The Erlang B traffic model
is used by telephone system designers
the number of lines
required for PSTN
connections (CO trunks
) or private wire connections
. The three variables
involved are Busy Hour Traffic (BHT
), Blocking and Lines:
- Busy Hour Traffic (in Erlangs) is the number of hours of call traffic there are during the busiest hour of operation of a telephone system.
- Blocking is the failure of calls due to an insufficient number of lines being available. (for example 0.03 mean 3 calls blocked per 100 calls attempted, or 3 percent of the calls).
- Lines is the number of lines available to the telephone system.
For Erlang B calculations to be considered accurate, three assumptions must be made:
- Holding times are constant and exponential
- Blocked calls are cleared (i.e. busy calls are not put on hold)
- Calls are taken in random order.