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Erlang - a
unit of traffic
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| An Erlang is a unit of telecommunications traffic
measurement. Strictly speaking, an Erlang represents the continuous use of one voice
path. In practice, it is used to describe the total traffic volume of one hour. For
example, if a group of user made 30 calls in one hour, and each call had an average call
duration of 5 minutes, then the number of Erlangs this represents is worked out as
follows:
| Minutes of traffic in the hour |
= |
number of calls x duration |
| Minutes of traffic in the hour |
= |
30 x 5 |
| Minutes of traffic in the hour |
= |
150 |
| Hours of traffic in the hour |
= |
150 / 60 |
| Hours of traffic in the hour |
= |
2.5 |
| Traffic figure |
= |
2.5 Erlangs |
Erlang traffic measurements are made in order to help telecommunications network
designers understand traffic patterns within their voice networks. This is essential
if they are to successfully design their network topology and establish the necessary
trunk group sizes.
Erlang traffic measurements or estimates can be used to work out how many lines are
required between a telephone system and a central office (PSTN exchange lines), or between
multiple network locations. |
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| Several traffic models exist which share their name with the Erlang unit
of traffic. They are formulae which can be used to estimate the number of lines
required in a network, or to a central office (PSTN exchange lines). A formula also
exists to model queuing situations, and lends itself well to estimating the agent staffing
requirements of call centers. The main Erlang traffic model are listed below, with links
to the free online calculators on this Web site:
- 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.
- 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.
- 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.
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To investigate the traffic unit Erlang, and the Erlang traffic models, we
suggest the following sources of information on this Web site:
- Free online Erlang traffic Calculators
These online calculators allow you to perform Erlang traffic calculations
now. The Call Minutes Calculator
does not even require an understanding of the Erlang traffic unit, and allows entries in minutes
rather than Erlangs. Detailed information is available in the Help
area (press the Help button).
- Dimensioning Trunk Groups
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 uses the Erlang B traffic model.
- Call Centre Design
This white paper describes the steps involved in assessing the staffing
requirements of a call centre and estimating the number of trunks (central office lines)
required to serve a call centre for incoming calls. The suggested method uses both
Erlang B and Erlang C.
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 Agner Krarup Erlang was born in 1878 in
Lønborg, Denmark. He was a pioneer in the study of telecommunications traffic and,
through his studies, proposed a formula to calculate the fraction of callers served by a
village exchange who would have to wait when attempting to place a call to someone outside
the village.In 1909, he published his first work: The Theory of Probabilities and
Telephone Conversations. He gained worldwide recognition for his work, and his
formula was accepted for use by the General Post Office in the UK.
Erlang never married. He worked for the Copenhagen Telephone Company for twenty
years, until his death in 1929. During the 1940s, the Erlang became the
accepted unit of telecommunication traffic measurement, and his formula is still used
today in the design of modern telecommunications networks. |
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| If you have any comments on this document, please send us an email. Why not try our Telecom Design Forum? It is an interactive
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| This document
should not be viewed as a consultative document. It is the readers' responsibility to
ensure that the most appropriate telecommunications strategy is applied to his or her
business. No liability is accepted by the authors for omission or error. |
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