The Erlang unit is a measure of communication traffic or trunk occupancy (from telephone parlance). 1 Erlang = 3600 call-seconds = full occupancy of a trunk for one hour. Telephone trunk traffic tables set out the number of trunks required to give a desired grade of service (GOS)(= number of calls delayed more than a specified number of seconds in the study hour) (or = number of calls abandoned after a specified number of seconds in the study hour), for calls of a given average trunk occupancy (Erlangs, or call-seconds) per call.
There are two "Erlang formulas" in general use: Erlang B (for delayed calls abandoned) and Erlang C (for delayed calls held) Erlang C is also known as Molina or Poisson
Both Erlang B and Erlang C formulas assume calls are (traffic is) offered to the trunk group completely randomly from an infinite population source. While this approximately true for large telephone systems, it is definitely not true for small systems and usually not for trunked radio systems (where it is often incorrectly applied). For small systems, with non-random traffic the Erlang formulas generally give overly-conservative results (i.e., call for too many trunks for the given traffic), but in fact the formulas are not useful for non-random traffic and modelling is the preferred method.