University reduces AP density - for voice

* U of Arizona adds RSSI to design variables

It's common wisdom that shops wishing to run voice over wireless LANs (VoWLAN) must deploy more access points than they would if installing Wi-Fi for data-only transmission. The denser deployments, of course, help plug coverage gaps that could otherwise cause live calls to disconnect. The University of Arizona, however, discovered that a too-dense AP deployment carries its own set of problems.

When the university initially installed its 5000 Cisco lightweight APs across 65 buildings, it did so with both data and voice in mind. “Rather than having employees at desks using their cell phones [while ignoring] an $800 desk phone,” the university wanted to buy users one Wi-Fi phone and avoid usage charges for local mobile calls, says Justin Miller, network systems analyst at the University of Arizona in Tucson.

The university used a 25dB signal-to-noise ratio (SNR) measurement scheme to determine the number and placement of APs to accommodate voice on the advice of a third-party project manager. However, the plan didn’t account for received signal strength indication (RSSI) levels. The university also initially used a third-party planning tool that didn’t allow for whitespace in channels adjacent to active ones to alleviate interference.

The result? “We found the infrastructure to be 20% to 50% overbuilt,” Miller says, which caused a large volume of client-to-AP re-associations and general voice-session instability. The too-dense AP deployment meant that the Wi-Fi phones would ping-pong their associations between APs and drop calls, he explains.

So the university switched to AirMagnet WLAN Planner tool for its 802.11a/b/g networks. “We learned by using the tool that our initial planning techniques were flawed,” Miller says. His redesigned the AP layout to ensure -68dBm signal strength, and now the network “works flawlessly,” he says.

The university runs Cisco Aironet 1130 802.11a/b/g APs that cover about three-fourths of the university’s student population and four Cisco Draft N 802.11n 1250 APs outdoors. For voice, university personnel use Cisco, Nokia, and T-Mobile Wi-Fi-enabled handsets.

For use of its own 7921 Wi-Fi phones, Cisco recommends a minimum planned signal level threshold of -67dBm and an SNR of 25dB to ensure a good user voice experience.

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