Polycom SpectraLink 8020 Deployment Manual page 12

Deploying SpectraLink 8020/8030
Wireless Telephones
July 2009
Figure 4 - 802.11a Non-interfering Channels with Overlapping Cell Coverage
There are some deployment scenarios that require limiting the number of 802.11a channels. A key
reason is to improve roaming performance. With 802.11a there are four channel bands available to
choose from. These channel bands comprise a number of individual channels over a specific range of
frequencies in the 5GHz range. These bands include UNII-1 (5.15 – 5.25GHz), UNII-2 (5.25 – 5.35GHz),
UNII-2 Extended (5.47 – 5.725GHz) and UNII-3 (5.725 – 5.825GHz). The two UNII-2 bands are DFS
(Dynamic Frequency Selection) bands. The 802.11a specification identifies DFS bands as overlapping
with the frequencies utilized globally by radar systems. Because of this shared use for these two
frequency ranges the 802.11a standard calls for a zero contention behavior from wireless devices on the
channels in these bands. This means that a DFS channel can possibly become unavailable due to the
detection of radar signals by an access point on one of the DFS channels as required by the standard.
Also, the full set of channels available in the U.S. may not be available outside the U.S. Refer to your
local RF governing body for specific channel availability. In some cases where use of DFS channels is
either not allow due to legal restrictions or use of DFS channels is not desired, an eight-channel plan is
recommended as depicted in Figure 5. As illustrated, there is still separation of adjacent channels by at
least 1 cell. Same channel separation can now be a minimum 1 cell in a single plane, rather than in three
dimensions, because only eight channels are being utilized instead of all 23. Many sites use this pattern
with no reported issues.
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Best Practices Guide