Models. A set of traffic engineering techniques utilized to determine numbers of facilities required in various telecommunications scenarios. Developed by Danish mathematician A.K.Erlang in early 1900s. Erlang B is used to determine required facilities in an “all calls cleared” situation such as automatic route selection in a PBX. Extended Erlang B is a modified technique used when there is measurable retry of calls taking place when calls are blocked. Erlang C assumes blocked calls will wait in queue and is therefore the Erlang technique used to determine staffing needs in a typical “hold for the next agent” contact center scenario.