WMM protocol

The Distributed Coordination Function (DCF) in 802.11 requires APs and clients to use the carrier sense multiple access with collision avoidance (CSMA/CA) access mechanism. APs or clients listen to the channel before they hold the channel for data transmission. When the specified idle duration of the channel times out, APs or clients randomly select a backoff slot within the contention window to perform backoff. The device that finishes backoff first gets the channel. With 802.11, all devices have the same idle duration and contention window. Therefore, they are equal when contending for a channel.

To provide QoS services, WMM divides data traffic into four ACs that have different priorities. Traffic in an AC with a high priority has a better chance to use the channel.

• Enhanced distributed channel access—EDCA is a channel contention mechanism defined by WMM to preferentially transmit packets with high priority and allocate more bandwidth to such packets.

• Access category—WMM defines the following ACs: AC-VO for voice traffic, AC-VI for video traffic, AC-BE for best effort traffic, and AC-BK for background traffic. The priorities of the four ACs are in descending order.

• Connect Admission Control—CAC limits the number of clients that can use high-priority ACs (AC-VO and AC-VI) to make sure there is enough bandwidth for these clients.

• Unscheduled automatic power save delivery—U-APSD is a power saving method defined by WMM to save client power.

• Arbitration inter-frame spacing number—In 802.11-based WLAN, each client has the same idle duration (DIFS), but WMM defines an idle duration for each AC. The idle duration increases as the AIFSN increases.

• Exponent form of CWmin/Exponent form of CWmax—ECWmin/ECWmax determines the backoff slots, which increase as the two values increase.

• Transmission opportunity limit—TXOP limit specifies the maximum time that a client can hold the channel after a successful contention. A larger value represents a longer time. If the value is 0, a client can send only one packet each time it holds the channel.

CAC requires a client to obtain permission from an AP before it can use a high-priority AC for transmission. This guarantees bandwidth for the clients that have gained access. CAC controls real time traffic (AC-VO and AC-VI traffic) but not common data traffic (AC-BE and AC-BK traffic).

If a client wants to use a high-priority AC (AC-VO or AC-VI), it must send a request to the AP. The AP returns a positive or negative response based on either of the following admission control policies:

• Channel usage-based admission policy—The AP calculates the total time that the existing high-priority AC queues occupy the channel per unit time, and then calculates the time that the requesting traffic will occupy the channel per unit time. If the sum of the two values is smaller than or equal to the maximum hold time of the channel, the client can use the requested AC queue. If it is not, the request is rejected.

• Client-based admission policy—If the number of clients using high-priority AC queues is smaller than the maximum number of high-priority AC clients, the request is accepted. If it is not, the request is rejected. During calculation, a client is counted as one client if it is using both the AC-VO and AC-VI queues.

If the request is rejected, the AP assigns AC-BE to clients.

U-APSD enables clients in sleep mode to wake up and receive the specified number of packets only after receiving a trigger packet. U-APSD improves the 802.11 APSD power saving mechanism.

U-APSD is automatically enabled after you enable WMM.

WMM defines the following ACK policies:

• Normal ACK—The recipient acknowledges each received unicast packet.

• No ACK—The recipient does not acknowledge received packets during wireless packet exchange. This policy improves the transmission efficiency in an environment where communication quality is strong and interference is weak. If communication quality deteriorates, this policy might increase the packet loss rate. For A-MPDU packets sent by 802.11n clients, the No ACK policy does not take effect.