Most of today's 802.11 networks rely on the CSMA/CA MAC (media access control) protocol in conjunction with DCF (Distributed Coordination Function). The 802.11 standard provides for an alternative PCF (Point Coordination Function), which provides basic QoS capabilities, but it is generally considered too inefficient and too limited to support emerging needs. The 802.11e draft provides for two more mechanisms: EDCF (Enhanced DCF) and HCF (Hybrid Coordination Function). EDCF improves upon the existing DCF MAC specification by supporting up to eight priority traffic classes that map directly to the RSVP protocol priority levels.
Although stations are constantly transmitting and attempting to gain access to the medium, there are idle times between transmissions called interframe spaces. Several such spaces are already defined in 802.11, including SIFS, PIFS and DIFS. EDCF adds AIFS (Arbitration Interframe Space). Higher priority traffic will have lower AIFS idle times, providing media access priority over other stations. Even if there are several stations within the same priority traffic class, there is still a variable length contention window with its own countdown. HCF builds upon PCF's round-robin polling mechanism by providing more intelligent polling algorithms.
Since the IEEE 802.11e group is not expected to approve a QoS standard until mid-2004 at the earliest, interim QoS industry standards have begun to appear, in large part to address the needs of developers wishing to distribute multimedia content over WLANs. The WME (Wireless Multimedia Enhancements) standard has been touted as a stand-in for the uncompleted 802.11e. Already, Atheros Communications, Broadcom and Intersil are producing chipsets that include support for the WME standard. Cisco has pushed the envelope a bit by implementing a November 2002 draft specification of 802.11e in its newer products and firmware sets.
Most of the systems we reviewed took one of two approaches. SpectraLink has for several years offered its own voice-priority specification, SVP (SpectraLink Voice Priority), and support for the standard has been incorporated into many products from vendors such as Airespace, Avaya, Cisco, Enterasys, Intermec, Proxim and Symbol. SVP operates through a queuing mechanism built into the access points that provides priority to packets like voice that require real-time processing.
An alternative approach, such as the one used by Cisco on its upstream connection, involves lowering the value of the 802.11DCF CWmin (minimum contention window) parameter, which controls the idle wait time between frames used to determine when devices can transmit. Wireless devices choose a random value between 0 and CWmin before transmitting. Most wireless NICs and other devices have a CWmin value of 32 defined, so that on average, they must wait for 15.5 idle slots to pass by before they can transmit. Wireless phones that want early access to the medium merely lower their CWmin to 0 or 1, so that at most they need to wait only 1 idle slot before transmitting. This works well if there is only one device that has a small CWmin, but if there are multiple devices requiring prioritized access, it can lead to high collision rates. --Frank Bulk
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