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This white paper discusses a method of optimising the
number of lines in a trunk group based on the traffic carried by that trunk
group. This is known as dimensioning a trunk group. |
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In order to help you with your
calculations, we have provided free
online traffic calculators at this Web site which you can use now. A Windows 95 / 98 / NT version is available for immediate download
at 80 US Dollars and offers increased speed, capacity and convenience. |
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A network cannot be properly designed without
understanding the traffic patterns which that network will carry. In order to carry out a
complete network design, a matrix of traffic figures between every combination of site on
that network should be gathered. For each site, a traffic figure should be obtained for
calls to every other site, and this information can be used to calculate between which
sites network links should be installed. Westplan carries out
network design in this way. |
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| Analysing using existing traffic data |
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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:
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- 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.
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| Dimensioning trunks using Erlang B |
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Of course, this is a rather idealistic view. In the
real world, links cannot be introduced and removed regularly and the voice network layout
may depend upon other factors such as data traffic carried over a network with voice and
data integration. 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
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| Busy Hour Traffic (B.H.T.) |
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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. |
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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
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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. |
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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.
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| An example trunking calculation |
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- BHT = 17.986 Erlangs
- Blocking = 0.01
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Pressing the Calc button reveals that 27 lines will be
required during the hour in question. Performing this calculation using our Windows 95 / 98 / NT product, Westbay Traffic Calculators is
similar, but please refer to the user guide or help system for detailed instructions. |
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The Erlang B models makes certain assumptions about the
nature of the call arrivals. Amongst them is the assumption that call arrivals are random
(Poisson arrivals). Although this is quite reasonable in most applications, it can cause
inaccurate results when there is a sudden peak of calls. This type of peak can be produced
by a radio or television advertisement being shown and here drastic call peaks are
expected, over-engineering of trunks and call center agents should always be carried out -
always be on the safe side! 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. |
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This document has demonstrated how to use our online and
Windows traffic calculators to dimension trunk groups. If you have any questions, please send us an email. Why not try our Telecom Design
Forum? It is an interactive newsgroup which you can use to exchange ideas about
telecoms design, and our products.
Return to the technical document index
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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 Westbay
Engineers Ltd. for omission or error. |
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| Last modified: 05 July 1999 |
Copyright © 1999. All rights reserved. |
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