The Erlang B Traffic model is an analytical model for calculating call blocking and is the basis for this calculator.
The model 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 are overflowed to another trunk group (for instance, calls to a private network link are overflowed 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.