|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
Busy Hour Traffic
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.
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
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.
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.
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 28 lines will be required during the hour
in question. Performing this calculation using our Windows
95, Westbay Traffic Calculators is similar, but please refer to the user guide or help
system for detailed instructions.