SpectraLink NetLink Wireless Telephones Best Practices White Paper Wireless Telephone - Manual (Page 8)

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Page 7
2.3.2 Push-to-Talk
Multicasting
Considerations
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.
2.3.3 Telephone
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
0.30 Erlangs, 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: