# Probability of Wait

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This equation represents the offered load and is measured in Erlangs.  Erlangs are a dimensionless unit often used in telephony to measure carried load.  It is an element frequently used in queuing models.

The offered traffic measured in Erlangs is related to call arrival rate, lambda and average call holding time, h, by1 :

E = lambda * h

For example, a single cord circuit has the capacity to be used for 60 minutes in one hour. If one hundred six-minute calls are received on a group of such circuits, then assuming no other calls are placed for the rest of the hour, the total traffic in that hour will be six hundred minutes, or 10 erlangs.2