Wireless Telephone Capacity; Access Point Bandwidth Considerations - Polycom SpectraLink 8020 Deployment Manual

Deploying SpectraLink 8020/8030
Wireless Telephones
July 2009
After a site survey is complete, coverage issues can be resolved by adding and/or relocating APs if
necessary. Overlap issues may be resolved by reassigning channels or by relocating some access
points. When adjustments are made to the WLAN configuration an additional site survey or site
verification should be performed to ensure that the changes are satisfactory and have not had an adverse
impact in other areas of coverage.
2.5

Wireless Telephone Capacity

Network capacity requirements factor into the number of APs required, although in most cases the
coverage area is the primary factor. Data traffic is often very "bursty" and sporadic. This is typically
acceptable because data applications can tolerate network congestion with reduced throughput and
slower response times. Voice traffic cannot tolerate unpredictable delays, where the bandwidth
requirements are much more constant and consistent. Voice traffic can also be predicted using
probabilistic usage models, allowing a network to be designed with high confidence in meeting anticipated
voice capacity requirements. Beyond the standard IP telephony design guidelines, there are several
additional considerations that should be addressed for VoWLAN with SpectraLink handsets.
When SVP is the selected QoS method for the handset, the SVP Server prevents oversubscription of an
AP and improved load balancing by limiting the maximum number of active calls per AP. Recommended
settings are AP specific and can be found in the
The maximum number of active calls must be defined for each of the three possible handset radio types –
802.11b, 802.11g and 802.11a. The SVP Server determines the maximum number of wireless
telephones in-call on a given AP and forces handsets to handoff when capacity maximums are reached.
Although 802.11g and 802.11a networks theoretically provide increased available bandwidth for support
of additional simultaneous call volume, the practical call volume limitations will depend on many factors
such as data rates used, competing network traffic, and network performance. Overall, the calls per AP
specified is often lower than the maximum number an individual AP may be able to support. This allows
some handsets to work at lower rates (802.11b at 1Mbps and 2Mbps) and some at the highest data rates.
For Wi-Fi Standard QoS or CCX, WLAN admission control techniques are used for AP bandwidth
management. When the handset is configured to use WMM it will be necessary to ensure the access
point also supports WMM Admission Control. This mechanism is responsible for ensuring the AP does
not become overloaded with any particular type of traffic. Depending on the AP manufacturer it may be
possible to adjust the settings for individual WMM traffic classifications. Details for making these sorts of
changes to APs are available in the
Section 4.2.3 for additional details.
2.5.1

Access Point Bandwidth Considerations

There are several factors which determine the AP bandwidth utilization during a telephone call. The first is
the VoIP protocol used and its characteristics. The type of codec utilized combined with the packet rate
will determine the size of the voice packets along with any additional overhead information required for
the protocol. Payload data will generally account for 30-50%of a typical voice packet, with 802.11 and IP
protocol overhead filling the rest. The 802.11 protocols include timing gaps for collision avoidance, which
means bandwidth utilization is more accurately quantified as a percentage of available throughput rather
than actual data throughput.
The percentage of bandwidth required is greater for lower 802.11a/b/g data rates; however it is not a
linear function because of the bandwidth consumed by the timing gaps and overhead. For example, a call
using standard 64 Kbps voice encoding (G.711) utilizes about 4.5 percent of the AP bandwidth at 11
Mbps, and about 12 percent at 2 Mbps. In this example, four simultaneous calls on an AP would consume
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