the design capacity then there will be pockets of congestion.

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2. 530 TELETRAFFIC in conversation and also the time prior to conversation when the call being set-up; is holding time is the total time for which the network is in use 0 the total number of data characters conveyed All the above statistics impact on the performance of teleconmmunication networks, of but the holding time is particularly influential to the carrying capacity and congestion telephone and other circuit-switchednetworks. As far as the science of teletraffic is concerned, the traffic volume is normally defined the third definition above. In other by words, the total trafic volume is equal to the total network holding time. The traffic volume, however, cannot be directly used in the determination of exactly how many circuits on a trunk or whatsize of exchanges will need to be provided. What we need is some idea of the maximum usage of the network at any one time. For this reason, it is normal to measure the traffic intensity of circuit-switched networks. 30.2 TRAFFIC INTENSITY (CIRCUIT-SWITCHED NETWORKS) The trafic intensity of a circuit-switched network is defined to be the average number of calls simultaneously in progress during a particular period of time. It is measured in units of Erlangs. Thus an average of one call in progress during a particular period wouldrepresenta trafic intensity of one Erlang. The traffic intensity on any route between two exchanges can also be quoted in Erlangs. It is measured by first summing the total holding time of all the circuits within the route and then dividing this by the time period T, over which the measurement was made. In some countries, including the United States, trafic intensity is measured not in Erlangs but in units called CCS (hundred call seconds). CCS is a measureof the totalcall holding time during the network or route busy hour. The two units, CCS and Erlang are very simply related because 1 Erlang = 3600 call seconds = 36 CCS The definition of trafic intensity is not restricted to traffic between exchanges. Cross- exchangetraffic (that passingacross an exchangefromincoming ports to outgoing ports) can also be measured and quoted in Erlangs. As an example, the route between exchanges A and B in Figure 30.1 consists of five circuits. The chart in Figure 30.1 also shows the periods of usage of each of these circuits during a 10 minute period. It will be seen from the chart in Figure 30.1 that the total number of minutes of cir- cuit usage during the 10 minute period was 35 minutes. This is the sum of the individual circuit holding times, respectively: 6min, 7.5 min, 7.5 min, 8 min, 6min. Dividing the total of 35 minutes by the length of the monitoring period (10 minutes) we may deduce that the trafJic intensity on the routewas 3.5 Erlangs. In other words, an average of 3.5 circuits were in use throughout the whole of the period. However, usually the challenge facing a telecommunications network planner is how many circuits to provide to carry a given volume of traffic. The general principle is that more circuits will be needed on a route than the numerical value of the route traffic intensity measured in Erlangs. It would be easy to succumb to the mistaken belief that a
3. PRACTICAL TRAFFIC INTENSITY MEASUREMENT (ERLANG) 531 S S circuitroute L 5 Exchange Exchange Circuit 1 number 2 3 L = Circuit in use 5 1 2 3 L S 6 7 8 9 10Tlme Figure 30.1 Circuit usage on a route between exchanges trajic intensity of 3.5 Erlangs (3.5 simultaneous calls) could be carried on four circuits. Figure 30.1 clearly illustrates an example in which this is not the case. All five circuits were simultaneously in use twice, between times 2 and 3, and between times 5.5 and 6. Here we must digress briefly to examine the concepts of ofered and carried trafic, though their names may give them away. Offered trafic is a theoretical concept. It is a measure of the unsuppressed trajic intensity that would be transported on a particular route if all the customers’ calls were connected without congestion. Carried trajic is that resultantfromthecarried calls, and it is the value of traffic intensityactually measured. For a network without congestion the carried traflc is equal to the offered trafic. However, if there is congestion in the network, then the offered traffic will be higher than that carried, the difference being the calls which cannot be connected. Teletraffic theoryleadsto a set of tables andgraphs which enabletherequired network circuit numbers to be related to the ofered trujic in Erlangs (i.e. the demand). 