Dont Put Voice over Wi-Fi on Hold

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.