Tests indicate VoIP in 802.11b clobbers performance

The Tolly Group has just posted the results of its evaluation of the Airespace 4000 Wireless Switch and 1200 Access Point for its support of voice over IP over 802.11b wireless LANs. Airespace commissioned the tests.

The Tolly Group results indicate that the Airespace gear can successfully prioritize voice and hand off calls within and between subnets with no interruptions. The tests determined that the gear could support 14 simultaneous VoIP conversations – Spectralink IP phones were used – within one basic service set, or BSS (a network with one AP running).

But a big takeaway is the performance hit that VoIP sessions impose on WLAN data. Because WLANs throttle back non-real-time traffic when running quality-of-service (QoS) for VoIP, data performance suffers in the relatively low-speed 802.11b environment.

By way of example, in the Tolly Group-Airespace tests, when a wired PC generated nearly 9.77M bit/sec of traffic, wireless clients received just 2.68M bit/sec of that traffic when VoIP sessions were active. But when VoIP traffic went away, wireless clients received 6M bit/sec.

This is one reason it’s a good thing that 54M bit/sec WLAN options – particularly those in the 5 GHz frequency, which provide lots of additional nonoverlapping channels – are becoming available.

The Tolly Group didn’t actually test VoIP quality (how good the conversation sounded). The test simply presumed that because bandwidth for non-real-time traffic throttles back to provide sufficient bandwidth for VoIP, good voice quality will be present.

It was a little confusing that both Airespace’s own proprietary QoS mechanisms (three service classes) and the proprietary SpectraLink Voice Prioritization (SVP) protocol, which runs in SpectraLink phones and gateways, were activated. The report implies that SVP and Airespace’s own queuing mechanisms worked together to provide sufficient bandwidth for VoIP.

However, Jeff Aaron, Airespace’s senior manager of product marketing, says, “All performance metrics gathered by Tolly used Airespace’s own QoS. While we demonstrated support for SVP in the test bed, all the actual tests and the subsequent results… where handled via our own QoS scheme.”

He said the reason Spectralink handsets and gateways were part of the testbed was that “Spectralink is the clear leader in VoWLAN gateways and handsets. We wanted to show that we are fully interoperable with their solution.”