Polycom SpectraLink 8020 Deployment Manual page 7

Deploying SpectraLink 8020/8030
Wireless Telephones
July 2009
The WLAN layout must factor in the transmission settings that are configured within the APs. The
transmission of voice requires relatively low data rates and a small amount of bandwidth compared to
other applications. The 802.11 standard includes automatic rate switching capabilities so that as a user
moves away from the AP, the radio adapts and uses a less complex and slower transmission scheme to
send the data. The result is increased range when operating at reduced transmission data rates. When
voice is an application on the WLAN, APs should be configured to allow lower transmission rates in order
to maximize coverage area. If a site requires configuring the APs to only negotiate at the higher rates, the
layout of the WLAN must account for the reduced coverage and additional APs will be required to ensure
seamless overlapping coverage.
SpectraLink handsets perform Dynamic Channel Assessment (DCA) in between the transmission of
packets to learn about neighboring APs. It takes about two seconds for a DCA cycle to complete in an
802.11a eight channel deployment and approximately one second for a standard three channel
deployment for 802.11b/g. In order to ensure a DCA cycle can complete within the assessment area (see
Figure 1), a person moving through the assessment area must be within the area for at least 4-5 seconds
to make sure the DCA starts and ends within the assessment area. Failure to complete the DCA cycle
within the assessment area can lead to lost network connectivity resulting in a hard handoff, lost audio,
choppy audio or potentially a dropped call.
The handset compares the signal strength of neighboring APs to determine whether to roam from the
current AP. In order to roam, the handset has to determine whether other APs are either five decibels
(dB) (for any first attempt associating with an AP) or ten decibels stronger (to roam back to the previous
AP) than the current AP‟s signal. In most cases the handset only needs five decibels of signal difference
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Figure 1 - Dynamic Channel Assessment (DCA)
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Best Practices Guide