DTPC may hobble VoWLAN

As old battles come to an end (Fred Ikle, a high-level Pentagon official in the Nixon years, captured this fact in the title of his acclaimed book, Every War Must End), new battles erupt. The battle over thin vs. fat APs has largely been settled in favor of distributed switching architectures (see "Spotlight" below), so now a new disturbance looms for WLANs: The advantages and disadvantages of dynamic transmit power control (DTPC) capabilities. 

Joanie Wexler writes that some of the RF fine-tuning capabilities built into WLAN gear may hinder WiFi networks optimized for VoIP. In evidence: The DTPC capability in Cisco and other vendors' WLAN products. The DTPC, for example, helps to keep a network going if an AP fails by increasing the power of neighboring APs, thus allowing these APs to compensate for their fallen comrade. Things get a bit more complicated in voice-enabled WLANs.

Kurt Mensch, senior product manager at SpectraLink, explains that the reason for the problems is that voice transmission demands consistent power levels across all APs for optimum performance. Wexler quotes Mensch to say that "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."

Talk about a Catch-22: The opposite is also true, as Mensch admits that if an AP fails and the neighboring APs do not compensate (because DTPC is disabled), then the WLAN may cover an area insufficiently, which could also hobble VoIP quality. "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."

For more on DTPC
- see Joanie Wexler Techworld report
- Cisco's information webpage 
- and Anmol Sheth's University of Colorado White Paper

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