The Erlang B Traffic Model

The Erlang B Traffic Model is an analytical model for calculating call blocking. It makes the following assumptions:

•	Calls are presented randomly to a trunk group (Poisson arrivals)

•	Call times are fixed or are exponentially distributed

•	Blocked calls are cleared

The assumption that blocked calls are cleared is only valid if calls overflow to another trunk group (for instance, calls to a private network link overflow to the public PSTN telephone network if all trunks on the private network are in use). In practice, this is often not the case. However, if blocking is low the error introduced by this assumption is small. The Extended Erlang B model specifically addresses issues of caller retries and can produce more accurate results under these circumstances.