30.3 PRACTICALTRAFFIC INTENSITY (ERLANG) MEASUREMENT The trafic intensity on a given route may be measured using one of two main methods, either by sampling or by an absolute measurement. The latter method was used in the example of Figure 30. I. Historically, however, electro-mechanical exchanges did not lend themselves to easy calculation of the absolute value of the total holding time. Instead, it was common practice to use a sampling technique. We can obtain an esti- mate of the total circuitholding time by looking at the instantaneous state of the circuits at a number of sample points in time.If we take ten samples, after imin, limin,
4. 532 TELETRAFFIC THEORY Table 30.1 Samples of number of circuits in use Elapsed time 1 1 ; 2; 3; g 5; 6f 7f 8; 9; Circuits in use 3 5 5 3 4 5 4 2 4 4 2$min, . . . ,9$min, we could assume that this was representative of the respective time periods, 0-1 min, 1-2min,. . . , etc. Thus, because after $minute has elapsed, we find that three circuits are in use; we assume that three circuits will be in use for the whole period of the first minute. By this method we obtain 10 ‘snapshots’ of the number of circuitsinuse,corresponding tothe 10 individualoneminuteperiodswhich we sampled. The results are as shown in Table 30.1. Averaging the sample values will give us an estimate of the traffic intensity over the whole period. This value comes out anestimated 3.9 Erlangs. This is calculated as the at sum of the ten sample values (39), divided by the overall duration (10). Compare this with the actual value of 3.5 Erlangs. The error arises from the fact that we are only samplingthecircuitusageratherthanusingexactmeasurement.Theerrorcan be reduced by increasing the frequency of samples. Table 30.2 gives the values calculated at a$ minute (as opposed to a l-minute) sampling rate (sometimes also calledscan rate). 3.5 The imreased sampling rate Table 30.2 estimates the traffic correctly as Erlangs of (70 X 0.5/10). The exactness of the estimate on this occassion is fortuitous. Nonethe- less, the principle is well illustrated that tooslow a sampling rate will produce unreliable traffic intensity estimates and that the estimate is improved in accuracy by a higher sampling rate. A good guide is that the sample period length should be no more than about one-third of the average call holding time. In the example of Figure 30.1, the average call holding time is 35 min/l7 calls = 2.06 min, so a sample should be taken at least every 0.7min to derive a reliable estimate of the traffic intensity. The minimum monitoring period should be about three times the average call holding time. This helps us to avoid unrepresentative peaks or troughs in call intensity. Nowadays, stored program controlled ( S P C ) (i.e. computer-controlled) exchanges have made the absolute measurement of call holding times relatively easy. So today many exchanges produce the measurements of traffic intensity which exact.They do are so by calculating the value using data records storing the start and end time of each individual call or connection. Table 30.2 ;minute scan rate time Elapsed 1 4 3 14 ‘3 24 23 3; 39 4 + in Circuits use 3 3 4 4 5 5 3 3 4 4
5. THE BUSY HOUR 533 30.4 THE BUSY HOUR In practice, telecommunications networks are found to have a discernible busy hour. This is the given period during the day when the trufic intensity is at itsgreatest. Traditionally measurements of traffic intensity have been made over a full 60-minute period at the busy hour of day, to calculate the busy hour trufic (a shortened term for the busy hour trufic intensity). By taking samples over a few representative days, future busy hour trufJic can usually be predicted well enough to work out what the equipment quantities and route sizes of the network should be. The dimensioning is made by using the predicted busy hour traffic as an input to the Erlang formula(presented later in this chapter). When that formula has pronounced the scale of circuits and equipment that are going to be needed at the busy hour, we can feel secure at less hectic times of day. In more recent times it has been found that exchanges are developing more than one busy hour; maybe as many as three, including morning, afternoon and evening busy hours. The morning and afternoon busy hours are usually the result of business traffic. The evening one results from residential, international traffic and nowadays also from Internet access traffic (dial-up connections from home PC-users to Internet servers and bureaux. Some examples of daily traffic distribution are given in Figure 30.2. The first distributioninFigure 30.2 is atypical business-serving exchange,with morning and afternoon busy hours. The second example shows the traffic in a residen- tial exchange, where morning and afternoon busy hours are less than the evening busy Traffic I ( a ) business area 0000 0800 1600 2Loo Traffic ( b l residential area Traffic (C) U.K-Austrolio (limited by tlmezonedifference 1 Time of day Figure 30.2 Some typical daily traffic distributions
