Chapter 3 The Tariscope program
3.10 Telephone traffic measurement
3.10.1. Brief history
Problem of estimate of telephone traffic originated in the beginning of the 20th century. In 1909 the Danish mathematician Agner Krarup Erlang published research "The Theory of Probabilities and Telephone Conversations". In this research Erlang deduced the formula that allows to define required number of telephone lines to service subscribers with required quality of service. The mentioned research became the starting point of creation of new science named the Queueing theory, which found a use in very different fields. In telecommunication this theory is named Teletraffic theory.
In the Teletraffic theory the word 'traffic' is usually used to denote the traffic intensity, i.e. traffic per time unit. According to ITU-T: The instantaneous traffic intensity in a pool of resources is the number of busy resources at a given instant of time. The ITU-T recommendation also specifies that the unit usually used for traffic intensity is erlang (E). The unit is dimensionless. The total traffic carried in a time period is a traffic volume, and it is measured in erlang-hours (Eh), or if more convenient, for example, erlang-seconds. It is equal to the sum of all holding times inside the time period.
The erlang (1 El) corresponds to continuous holding of one voice channel for 1 hour. For calculation of intensity of traffic in Erlangs it is necessary to take the general duration of calls in seconds in any hour and divide it for 3600 seconds. The received value will show average of the lines which are continuously holded within an hour.
To measure the telephone traffic intensity typically it is also used Centum Сall Seconds (CCS). This unit is based on a holding time of 100 seconds and can still be found, e.g. in USA.
1 Eh = 36 CCS
In spite of the fact that the units Erlang and CCS the most often used units for determination of telephone traffic, it is possible to use also other temporary periods, such as 1 minute or 1 second for determination of traffic for these periods. It is natural that reduction of the temporary period leads to more exact determination of value of loading.
