In electricity, we count the amps or volts; in meteorology, we measure the pressure; in telecommunications, we count the Erlangs.
The telephone came into existence in 1870. Most of the concepts and notations were derived during this period. Looking at a telephone connection over a time period of length T, we define its observed traffic flow as the percentage of time during which the connection is busy
A priori, traffic is a dimensionless quantity since it is the ratio of the occupation time to the total time. However, it still has a unit, Erlang, in remembrance of Erlang who, along with Palm, was one of the pioneers of the performance assessment of telephone networks. Therefore, a load of 1 Erlang corresponds to an always busy connection.
Looking at several connections, the traffic carried by this trunk is the sum of the traffic of each connection
This is no longer a percentage, but we can give a physical interpretation to this quantity according to the ergodic hypothesis. In fact, assume that the number of junctions is large, ...