Understanding Wi-Fi Multimedia

The Wi-Fi Multimedia (WMM) specification provides prioritization of packets for four types of traffic:

Voice—High priority queue with minimum delay. Time-sensitive data such as VoIP and streaming mode are automatically sent to this queue.
Video—High priority queue with minimum delay. Time-sensitive video data is automatically sent to this queue.
Best effort—Medium priority queue with medium throughput and delay. Most traditional IP data is sent to this queue.
Background—Lowest priority queue with high throughput. Bulk data that requires maximum throughput but is not time-sensitive (for example, FTP data) is sent to the queue.

Priority is based on the 802.11 Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. When multiple devices try to access the wireless medium at the same time, packet collisions can occur. To minimize the chances of packet collision, a client must wait for a randomly selected time and then check to see if any other device is communicating on the medium before it starts to transmit.

The wait time consists of a fixed period called the arbitration interframe spacing (AIFS) followed by a random period called the contention window. You can specify the minimum and maximum contention window values.

The WMM specification suggests different wait times for each of the traffic queues so that applications that are sensitive to packet delays have less time to wait and therefore have a better chance of transmitting on the network. To allow consistent QoS across both wireless and wired networks, the queues defined in the WMM specification map to IEEE 802.1d prioritization tags (see Table 4).

Table 4: WMM Queues to IEEE 802.1d Tag Mapping

WMM Queue	IEEE 802.1d Tag
Voice	7, 6
Video	5, 4
Best Effort	0, 3
Background	2, 1

Understanding Wi-Fi Multimedia

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