* Cisco expert recommends voice site surveys
I recently spoke with Jon Leary, a product line manager in Cisco’s wireless networking business unit, about what enterprises might do to ensure success with voice over wireless LAN deployments.
VoIP’s fundamental sensitivities to latency and jitter are tricky enough to accommodate in a cabled packet-switching environment. The situation becomes more challenging when using the less-predictable radio-frequency medium, and delays caused by inter-access point (AP) handoffs enter the picture.
Leary recommends conducting a special “voice site survey.” One reason, he said, is because voice users will likely use the network in locations where data users don’t, such as in hallways and on staircases. Similarly, he said, a voice site survey will help you achieve more overlap among cells than you’ll typically need in a data-only WLAN – preferably 15% to 20% overlap – so your voice coverage will be seamless.
“This overlap also allows for load-balancing [among APs] to occur more easily,” Leary said, adding that a practical number of concurrent VoIP calls in an 11M bit/sec 802.11b LAN using a G.711 VoIP codec is about seven per cell.
In today’s environment, VoIP phones are primarily limited to 802.11b technology. One forthcoming exception is Motorola’s dual-mode GSM/802.11a CN620 phone, announced in July but not due to ship until the fourth quarter.
Leary, in fact, wonders whether 54M bit/sec WLANs (802.11a and 802.11g) will see much voice action because the higher the transmission rate, the higher the signal-to-noise ratio (SNR) required. Minimum SNR in an 802.11b network, he said, should be 25 decibels.
Given the current 802.11b-centric VoIP climate, here are some other noteworthy tips from Leary:
* In Cisco WLANs using Cisco 7920 phones, he advises to set APs and phones to transmit at the same power levels to prevent “one-way audio.” If APs are configured to transmit at varying power levels, set phones to the highest AP transmit power in use.
* In an ideal RF environment, non-overlapping 802.11b cells are a minimum of five channels apart (i.e., adjacent cells in the U.S. would run on channels 1, 6 and 11; in Europe they would run on 2, 7 and 12).
* He recommends no more than one AP per overlapping channel set to avoid additive noise.
