SpectraLink NetLink Wireless Telephones Best Practices White Paper Wireless Telephone manual

2.3.2Push-to-Talk Multicasting Considerations

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The push-to-talk (PTT) mode of the NetLink i640 Wireless Telephone uses SpectraLink’s proprietary SpectraLink Radio Protocol (SRP) ADPCM encoding. If a PTT broadcast is active (i.e. a user presses the PTT button), the feature will use the bandwidth as indicated in the table above for the single transmitting i640 Wireless Telephone and one half of the bandwidth for all of the receiving i640 Wireless Telephones. The data rate used for PTT depends on the AP’s settings for multicast traffic. This bandwidth used is independent of the number of handsets receiving the PTT call. Because the PTT mode uses IP multicasting, all APs on the subnet will transmit a PTT broadcast unless the network is running Internet Group Management Protocol (IGMP), in which case the broadcast will only go to those APs that are associated with NetLink i640 Wireless Telephones with the PTT feature enabled.

2.3.3Telephone Usage

Because the data rate and the packet rate are constant, 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 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 one-hour period. Heavy telephone users may generate 0.20 to

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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