Vo-Fi and dynamic power control: A possible mismatch?

* Irregular AP transmit levels can degrade voice

The RF fine-tuning capabilities built into wireless LAN equipment help keep WLANs running at optimal performance and build in access point redundancy. But some of these capabilities can actually hinder Wi-Fi networks optimized for VoIP.

Take, for example, the dynamic transmit power control (DTPC) capability in Cisco and other WLAN system vendors’ products. This is a valuable capability for continued network uptime in the event of an AP failure; in such cases, with DTPC enabled, surrounding APs will increase their power to fill in the gap created by the coverage loss.

However, in voice-enabled Wi-Fi networks, DTPC might cause problems.

“You cannot have dynamic adjusting [in a Vo-Fi environment]; APs must be static,” says Joe Fearday, telecommunications manager at Trinity Health in Farmington Hills, Mich. The healthcare provider uses Cisco Wi-Fi infrastructure equipment with SpectraLink wireless phones. “The benefits [of dynamic AP adjustment] go away in a voice-optimized network.”

The reason? Voice traffic requires consistent power levels across all APs for optimum performance, explains Kurt Mensch, senior product manager at SpectraLink.

“We recommend that all APs are set to the same power level for optimal performance. If you use DTPC, the wireless network coverage will be inconsistent; the coverage overlaps will no longer be according to the intended original layout. We also actively scan for APs to hand off to, but we don’t know the AP power until we authenticate to it so we need to make assumptions that the power output is the same on all APs.

"In addition, in many cases the AP needs to turn the radio off to change the power,” he continues. “This can cause dropped calls or audible drops in service.”

If an AP fails and the surrounding APs don’t compensate because DTPC is disabled, you do run the risk of having insufficient coverage in that area, which could also affect VoIP quality, Mensch acknowledges.

“But cranking up the power on the adjacent APs can cause other problems from interfering with other APs on the same channel,” he says. “The preference is to know about a failed AP as soon as possible and deal with it rather than trying to compensate for it and creating problems in other areas.”

Copyright © 2006 IDG Communications, Inc.

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