In addition to prioritized access, 802.11e will provide scheduled access, where applications can reserve bandwidth resources. This HCCA (Hybrid Coordination Function Controlled Channel Access) is a centralized scheduling mechanism that provides more deterministic network access. A VoIP app, for instance, would send a resource-reservation request to the AP. The AP would then provide an appropriate assignment based on predefined metrics, including data rate, PHY rate, packet size, service interval and burst size.
WMM and 802.11e are major developments for WLANs, but QoS alone isn't enough. No matter how sophisticated your QoS parameters, if the WLAN is poorly designed and lacks sufficient capacity, your apps won't perform and users will notice. Most enterprises need a multiband 802.11 a/b/g wireless infrastructure as well with densely positioned APs. It's this combination of smart design and QoS that will make wireless the right infrastructure for voice and other applications.
Dave Molta is a Network Computing senior technology editor. He is also assistant dean for technology at the School of Information Studies and director of the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected].
802.11 MAC relies on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), a slightly more sophisticated variation of the Ethernet CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol. When the pipe on an 802.11 network becomes available, a node accesses it by observing two timing parameters: the interframe space, a predefined dead time between packets; and the contention window (sometimes called the random back-off wait), the period when nodes contend for access to the medium.
Because even legacy 802.11 networks need to prioritize acknowledgment and management frames over normal data frames, the 802.11 spec defines multiple interframe spaces used by different types of packets. The shorter the interframe space a node needs to observe, the higher its priority. So because a frame acknowledgment's interframe space is shorter, it always gets higher priority access to the medium.
