This white paper describes how to use our online calculators or Windows products to work out how many lines are required in a trunk group. Westbay Traffic Calculators is a Windows 95/NT programme which contains four calculators.  Online calculators are also hosted at this Web site and can be used free of charge now.

• Purpose
• Software tools required
• Designing networks
• Dimensioning trunks using Erlang B
• Reasons for caution
• Conclusion

It is also common for traffic figures for calls carried over particular trunk groups to be retrieved from loggers. With existing networks, these figures are often used to calculate the optimum number of trunks for a group. Our opinion is that this practice is not a sound basis for network design for the following reasons:

• Call loggers can give a distorted view of the traffic on a link as they measure carried traffic rather than offered traffic.  Changes to link capacities based on these figures can often be under-estimated because traffic models base their results on offered traffic. In other words, increasing the number of lines generates more traffic which needs more lines! Link traffic does not include those calls from callers who do not use the direct links because the grade of service is bad.
• Tweaking links as necessary avoids the central issue of network design which is to produce the most economical network layout. To design a network, consideration should be given to locations between which links should be introduced or removed rather than changing the size of existing links.

So, a way of estimating the number of lines required for a known value of offered traffic is required. This is available in the form of the Erlang B traffic mode which requires the following inputs:

• Busy Hour Traffic
• Blocking

Busy Hour Traffic (B.H.T.)
This figure represents the quantity of traffic expressed in a unit called Erlangs. For the purposes of these calculations, 1 Erlang can be considered equivalent to 1 hour of calls.

You will need to provide an estimate for this figure, which represents the number of hours of traffic which is offered to a trunk group in its busiest hour. For example, if you know from your call logger that 350 calls are made on a trunk group, and the average call duration is 180 seconds, then the busy hour traffic will be:

BHT = Average call duration (s) * Calls per hour / 3600
BHT = 180 * 350 / 3600
BHT = 17.5 Erlangs

Blocking
The blocking figure describes the calls which cannot be completed because insufficient lines have been provided. A figure of 0.01 means that 1% of calls would be blocked; this is a normal figure to use in traffic engineering. For some applications, 0.03 (3%) blocking is used.

Example calculation
Having established these two parameters, an estimate of the number of lines required can be made using the Erlang B Traffic Model. You can use our online calculator to work through this example now.

• BHT = 17.986 Erlangs
• Blocking = 0.01

Pressing the Calc button reveals that 27 lines will be required during the hour in question. Performing this calculation using our Windows product, Westbay Traffic Calculators is similar, but please refer to the user guide or help system for detailed instructions.

Our suggestion has been to obtain traffic figures from call loggers. Care must be taken when using this method. Extracting a figure for the traffic carried on a trunk group will often be sufficient, but it should be borne in mind that this figure would represent the traffic carried over a trunk group and not the traffic offered to a trunk group (that is, it would not include the traffic currently being blocked) - be careful!

Lastly, it is important to note that the busy hour traffic figure should represent the busiest traffic load a trunk group will ever be offered. The trunk group being designed must be large enough to cater not just for today's peak, but for every peak. Therefore, extreme caution should be exercised when calculating BHT.

Why not try our Telecom Design Forum?   It is an interactive newsgroup which you can use to exchange ideas about call centre management and staffing.

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This document should not be viewed as a consultative document. It is the readers' responsibility to ensure that the most appropriate telecommunications strategy is applied to his or her business. No liability is accepted by the authors for omission or error.