| 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
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. |
