The Erlang unit reference article from the English Wikipedia on 24-Jul-2004
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Erlang unit

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The dimensionless unit named the erlang is a statistical measure of telecommunications traffic used in telephony. It is named after the Danish telephone engineer A. K. Erlang, the originator of queueing theory.

In the traffic calculation, one Erlang implies a single resource in continuous use (or two channels at fifty percent use, and so on, pro rata). For example, if a bank has two tellers and during the busiest hour of the day they're both busy the whole time, that would represent two erlang of traffic.

Typically erlang might be used to determine if a system is over- or under- provisioned (has too many or too few allocated resources).

It might be used to measure traffic on a T-1 or E-1 line, to determine how many voice lines are in use at the busiest hour of the time period being examined; for 24 channels, if only 12 are ever in use, the other 12 might be made available as data channels.

Traffic calculations measured in erlang can also be used to calculate grade of service (GoS) or quality of service (QoS). The GoS or QoS of a particular resource is the probability of traffic being offered to a resource meeting a condition where it cannot be served now. GOS is calculated from the perspective of the resource and not the perspective of the request.

There are a range of different Erlang formulae:

The formula for Erlang B is

Eb(0,t) = 1

Eb(r, t)= tEb(r − 1,t)/ (r + tEb(r − 1,t))

where Eb is the probability of blocking, t is the number of Erlang offered, and r is the number of resources.

The "Erlang C" calculation is often used to calculate the number of agents or customer service representatives needed to staff a call center.

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