How to use our online calculators or Windows products to work out how many lines are
required in a trunk group.
Contents
Purpose
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 and can be used to work out how many lines are required into a call
centre facility.
Designing networks
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.
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.
Dimensioning trunks using Erlang B
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.
Reasons for caution
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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