SpectraLink Corp.'s proprietary SVP (SpectraLink Voice Priority) technology gets around this problem by making media access less fair. VoIP phones supporting SVP can jump to the head of the line. Most WLAN infrastructure vendors have implemented SVP in their devices.
SVP provides wireless QoS by specifying a back-off value of zero for voice packets. That gives voice packets priority over normal data packets. The downside is that if multiple SVP VoIP phones attempt to transmit at the same time, you'll get collisions. If too many phones exist in a given cell, the phones can monopolize available bandwidth and thus starve out conventional data users. To get around this, limit the number of handsets per cell to a dozen or less. For the nuts and bolts of how media access is controlled on 802.11 networks, see "How 802.11 Media Access Works," below.
On the wired side, SVP also adds priority queuing to the AP--using a unique protocol type in the IP header--to give voice packets priority.
SVP is the de facto QoS standard for WLANs, but SpectraLink is well-aware that vendor-neutral standards are key to widespread adoption. So the company is backing efforts by the IEEE to enhance the 802.11 MAC with QoS support. The IEEE's 802.11e task group, which has been working on this for about five years, is expected to release a final standard early next year.
The snail's pace of the IEEE QoS effort spurred the Wi-Fi Alliance to develop a prestandard subset of QoS functions for the short term that will work with the eventual 802.11e standard. This strategy is much like the Wi-Fi Alliance's effort with the WPA (Wi-Fi Protected Access) security scheme, a subset of the capabilities that eventually emerged in the 802.11i security standard.