6. 534 TELETRAFFIC THEORY hour. The third exampleis of an international route,where there is often a single peak, constrained in duration by the time zone difference between the countries. Multiple busy hours necessitate the calculation of separate busy-hour traffic values for each individual busy hour,and design of the network to meet each one. It is not enough to use only one of the sets of busy-hour traffic values. In Figure 30.2 for example, the overall effect of distributions (a) and (b),when combined, is a two-humped pattern with overall busy hours in morning and afternoonof approximately equal magnitude. How- ever, if we were to design the whole network on the basis of its morning and afternoon busy hour traffic values alone, we would not provide enough circuits at the residential exchange to meet its evening busyhour. Normal practiceis therefore to dimension each exchange matrix according to its own exchange busy hour, and each route according to its individual route busy hour. Another point to consider when dimensioning networksis that a route or exchange is unlikely to be equally busy throughout the entire 60 minute busy hour period.To meet the peak demand, which might besignificantly higher than thehour’s average traffic, it is sometimes convenientto redefine the busy hour asbeing of less than 60 minute duration. It may seem paradoxical to have a busy hour of less than 60 minutes, but Figure 30.3 illustrates a case oftraffic which has a short duration peak between 15 and 30 minutes. of In the example of Figure 30.3, we were to use a 60 minute busy hour measurement if period, we would estimate a busy-hour traffic of value Q as shown in the diagram. This would be a gross underestimate of the actual traffic peak, and would guarantee that the busiest period would be heavily congested. By redefining the busy hour to be of only 30min duration we obtain an estimate P which is much nearer the actual traffic peak. Surprisingly,however,the useof much shorter busy hourperiods and attempts to pinpointpeaks of traffic whichlastonlya few minutesarenot really important; customers who suffer congestion at this time are more than likely to get through on a repeat call attempt within a few minutes anyway! To sum up, it is usual to monitor exchange busy hour and route busy hour traffic values as a gauge of current customer usage and network capacity needs. Future net- of work design and dimensioning can then be based on our forecasts what the values of these parameters will be in the future (forecasting methods are covered in Chapter 31). Actual I peak *- I Number of clrcuits in use Short-term fluctuations Time Figure 30.3 A short duration busy hour
7. MULA THE TRAFFIC INTENSITY 535 30.5 THE FORMULA FORTRAFFIC INTENSITY Our next step is to develop the formula for traffic intensity, as a basis for subsequent discussion of the Erlang method of network dimensioning. Recapping in mathematical terms, the traffic intensity is given by the expression Trafic intensity= the sum of circuit holding times (carried traffic) the duration of the monitoring period Now let A =the traffic intensity in Erlangs T = the duration of the monitoring period h; = the holding time of the ith individual call c = the total number of calls in the period of mathematical summation Then, from above Now, because the sum of the holding times is equal to the number calls multiplied by of the average holding time, then where h =average call holding time, and therefore It is interesting to calculatethe call arrivalrate, in particularthenumber of calls expected to arrive during the average holding time. Let N be this number of calls, then N = no of call arrivals during a period equal to the average holding time =h X call arrival rate per unit of time =hxc/T = ch/T = A In other words, the number of calls expected to be generated during theaverage holding time of a call is equal to the traffic intensity A . This is perhaps a surprising result, but one which sometimes proves extremely valuable.
