Polycom H340 - Manual (Page 6)

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Deploying SpectraLink e340, h340 and i640 Wireless Telephones BEST PRACTICES GUIDE
October 2010
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 previous 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 ten decibels higher for the handset to return to the
previously associated AP. This behavior requires that the assessment area must have at least a ten decibel
difference to enable good roaming behavior for all cases.
Corners and doorways pose a particular design issue. The shadowing of corners can cause steep drop-offs in signal
coverage. Make sure to have adequate cell overlap at and around corners so that the audio stream is not impacted
by a user going around corners. This may require placement of AP at corner locations to ensure appropriate cover
and prevent RF shadows.