Erlang (unit)

Erlang is a dimensionless auxiliary unit of measurement used in the transport theory for the market value in a communication network ( Erl, erl ). She was named at the suggestion of David George Kendall to Agner Krarup Erlang, which came to prominence as first thought about queuing problems in telephony.

An Erlang corresponds to the permanent full capacity of a communication channel or another resource.

Are about when an ISDN connection permanently both B- channels are busy, then this corresponds to 2 Erl If a channel is occupied only 8 hours a day, so this corresponds to 1/3 Erl

1000 calls of 2 minutes each within an hour are; for such a call volumes so at least 34 news channels would be necessary. Due to the Erlang distribution of practical value is higher.

Usually, the fair market value - calculated in practice over an observation period of one hour - in Erlang. Accordingly, the market value is for continuous occupancy of a voice channel over an hour equal to one Erlang. The maximum market value is achieved and measured in the rush hour.

Practical application finds the unit, for example on the status panel of the EWSD and provides information on the utilization of the communication processor (CP).

Calculation values

The optimal case, the assignment is practically hardly achieved because the attempts at conversation follow approximately the Erlang distribution. A basis for planning the establishment of networks, there are various models to calculate the break or waiting probabilities for given call loads.

The Erlang B formula describes the abort ratio of blocking queues that access leads to an already underutilized resource in an immediate termination. In a mobile conversation this is the case if a call attempt, all channels are already occupied and a busy tone, after the caller hangs.

The Extended Erlang B formula also describes blocking queues, but take into account that crashes often have immediate access attempts new result, such as by redialing in telecommunications.

The Erlang C formula describes queues with holding patterns, in which an initially unsuccessful access at full capacity still remains a certain amount of time in queue until it can be assigned to the resource you want or if there is a crash. This model is used inter alia to calculate the capacity of call centers.

The above-mentioned Formulas are based on a known market value for a single resource ( a radio channel, an outside line ) and allow the calculation of fair value for a larger bundle of resources ( number of radio channels, several connection cables).

• Auxiliary unit
• Traffic Theory