Erlang (unit)
Erlang (Erl, Erl) is a dimensionless auxiliary unit of measurement used in traffic theory for the traffic value in a communication network . At the suggestion of David George Kendall, it was named after Agner Krarup Erlang , who was the first to think about queue problems in telephony .
1000 calls of 2 minutes each are within an hour ; According to the Erlang-B formula, at least 45 message channels would be necessary for such a volume of calls with a blocking probability of 1% .
In practice, the market value in Erlang is usually calculated over an observation period of one hour. Accordingly, the traffic value for uninterrupted occupancy of a voice channel for an hour is equal to one Erlang. The maximum traffic value is reached and measured during the rush hour .
In the practical environment, the market value, which represents the desired or requested occupancy, is often equated or confused with the actual occupancy; however, the two values are only identical if there is no blocking, i.e. H. each participant receives his line when he asks it.
In practice, the unit Erlang is z. B. on the status panel of the EWSD , there it provides information about the utilization of the communications processor (CP).
Calculation values
In practice, the optimal case of occupancy is hardly achieved, since the conversation attempts approximately follow the Erlang distribution . As a basis for planning when setting up networks, there are various models for calculating the probabilities of interruption or waiting for a given call load.
The Erlang-B formula describes the termination ratio for blocking queues in which access to an already busy resource leads to an immediate termination. In the case of a mobile phone call, this is the case if all channels are already busy when a call is attempted and a busy tone sounds, whereupon the subscriber hangs up.
The extended Erlang-B formula also describes blocking queues, taking into account, however, that aborts often result in immediate new access attempts, such as through redialing in telecommunications.
The Erlang-C formula describes queues with queues in which an initially unsuccessful access remains in the queue for a certain period of time at full capacity until the desired resource can be allocated to it or it is terminated. This model is used, among other things, to calculate the capacity of call centers .
The above Formulas are based on a known traffic value for a single resource (a radio channel , a trunk line ) and allow the calculation of the traffic value for a larger bundle of resources (several radio channels, several connection lines).