8. 536 TELETRAFFIC THEORY 30.6 THE TRAFFIC-CARRYINGCAPACITY OF A SINGLE CIRCUIT In this section we discuss the traffic-carrying capacity of a single circuit. This leads on to a mathematical derivation of the Erlang formula, which is the formal method to calculate the traffic-carrying capacity of a circuit group of any size. For our explanation let us assume we have provided an infinite number of circuits, laid out in a line or grading, as shown in Figure 30.4. The infinite number provides enough circuits to carry any value of traffic intensity. Now let us further assume that each new call scans across the circuits fromthe left-hand end until finds a free circuit. it Then let us try to determine how much traffic each of the individual circuits carries. First, let us consider circuit number one. Figure 30.5 shows a timeplot of the typical activity we might expect on this circuit, either busy carrying a call, or idle awaiting for another call to arrive. The timeplot of Figure 30.5 starts with the arrival of the first call. This causes the circuit to become busy for the duration of the call. While the circuit is busy a number of other calls will arrive, which circuit number one will be incapable of carrying. These other calls will scan across towards a higher-numbered circuit (circuit number two, then three, and on) until the first free circuit is found. Finally, at the end so of the call on circuit number one, the circuit will be returned to the idle state. This state will prevail until the next new call arrives. Let us try to determine the proportion of time for which circuit number one is busy. For this purpose, let us assume each callis of a duration equal to the average call hold- ing time h. This is not mathematically rigorous but it makes for simpler explanation. Let us also invent an imaginary cycle of activity on the circuit. Circuit outlets Circuitnumber New calls start at this end scon and ocross the circuits turn in until1 finding o free circuit Figure 30.4 Scanning for a freecircuit T ldle Idle I dle Arrival time of the first call Figure 30.5 Activity pattern of circuit number l
9. THE TRAFFIC-CARRYING CAPACITY OF A SINGLE CIRCUIT 537 Cycle r,- Repeatcycle -p -- Repeat cycle ,Repeat - - L -, - cycle T Ime Nextcall arrival Figure 30.6 Average activity cycle on circuit number 1 Our imaginary cycle is as follows. After the arrival of the call, we expect circuit number one to be busy for a period first of time equal to h. As we learned in the last section we can expect a total of A calls to arrive during the average holding time, where A is the offered traffic. ( A - 1 ) of these calls (i.e. all but the first) will scan over circuit number one to find a free circuit among the higher numbered circuits. At the end of the first call circuit number one will be released, and an idle period will follow until the next call arrives (i.e. the ( A + 1)th). We can imagine this cycle repeating itself over and over again. As we can see from our imaginary ‘average’ cycle, shown in Figure 30.6, the total + number of calls arriving during the cycle is A 1. The total duration of the cycle is therefore A + 1 - h ( A + 1) hx- - ~ A A We also know that circuit number one busy during each is cycle for a periodof duration h. of Therefore the average proportionthe time for which circuit number one busy is given is by A - A occupancyofcircuitnumber 1 =h X ~ - h(l+A)-l+A This value is the so-called circuit occupancy of circuit number one. It is numerically equal to theaverage number of calls in progress on circuit number one, and is therefore the intensity of the traffic carried on circuit number one, and is measured in Erlangs accordingly. Thus if one Erlang were offered to the grading ( A = l), then the first circuit would carry half an Erlang. The remaining half Erlang is carried by other circuits. Taking one last step, if we assume that new calls arrive at random instants of time, then the proportionof calls rejected by circuit number one is equal to the proportion of time during which the circuit is busy, i.e. A / ( 1 + A ) . In our simple case, if only one + circuit were available, then A / ( 1 A ) proportion of calls cannot be carried. This is called the blocking ratio B, and is usually written A B(b1ocking ratio - for one circuit) = - l+A where A = offered traffic.
