Voicing support

By Andrew Garcia  |  Posted 2005-10-31 Print this article Print

Many wireless networks were designed for data access only, so organizations looking to deploy VOW must first determine whether their WLAN (wireless LAN) will adequately support voice for a large user base.

The WLAN will need to provide sufficient RF (radio frequency) coverage to minimize dead spots and be able to accommodate heavy user densities in many locations. Scalability, security and QOS (quality of service) features will also be of paramount importance.

Unlike wireless devices designed for data, VOW phones do not require much bandwidth. The phones weve seen to date generally support the G.711 a-law and u-law and G.729 codecs, so administrators should expect only 8K- or 64K-bps (unidirectional) throughput per call.

Handoff times and roaming performance will be critical metrics by which wireless phones will be judged. Handoff times can be measured by the amount of time it takes between data transmissions through different access points—the time between the last packet sent through access point A and the first packet sent through access point B.

During this interval, a wireless client is probing for an access point with a stronger signal and then associating and authenticating to this access point.

The human ear can detect latency and delay issues that take longer than 50 to 80 milliseconds, and extended handoff times (500 ms or more) can lead to dropped calls.

Gearing up for this wireless phone test, we performed a number of practice tests on wireless clients designed for data access. In these informal tests, we saw excellent sub-30-ms handoffs from some high-end adapters and sluggish times (well over 1 second) from some consumer-grade and integrated wireless devices.

Our final tests showed similar variability in our small sample group, as well as conclusive evidence that a very common brand of wireless adapter makes for a poor roaming soft phone.

Next Page: To the test.

Andrew cut his teeth as a systems administrator at the University of California, learning the ins and outs of server migration, Windows desktop management, Unix and Novell administration. After a tour of duty as a team leader for PC Magazine's Labs, Andrew turned to system integration - providing network, server, and desktop consulting services for small businesses throughout the Bay Area. With eWEEK Labs since 2003, Andrew concentrates on wireless networking technologies while moonlighting with Microsoft Windows, mobile devices and management, and unified communications. He produces product reviews, technology analysis and opinion pieces for eWEEK.com, eWEEK magazine, and the Labs' Release Notes blog. Follow Andrew on Twitter at andrewrgarcia, or reach him by email at agarcia@eweek.com.

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