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Deploying SpectraLink 8020/8030
Wireless Telephones
May 2009 Best Practices Guide
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 one second for a DCA cycle to complete for a
standard three channel deployment for 802.11b. 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.
Figure 1 - Dynamic Channel Assessment (DCA)
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 current
AP) than the current AP’s signal. In most cases the handset only needs five decibels of signal difference
between APs to make a decision to roam. But to prevent ‘ping-pong’ behavior the separation needs to be
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