10. 538 TELETRAFFIC Though not proven above in a mathematically rigorous fashion, the above result is the foundation of the Erlang method of circuit group dimensioning. Before going on, however, it is worth studying some of the implications of the formula a little more deeply for two cases. First, take the case of one Erlang of offered traffic to a single circuit. Substituting in + our formula A = 1, we conclude that the blocking value is 1/(1 1) = 1/2. In other words,halfofthecallsfail(meetingcongestion),andonlyhalfarecarried.This confirms our earlierconclusion thatthe circuitnumbers needed tocarry a given intensity of traffic are greater than the numerical value of that traffic. With traffic intensity of A = 0.01, then the proportion of blocked calls would have been only O.Ol/l.Ol (=O.Ol), or about one call in one-hundred blocked. This is the proportion of lost calls targetted by manynetworkoperatingcompanies.Putin practical terms, the carrying capacity of a single circuitinisolation is around only 0.01 Erlangs. Next let us consider a very large traffic intensity offered to our single circuit. In this case most of the traffic is blocked (if A = 99, then the formula states that 99% blocking is incurred, i.e. is not carried by our particular circuit). However, the corollary is that the traffic carried by the circuit (equalto the proportion of the time for which the circuit is busy) is 0.99 Erlangs. In other words the circuit is in use almost without let-up. This is what we expect, because as soon as the circuit is released by one caller, a new call is offered almost immediately. In the appendix the full Erlang lost cull formula is derived using a more rigorous mathematical derivation to gain an insight into the traffic-carrying characteristics of all the other circuits in Figure 30.4. For the time being, however, Figure 30.7 simply states the formula. To confirm the result from our previous analysis, let us substitute N = 1 into the formula of Figure 30.5. As before, we obtain a proportion of lost calls for a single circuit (offered traffic A ) of A/(1 + A ) - A B(1,A) = - 1 l+A E(N, A ) or 2! 3! = c /! ( l + A + - +A-2+ .A 3 N . . + -N B(N, A ) where E(N, A ) = proportion of lost calls, and probability of blocking A = offered traffic intensity N = available number or circuits N ! = factorial N Figure 30.7 TheErlanglost-callformula
11. RCUIT-SWITCHED DIMENSIONING 539 Circuit 5 erlangs (Total area under plot 1 occupancy (in erlangs) I I Areashadedrepresents traffic carried by the Elh clrcult I 11 circuits 1 2 3 L 5 6 7 8 9 Circuit number ( i n order of selection) Figure 30.8 Circuit occupancies This of course is also equal to the circuitoccupancy (the traffic carried by it). The advantage of our new formula is that we may now calculate the occupancy of all the other circuits of Figure 30.4. By calculating the lost traffic from two circuits we can derive the carried traffic. Subtracting the traffic carried on circuit number onewe end up with that carried by circuitnumbertwo. In asimilarmanner,the traffic carrying contributions of the other circuits can be calculated. Eventually we are able to plot the graph of Figure 30.8, which shows the individual circuit occupancieswhen 5 Erlangs of traffic is offered to an infinite circuit grading. As expected,thelow-numberedcircuitscarrynearly 1 Erlangandare innear- constant use, whereas higher numberedcircuitscarry progressively less traffic. The traffic carried by the first eleven circuits is also shown. From the formula Figure 30.7, of this is the number of circuits needed to guarantee less than l % proportion of calls lost. Thus the right-hand shaded area in Figure 30.10 represents the small proportion of lost calls if 11 circuits are provided. The ability to calculate individual circuit occupancies is crucial to grading design (see Chapter 6). 30.7 DIMENSIONING CIRCUIT-SWITCHED NETWORKS Thefuture circuitrequirementsforeachroute of a circuit-switched network(i.e. telephone, telex, circuit switched data) may be determined from the Erlang lost call formula. We do so by substituting the predicted ofleered traffic intensity A , and using trial-and-error values of N to determine valuethe which gives a slightly better performance than the target blocking or grade of service B. A commonly used grade of service for interchange traffic routes is 0.01 or 1 % blocking. It is not an easy task by direct calculation to determine the value of N (circuits required), and for this reason it is usual to use either a suitably programmed computer or a set of trafJic tables. In recent years, numerous authors organizations and haveproduced modified versions of the Erlang method, more advanced and complicated techniques intended
12. 540 TELETRAFFIC to predict accurately the traffic-carrying capacity of various sized circuit groups for different grades ofservice. All have their place but in practice it comes down to finding the most appropriate method by trying several for the best fit for given circumstances. In my own experience, the extra effort required by the more refined and complicated methods of dimensioningisunwarranted.In practicethetraffic demand mayvary greatly from one day or month to the and the practicality is such that circuits have next to be provided in whole numbers, often indeed in multiples of 12 or 30. The decision say then is whether 1 or 2, 12 or 24, 30 or 60 circuitsshouldbeprovided. It is rather academic to decide whether23 or 24 circuits are actually necessary when least 30 will at be provided. Table 30.3 illustrates a typical traffic table. The one shown hasbeen calculated from the Erlang lost-call formula. Down the left hand column of the table the numbercir- of