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Installation Guide

percent theoretical bandwidth utilized per call). Lower overall bandwidth is available when there are a

greater number of devices associated with an AP.

Even with all of the known variables, there are many other vendor-specific characteristics associated

with individual APs that make it difficult to quantify the precise number of concurrent calls per AP,

without thorough testing of specific configurations. Avaya-supported Polycom VIEW access point

Configuration and Deployment Guides that identify the maximum number of calls per AP for specific

models that have been tested to be compatible with the Avaya wireless telephones.

To allow for bandwidth to be available for data traffic, Avaya provides the ability to limit the number of

calls per AP within the Avaya Voice Priority Processor (AVPP). The "Calls per Access Point" setting

limits the number of active Avaya wireless telephone calls on each AP. Wireless telephones are free to

associate with other APs within range that have not reached the set maximum number of calls. Avaya

requires this setting to be equal to or below the maximum number of calls discussed in the previous

paragraph.

2.4.2 Push-to-Talk Multicasting Consideration

Some Avaya handset models deliver push-to-talk (PTT) functionality. Because the PTT mode uses IP

multicasting, all APs on the subnet will transmit a PTT broadcast. This can be limited to only the APs

that are handling one or more PTT-enabled handsets by enabling the Internet Group Management

Protocol (IGMP) on the wired infrastructure network.

When Avaya 3645 handsets are deployed on a network with previous versions of Avaya handsets

some interoperability considerations must be observed. The Avaya 3645 handsets have 25 PTT

channels available where the earlier 3626 model enabled only eight PTT channels. When PTT is

activated on a network using a mix of handset versions, only the eight common channels will be

available for the older handsets.

2.4.3 Telephone Usage

Because data rate and packet rates are constant with voice applications, Wi-Fi telephony calls may be

modeled in a manner very similar to circuit-switched calls. Telephone users (whether wired or

wireless) generally tend to make calls at random times and of random durations. Because of this,

mathematical models can be applied to calculate the probability of calls being blocked based on the

number of call resources available.

Telephone usage is measured in units of Erlangs. One Erlang is equivalent to the traffic generated by

a single telephone call in continuous use. A typical office telephone user will generate 0.10 to 0.15

Erlangs of usage during normal work hours, which equates to six to nine minutes on the telephone

during an average one-hour period. Heavy telephone users may generate 0.20 to 0.30 Erlangs, or an

average of 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 included in these averages.

The traffic engineering decision is a tradeoff between additional call resources and an increased

probability of call blocking. Call blocking is the failure of calls due to an insufficient number of call

resources being available. Typical systems are designed to a blocking level (or grade of service) of 0.5

percent to two percent 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 one

percent or less is desired, each AP can support approximately 13 moderate wireless telephones

users. If the AP coverage supports 12 simultaneous calls per AP, each AP can then support

approximately 39 moderate users.

Octiober 2008