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Certification Shelved for Voice QoS over Wireless: Page 3 of 4

HCCA was intended to provide assured delivery using "scheduled access"--a way for a wireless AP to grant time slots to clients for contention-free periods. The AP would receive reservation requests from stations, check resource availability, and grant or refuse the access. This is "admission control" and is often referred to as "parameterized QoS," because it receives the station requests in the form of parameters--what priority is required, amount of time needed, and so on--then controls transmission time and duration. The AP coordinates all access and contention-free periods. Clients must be aware of HCCA for the system to work. HCCA gives time-sensitive apps, such as voice, guaranteed access to the wireless medium.

HCCA isn't well-suited for unlicensed spectrum. Technology based on licensed spectrum has an advantage: In a managed RF environment, it's possible to guarantee access without concern for third-party interference. With unlicensed spectrum, tight coordination within your domain is impossible because nothing prevents another operator or technology operating in the same frequency from impacting the planned coordination. In many ways, HCCA provides a false sense of reliability. HCCA isn't suitable unless you are in a campus environment with a strict airspace policy or in a warehouse isolated from foreign interference. Interference in the unlicensed spectrum is the biggest obstacle to "guaranteed" QoS over wireless: The 802.11e standard states that "in unlicensed spectrum, true guarantees are often not possible. However, gradations of service are always possible, and in sufficiently controlled environments, QoS guarantees can truly be made."

The other hurdle with HCCA is that its implementation requires more memory and processing power on the AP, which increases costs; and, it's difficult to retrofit existing equipment. There must be a significant market driver for companies to implement HCCA; it's typically not a simple upgrade except on the newest hardware with sufficient memory and processing power. Additional intelligence is also required in roaming standards to handle HCCA, since it's controlled by the AP; unfortunately, that newer fast-roaming standard (802.11r) isn't ratified yet.

Stalled Certification

The Wi-Fi Alliance stated in 2004 and 2005 that it would begin to certify products based on HCCA in 2006, but that hasn't happened. The group quietly dropped WMM-SA in 2006. That means that the announced testing of WMM-SA has effectively been shelved.