cuits on a particular route are listed. Across the top the table various different grades of of service are shown. In the middle of the table, the values represent the maximum offered Erlang capacity corresponding to the route size and grade of service chosen. Thus a route of four circuits,working to a designgrade of service of 0.01, has a maximum offered traffic capacity of 0.9 Erlangs. We can also use Table 30.3 to determine how many circuits are required to provide a 0.01 grade of service, given an offered traffic of 1 Erlang. In this case the answer is five circuits. The maximum carrying capacity of five circuits at 1% grade of service is 1.4Erlangs, slightly greater than needed, but the capacity of four circuits is only 0.9 Erlangs. The problem with traffic routes of only a few circuits is that only a small increase in traffic is needed to cause congestion. It is goodpracticetherefore to ensure that a minimum number of circuits (say five) are provided on every route. Table 30.3 A simpleErlangtraffic table Grade of service ( B ( N ,A ) ) 1 lost call in Number 50 100 200 1000 of circuits (0.02) (0.01) (0.005) (0.001) ~~ ~~~~~~ Erlangs Erlangs Erlangs Erlangs 1 0.020 0.010 0.005 0.001 0.15 2 0.22 0.105 0.046 0.60 3 0.35 0.19 4 1.1 0.9 0.7 0.44 1.7 5 1.9 6 2.3 1.1 7 2.9 2.5 2.2 1.6 3.6 8 2.7 3.2 2.1 4.3 9 3.3 2.6 10 5.1 4.5 4.0 3.1 11 4.6 5.8 5.2 3.6 6.6 12 5.9 5.3 4.2
13. DIMENSIONING CIRCUIT-SWITCHED NETWORKS 541 Circuit requirement 12 11 10 Curvesrepresent : 9 1 lostcallin 50 (gos 0 . 0 2 ) 1 lost call in 100 (gos 0.011 1 lostcall in 200 (90s 0.005) 8 1 lost call in 1000 (gos 0.001) 7 6 Required circuit number L.7 ( 5 in practice1 5 L Intended grade of service better than 0.01 3 Offered traffic of 1 erlang 2 1 1 2 3 L 5 6 7 Capacity traffic in intensity(erlongs ) Figure 30.9 Graphical representation of the Erlang formula
14. 542 TELETRAFFIC Exchange C Exchange Exchange A B Secondchoiceroute First choiceroute L Figure 30.10 Overflow routing The information held in Table 30.3 is sometimes presented graphically, and Figure30.9 illustrates this. The traffic offered in Erlangs is usually plotted along the horizontalaxis, andthe circuit numbersupthe verticalaxis. A numberof different curvesthen correspond to different grades of service. To determine how many circuits are required for a given offered traffic, the offered Erlang value is read along the horizontalaxis, then a vertical line is drawn upwards to the curve corresponding to the required grade of service, and a horizontal line is drawn from this point to the vertical axis, where the circuitrequirementcan be read off. Our earlierexample is repeated on the figure, confirming that five circuits are needed to carry l Erlang at 1% grade of service. The traffic-to-circuit relationshipshowninTable30.3andFigure 30.9, andthe Erlang lost call formula on which they are based are only suitable for dimensioning full-availability networks (as defined in Chapter 6). For limited availability networks, slightly different formulae and traffic tables must be used. Furthermore, anotherslightly different set of traffic tables is called for when dimensioningnetworks which use overflow routing. Such tables are available from various publishers. Figure 30.10 shows an example of overflow routing in which traffic from exchange A to exchange B first tries to route directly, but is allowed to overjowl via exchange C if all direct circuits are busy. Only the link A-B in this network can be dimensioned by the simple Erlang lost- call formula (provided we specify an overflow grade of service, e.g. 10% of calls), but as we shall find out in Chapter 32, only a slight modification to the method is necessary to permit us to dimension all the other links. 30.8 EXAMPLE ROUTE DIMENSIONING As an example of the use of the Erlang lost-call formula in route dimensioning, let us calculate the number of circuits required to carry offered traffic A = 55 Erlangs at a grade of service B = 0.01. We shall show in Chapter 32 that an estimateof the number of circuits B required for 1% grade of service is given by the formulae N =6 + (A/0.85) where A < 75 N = 14 + (A/0.97) where 75 < A < 400 N=29+A where 400 < A
15. CALL WAITING SYSTEMS 543 so that for our case, A = 55 N(approx) = 70 circuits So far so good, but from here on we rely on trial and error and the formula in Figure 30.7. substituting A = 55, N = 70 we get B = 0.007 This is slightly better than we need, so let us try A = 55, N = 69. This time B = 0.009 Again A = 55, N = 68,then B = 0.012 68 circuits give a slightly worse grade of service than we need, so 69 circuits must be provided! 