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This white paper describes the steps involved in assessing
the staffing requirements of a call centre and the estimating the number of trunks
(central office lines) required to serve a call centre for incoming calls. |
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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. Ansapoint is a call centre analyser
which automates the design process discussed in this paper. It is available for
immediate download for just 160 US Dollars. |
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| Description of the design process |
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There are two distinct areas of design required in such an
application. The questions which must be answered are:
- How many call centre agents do I need?
- How many trunks do I need?
As call holding times depend upon average queuing times
(which depend upon the number of agents deployed), the two questions must be addressed in
the order shown. |
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| How many agents do I need? |
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Calculating the number of agents required is a continuous
process which will require regular reassessment as the circumstances of a call center
change. Assessments may be made for each working hour of a day, and should take such
factors as marketing campaigns and daily call peaks into account. We suggest performing a calculation for each working hour. In order to
estimate the number of agents required in a particular hour, the following information
relating to that hour is required as a minimum:
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| The minimum three entries required |
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- Number of calls received
- Average duration of these calls
- Average delay that you accept that incoming callers may
experience.
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Items 1 and 2 describe the incoming traffic levels and must be
established from call statistics or from estimates based on your understand of your
business. Item 3 is your performance criterion. Another performance criterion which can be
used defines call handling in terms of the percentage of calls answered within a target
queuing time (e.g. 85% of calls answered within 20 seconds of ringing). This can be more
meaningful and is supported by our Windows 95 / NT product, Westbay Traffic Calculators,
but is not yet supported by our online calculators. |
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Wrap up time (or wrap time) is the time an agent remains
unavailable to answer a call after a call has been completed. It is usually the time taken
to carry out administrative tasks relating to a call such as entering an order on a
terminal. For the purposes of Erlang B, wrap up time should be included in average call
duration. |
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Having established these three minimum parameters for an hour,
an estimate of the number of agents required can be made using the Erlang C Traffic Model.
You can use our online
calculator to work through this example now.
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| An example call centre calculation |
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- Calls received in the hour 350
- Average call duration 180 seconds (160 seconds duration + 20
seconds wrap up time).
- Average delay to all calls 25 seconds
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Pressing the Calc button reveals that 21 agents 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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| How many trunks do I need? |
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Whereas the number of agents required can (and should) be
dynamic, changing from hour to hour, the number of lines required to connect a call center
with a central office exchange is fixed (at least in traditional circuit switched
technology) and must cater for the maximum anticipated traffic levels which will be
encountered. Engineering the number of lines required is known as dimensioning a trunk
group. The Erlang B traffic model can be used to
estimate the number of lines required. This traffic model 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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The busiest hour must always be used for busy hour traffic
calculations. But, wrap up time is not included. In working out the number of lines
required, the busy hour traffic must be based on the duration of the calls and the queuing
times as these account for trunk occupancy; wrap up time does not occupy a trunk.Assuming our earlier call centre example
represents the busiest hour, the busy hour traffic is calculated as follows:
BHT = [ Average call duration (s) + Average delay
(s) ] * Calls per hour / 3600
The resulting figure shows the total trunk occupancy in
hours, including the average delay period during which calls are being queued in an ACD
and occupying trunks.
So, the busy hour traffic figure would be:
BHT = [ 160 + 25 ] * 350 / 3600
BHT = 17.986 Erlangs
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It is important to note that the busy hour traffic figure
should represent the busiest traffic load a call centre 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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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 28 lines will be
required during the hour in question. Performing this calculation using our Windows 95 / 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 and C traffic models make 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 (which can often be the reason for a call centre's existence in the first
place!) Where drastic call peaks are expected, over-engineering of trunks and call center
agents should always be carried out - always be on the safe side!
The Erlang C traffic model does not take abandoned calls
into account, but if your call center is engineered correctly, this should not be a
factor. It may cause a problem when attempting to use Erlang C to analyse an existing call
centre with poor performance. |
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This document has demonstrated how to use our online and
Windows traffic calculators to design some aspects of call center operations. 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
telecommunications design.
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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