Polycom H340 - Manual (Page 11)

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Deploying SpectraLink e340, h340 and i640 Wireless Telephones BEST PRACTICES GUIDE
October 2010
signals and displays the signal strength along with the AP channel assignments. The site survey mode may be used
to detect areas with poor coverage or interfering channels; check for rogue APs; confirm the Service Set
Identification (SSID) and data rates of each AP and include the security and QoS mechanisms supported by the AP;
and detect some AP configuration problems. With SpectraLink handsets, the entire coverage area must be checked
to ensure that at least one access point’s output meets the signal strength requirements summarized in Section 2.1.2
of this document. If the site-survey mode indicates that two APs are using the same channel within range of the
handset, it is important to adjust the channelization to avoid channel conflicts.
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.
2.3 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.
The SVP Server prevents oversubscription of an AP and improved load balancing by limiting the maximum number
of active calls per AP. Maximum settings, using the highest data rate, are AP specific and can be found in the VIEW
Configuration Guides on the Polycom web site. The SVP Server value determines the maximum number of wireless
telephones in-call on a given AP and forces handsets to handoff (roam) when capacity maximums are reached.
Overall, the calls per AP specified in the SVP Server is often lower than the maximum number an individual AP may
be able to support. This allows some telephones to work at lower rates (1Mbps and 2Mbps) and some at the highest
data rates.
2.3.1 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 802.11b 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 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.