Push-To-Talk Multicasting Considerations - Polycom SpectraLink 8020 Deployment Manual

Deploying SpectraLink 8020/8030
Wireless Telephones
July 2009
about 18 percent of the available bandwidth at 11 Mbps or about 48 percent at 2 Mbps or about 90
percent at 1Mbps.
The maximum number of simultaneous telephone calls an AP can support is determined by dividing the
maximum recommended bandwidth usage by the percentage of bandwidth used for each individual call.
Note that approximately 20 to 35 percent of the AP bandwidth must be reserved for channel negotiation
and association algorithms, occasional retries, and the possibility of occasional transmission rate
reductions caused by interference or other factors. Therefore, 65 to 80 percent of the total available
bandwidth should be used for calculating the maximum call capacity per AP. For example, if all calls on
an AP are using a theoretical 5.4 percent of the bandwidth at 11 Mbps, the actual number of calls
expected at that rate would be about 12 (65 percent of bandwidth available / 5.4 percent theoretical
bandwidth utilized per call). Lower overall bandwidth is available when there are a greater number of
devices associated with an AP or when lower data rates are used for the telephone call or calls.
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. Polycom‟s
number of calls per AP for specific models that have been tested to be compatible with the SpectraLink
handset.
When using SVP for QoS, Polycom provides the ability to limit the number of calls per AP with a
configurable setting in the SVP Server. The "Calls per Access Point" setting limits the number of active
calls on each AP and can be used to set aside bandwidth for data traffic. Wireless Telephones in-call are
free to associate with other APs within range that have not reached the set maximum number of calls.
Polycom requires this setting to be equal to or below the maximum number of calls recommended in
VIEW Configuration Guides. It is still possible for the number of phones associated to an AP to exceed
the maximum number of calls as would be the case with any additional clients associating to the same
AP. The maximum number of calls per AP will simply control how many of those associated phones will
be able to enter into a call. Additional phones beyond the maximum number specified will be forced by
the SVP Server to roam to a new AP that has not reached the maximum calls per AP. If no APs are
available any phones beyond the maximum will display an error message indicating there is insufficient
bandwidth to complete the call.
For systems configured to use Wi-Fi Standard QoS, the calls per AP are managed by the AP using
standards based methods. WMM Admission Control will ensure that AP capacity and bandwidth are
reserved for wireless clients based on the TSPEC (Transmission Specification) each device submits to
the AP to reserve bandwidth for that device‟s communications. The AP will keep track of the total
available bandwidth and will restrict access to network resources for clients if bandwidth becomes
unavailable.
2.5.2

Push-to-Talk Multicasting Considerations

SpectraLink 8030 handsets provide push-to-talk (PTT) functionality using the Polycom-proprietary
SpectraLink Radio Protocol (SRP) ADPCM encoding. 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 8030 handsets are deployed on a network with previous versions of SpectraLink handsets, some
interoperability considerations must be observed. The SpectraLink 8030 handsets have 24 PTT channels
plus one priority channel available. Earlier models enabled only eight PTT channels with no priority
channel. 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.
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Best Practices Guide
identify the maximum