30.9 CALL WAITING SYSTEMS Whereas in telephone and other circuit switched networks the dimensioning is carried outto a given grade-of-service (or call loss), this technique is not suitablefor dimensioning all types of network. Data networks and server-based services (such as Internet bureaux and operator switchrooms or telemarketing bureaux) are examples of queuing (i.e. bufSer) or call-waiting systems. A queue (or bufSer) minimizes, and may even eliminate, any loss of offered traffic. As we saw in Chapter 9, arriving packets from the separate virtual connections can be queued (buflered) in order until the line is free. Similarly, a caller accessing an Internet service must wait a little longer at times of heavy traffic demand on the network, but will be answered eventually. In the case of operator services or call-in telemarketing bureaux, incoming callers waitinaqueue listening to music or a recorded message untila human operator becomes free. In such networks the dimensioning techniques must be oriented towards ensuring thatthe capacity of adatalink,thenumber of servers or thenumber of human operators is sufficient to keep the queue waiting time within acceptable bounds. If there are insufficient operators the queue and the consequent waiting time will get longer and longer. We must also ensure that the queue capacity is long enough for all the people waiting or the data packets being temporarily stored in the bujier. Consider the operator switchroom or telemarketing bureaux first, because what hap- pens there follows on from the Erlang lost-call formula already discussed. Figure 30.11 illustrates a networkin which an operator switchroom is serving customers of the telephone network. In all, N operators (called servers in teletraffic jargon) are working and K places (confusingly also called servers) are available in the queue. The switchroom manager wants to make sure that the number operator positions of staffed (N) at any pointtime during the daygreat enough to cope thetraffic and of is with so keep down the queue waiting time. Furthermore, the switchroom designer wants to ensure that the queue capacity ( K ) is great enough to minimize the number of lost calls.
16. 544 TELETRAFFIC 7 I N operators Calls I Operator switchroom Figure 30.11 Call queueing in an operator switchroom Both NandK can be determined from adapted form the Erlanglost-cull method, an of using various forms of the Erlung Waiting Cull formula, as shown in Table 30.4. The formulae of Table 30.4 are unfortunately complex and this is not the place to describe their derivation or any of the more complex versions of them that have also been developed. We can, however, discuss their practical use, as follows. A tolerable delay t is set for thetime which telephone callers Figure 30.11 in will have to wait for an operator (e.g. 15 seconds), and we then decide what percentage of calls will be answered withinthisdelaytime(atypicaltargetwouldbe 95%). It is thenatrial-and-error exercise, using formulae (i) and (iv) of Table 30.4, to give the number of servers ( N ) which will meet the target constraints. The value N will always be greater than the offered traffic A . If it were not so, the operators would be reducing the queue at a rate slower than that at which the queue was forming. The queue and the delay could only become longer. (SubstituteN = A in formula (ii) and you will see that the average delay is infinite). Let us use an example to illustrate the method. We shall assume that the switchroom handles an average of 3000 calls per hour, and that they take an average of d = 60 seconds of the operator’s time (i.e. service time) to deal with. . . . Then the offered trufic, A (average number of calls in progress) = 3000 X 60/3600 = 50 Guessing a value of N = 55 to meet our target that 95% calls will be answered within 15 seconds, we calculate in order. . . A = 50 Erlangs (offered trufJic) N = 55 (active servers (operators)) B = 0.054 from the lost-cull formula C = 0.388 from formula (i) t = 15 seconds (target answer time) d= 60 seconds (average service time) Probability that delay exceeds 15 seconds from formula (iv) =0.11
17. CALL WAITING SYSTEMS 545 Table 30.4 The Erlang waiting call formula Probability of delay (i.e. that a customer will have to wait to be served) NB =c= N- A(l -B) This is called the Erlang call-waiting formula Average delay (held in queue) Average number of waiting calls Probability of delay exceeding t seconds Probability of; or more waiting calls Probability of X servers being busy (a) No queue, X = number of operators busy AX P(X) = P(0) 0 IX 5 N (b) Queue, all operators busy, J queue places taken X= N+j p ( N + j ) = c(1-\$) () i 1 Notation N = number of servers (those processing the calls, e.g. operators) j = number of calls in queue B = lost call probability if there were no queues derived from the Erlang lost call formula ~ = E(N, A) A = offered traffic in Erlangs d = average service time required by an active server to process a call As this proportion is higher than the target of 0.05, wewill need more servers, but probably not too many more because we are not too far adrift. Repeatingfor N = 56, we findtheprobability of answer in 15 seconds is 0.07. Almost, but not quite! Repeat again for N = 57, and the probability of answer taking longer than 15 seconds is only 0.04. Thus 57 operators will be needed in our switchroom.
18. 546 TELETRAFFIC THEORY Having determined the numberof operators ( N ) , the number of queue places needed is found by using formula (v) of Table 30.4. The planner first decides what small prop- ortion of calls it is acceptable to lose because there are no free places in the queue. Then he finds the value j from the formula which satisfiesthe condition that the probability of lost calls (more waiting calls than queue positions) is met. Going back to our example, let us aim to lose only1 % of calls because of inadequate queue capacity. Then the probability o f j or more waiting calls must be less than 0.01, where j is the designed capacity of the queue. The calculation is as follows.. . A = 50 Erlangs offered traffic N = 57 active operators B = 0.039 Erlang lost call formula C = 0.246 Erlang waiting call formula So, from formula (v) 0.01 = C ( A / N ) j rearranging j = (logO.01 - log C)/(log A - log N ) j = 24.4 rounding up, 25 queue places will be needed. Out of curiosity, why not calculate theaverage waiting time and the average number of callers inthe queue? The average waiting timeis 2.1 seconds, and the average number of waiting calls is 1.76. Calculating the number of operators needed to answer calls on a company’s office switchboard is carried out in an identical way to that used above. Theoretically, different formulae must be used on occasions where the offered traffic distribution does not follow Erlang’s detailed mathematical assumptions (given in the appendix). Inappropriateuse of the formulae may lead errorsin the available network to capacity, but on the other hand most practitioners prefer to use simple dimensioning methods and rely ontheirpracticalexperienceratherthanworrytoomuchabout complex statistical theory. If interested, a number of more complex methods and their detailed derivations are given in numerous specialist texts on queueing theory. 30.10 DIMENSIONING DATA NETWORKS Terminals and other devices sending information on data networks generally do so in a packet-, frame- or cell-oriented format over a packet-, frame- or cell-switched network. In the case of Figure 30.12(b), the data network system has been designed to flatten the profile of the traffic by using buffers and statistical multiplexing. Such a network demands the use of a call-waiting dimensioning technique to decide the required data link bit rate and buffer capacity.
19. DIMENSIONING DATA NETWORKS 547 Link speed restriction 4 Data speed demanded ( bit /s) (a) Figure 30.12 Peaked bursts of data demand The time for which individual packet, frame cell must wait in the an or buffer for the line to become free may be a significant proportion of the total time needed for it totraverse the network as a whole. Thus the waiting time in the buffer contributes significantly to the propagation time. The propagation time in turn may affect the apparent speed of response of a computer reacting to the typed other commands of the computer user. or Most computer applications can withstand some propagation delay, though above a given threshold value further delay may be unacceptable. Thus, for example, many packet-switched data networks are designed to keep delays lower than 200ms. The switch and network designers of a data network must ensure that the bitspeed of the line is greater than theaverage offered bitrate of all the packets, framesor cells. If the line were not fast enough, a build-up data would occur in the of buffer at the transmitting end, causingconsiderabledelay. Inaddition,the buffer must be largeenoughto temporarily store all waiting data. If the buffer were not big enough, then arriving packets will be lost at times when it is full and ‘overflows’. The formulae of Table 30.4 may be adapted for data network dimensioning by sub- situting N = 1. This corresponds to a single connection between points. It is equivalent to a single data link between terminals or packet-switched data exchanges. The link will have a given bit speed capacity, and the data flowing over the link could be thought of as being measured in Erlangs. For example, an average bit rate of 7500 bit/s being carried on a 14 400 bit/s link is equivalent to 7500/14 400 or 0.52 Erlangs. Table 30.5 The Erlang call-waiting formula applied to data networks _____ ~~ ~ (i) probability of delay = C = A =average line loading(typicallymonitored in %) (ii) average delay= D =D P (1/A - 1)L A* (iii) averagenumber of waitingpackets or frames = - l-A (iv) probabilityofpacketdelay(due to outputbuffering)exceeding t seconds = Aexp(-(l - A)tL/p) (v) probability o f j or more waitingpackets = A’+’ where p =data packet, frame or cell size in bits L = datalink line speed in bit/s