What is Erlang ?

Erlang :   An Erlang is a unit of telecommunications traffic measurement.  Strictly speaking, an Erlang represents the continuous use of one voice path.  In practice, it is used to describe the total traffic volume of one hour.  in other words " a one ckt  can one hour of traffic is known as 1Erlang"

For example, if a group of user made 30 calls in one hour, and each call had an average call duration of 5 minutes, then the number of Erlangs this represents is worked out as follows:

Minutes of traffic in the hour	=	number of calls x duration
Minutes of traffic in the hour	=	30 x 5
Minutes of traffic in the hour	=	150
Hours of traffic in the hour	=	150 / 60
Hours of traffic in the hour	=	2.5
Traffic figure	=	2.5 Erlangs

The main Erlang traffic model are listed below

  Erlang BThis is the most commonly used traffic model, and is used to work out how many lines are required if the traffic figure (in Erlangs) during the busiest hour is known.   The model assumes that all blocked calls are immediately cleared.

  Extended Erlang BThis model is similar to Erlang B, but takes into account that a percentage of calls are immediately represented to the system if they encounter blocking (a busy signal).  The retry percentage can be specified.

  Erlang CThis model assumes that all blocked calls stay in the system until they can be handled.  This model can be applied to the design of call center staffing arrangements where, if calls cannot be immediately answered, they enter a queue.