The biggest questions in Wi-Fi telephony involve call quality. Whereas security standards have at least been set, QoS specifications are still emerging. What's more, ensuring wireless voice quality involves more than just protocols and traffic prioritization. It also requires ubiquitous radio coverage and smooth handoffs between APs.
All Wi-Fi clients need to share the same airwaves, so some kind of traffic prioritization mechanism is a must for voice links. The most popular is SpectraLink Voice Priority (SVP), a proprietary protocol that only works with SpectraLink phones and requires the NetLink SVP Server, a management device that can prioritize up to 850 simultaneous calls.
To make SVP work with nonvoice devices, QoS needs to be enforced by APs so that voice is always given priority over data. Most enterprise-class APs made in the last year do support it, as SpectraLink licenses SVP free to AP vendors. Thanks to its heritage in non-IP cordless telephony, SpectraLink is such a dominant player in the Wi-Fi handset market that even competitors such as Cisco, Avaya, and Extreme Networks include SVP in their APs.
Still, SVP is unlikely to survive in the long term. The IEEE is developing its own QoS standard, 802.11e, which will go beyond prioritization. Instead of simply giving some packets preferential treatment, it will likely let APs provision bandwidth to simulate circuit switching. This proposal is somewhat controversial: 802.11 chip leader Atheros opposes it because it makes less efficient use of bandwidth than simple prioritization. However, the company has committed to implementing it if passed--something that seems likely given its support from Cisco. 802.11e will also add better power management, which isn't strictly related to QoS, but is helpful for VoIP because it makes the battery life of Wi-Fi devices nearly as long as cell phones'.
Vendor disagreements mean that the full 802.11e standard won't be ratified until mid-2005, but the part of it that handles SVP-style prioritization has been ready since early 2004. The Wi-Fi Alliance has branded this element as Wi-Fi Multimedia Extensions (WMM, also known as WME) and began certifying products for interoperability in August. The first of these should be shipping around now, which gives network managers planning Wi-Fi telephony another dilemma: Is prioritization good enough, or should they wait for bandwidth provisioning?
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