What is Erlang?

1. Erlang:
$ dimensionless quantity
$ continuous use of one circuit for one hour.
$ Types of Erlang traffic model:
i. Erlang B
This is the most commonly used traffic model, and is used to work out how many lines are
required if the traffic figure (in Erlangs) during the busiest hour is known. The model
assumes that all blocked calls are immediately cleared.
NOTE: Blocked Calls - call which are not successful due to busy traffic (busy signal)
ii. Extended Erlang B
This model is similar to Erlang B, but takes into account that a percentage of calls are
immediately represented to the system if they encounter blocking (a busy signal). The retry
percentage can be specified.
iii. Erlang C
This model assumes that all blocked calls stay in the system until they can be handled.
This model can be applied to the design of call center staffing arrangements where, if calls
cannot be immediately answered, they enter a queue.


2. Holding Time

3. Blocking Rate
$ The probability of blocking calls out of a specified no.

4. CAPS
$ Call Attempts per second
$ CAPS = Erlangs/Holding Time

5. BHCA
$ Busy Hour Call Attempts
$ BHCA = CAPS * 3600
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= ERLANG B =
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1. The Erlang B traffic model is used by telephone system designers to estimate the number of lines
required for PSTN connections (CO trunks) or private wire connections. The three variables involved
are Busy Hour Traffic (BHT), Blocking and Lines:
a. Busy Hour Traffic (in Erlangs) is the number of hours of call traffic there are during the
busiest hour of operation of a telephone system.
b. Blocking is the failure of calls due to an insufficient number of lines being available.
E.g. 0.03 mean 3 calls blocked per 100 calls attempted.
c. Lines is the number of lines in a trunk group.

2. If you know two of the figures, you can work out the third as follows:
a. Click on the Unknown radio button representing the unknown variable.
b. Enter the 2 known figures into their edit boxes.
c. Press Calc and the third figure will be calculated and displayed in the remaining edit box.

3. For example, if from your call logger, you know that the Busy Hour Traffic is 10 Erlangs, and you
want to know how many lines are required in this trunk group if you are prepared to tolerate 2 calls
being blocked in every 100 calls attempted then:
a. Press the Lines Unknown radio button (this is selected by default).
b. Enter 10 in the BHT edit box.
c. Enter 0.02 in the Blocking edit box.
d. When you press the Calc button, 17 will be displayed in the Lines edit box indicating that 17 lines would be required in this trunk group.