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2.3.2
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2.3.3Telephone Usage
Because the data rate and the packet rate are constant,
Telephone usage is measured in units of Erlangs. One Erlang is equivalent to the traffic generated by a single telephone call that lasts for one hour. A typical office telephone user will generate 0.10 to 0.15 Erlangs of usage, which equates to six to nine minutes on the telephone during a
0.30Erlangs, or 12 to 18 minutes of phone usage in an hour. Note that traffic analysis is based on the aggregate traffic for all users, so users with higher or lower usage are averaged out.
The traffic engineering decision is a tradeoff between additional call resources and an increased probability of call blocking. Typical systems are designed to a blocking level (or grade of service) of 0.5% to 2% at the busiest times. Traffic model equations use the aggregate traffic load, number of users, and number of call resources to determine the blocking probability. The blocking probability can also be used along with the aggregate traffic load to determine the number of call resources required. Traffic model equations and calculators are available at www.erlang.com.
Consider a system with APs that can support six active telephone calls. If a blocking probability of 1% or less is desired, each AP can support about 13 moderate wireless telephones users. If the AP coverage can support 12 simultaneous calls per AP, each AP can support about 39 moderate users.
The following table shows maximum users per AP, based on the AP’s ability to handle simultaneous calls:
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