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Collaboration, Voice & AV · Advanced

Erlang B

A traffic formula that calculates the number of trunks needed for a given busy-hour traffic and blocking probability.

Definition

Erlang B assumes blocked calls are cleared and do not retry or queue. Inputs are busy hour traffic in Erlangs and the desired grade of service, such as P.01 (1 percent blocking). It is used to size PSTN trunks, SIP trunk sessions and WAN voice capacity.

Example

With 10 Erlangs and P.01, Erlang B indicates about 18 trunks are needed.

Exam tip

Use Erlang B for trunks and Erlang C for agents in queues.

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