Glossary

What is Erlang B?

Erlang is a dimensionless unit for measuring telecommunications traffic. It is used to describe total volume passed in an hour. One Erlang is equivalent to one call, including call attempts and hold time. Erlang B also known as Erlang loss formula describes the probability of call losses for a group of identical parallel resources, including telephone lines, circuits, traffic and channels, also referred as M/M/c/c queue. Erlang B formula was derived by Agner Krarup Erlang. Since it describes the probability of queuing system, it is not only limited to telephone networks, this formula can be also used to calculate busy hour traffic (BHT), percentage of calls blocked, and number of lines in trunk group.

The Erlang B traffic model is based on the following assumptions:

· An infinite number of sources

· Random traffic arrival pattern

· Blocked calls cleared

· Hold times exponentially distributed

Today, almost every call center outsourcing company is using Erlang B formula to calculate the probability of call dropped or blocked. This probability is known as the Grade of Service (GoS) and it is always measured between number 0 and 1. The call center service providers are efficient at handling traffic when the probability of call drop or block is low, i.e., less than .05. Whereas with high GoS, i.e., more than 0.1, the probability of call drop and block is high leading to high rate of annoyed and impatient caller.