Networking Lesson: Prioritising wireless traffic

Learn how Wi-Fi multimedia can help your WLAN offer better service to users by prioritising traffic flows.

EDCA matured well before its contentious sibling. However, before 802.11e could be finalized, proprietary methods began to emerge. To nip interoperability problems in the bud, the Wi-Fi Alliance established the WMM certification program. Four years later, over 850 products have passed WMM tests for prioritizing airtime using EDCA. Network administrators have long used techniques like 802.1d and DiffServ to give selected traffic preferential treatment. As wireless LANs (WLANs) mature from handling casual Internet access to carrying challenging business applications and multimedia traffic, prioritization must be extended to the wireless edge. This can be accomplished using Wi-Fi Multimedia.

What is Wi-Fi Multimedia?

Wi-Fi Multi Media, also known as Wireless Multimedia Extensions (WME), is a subset of IEEE 802.11e MAC Enhancements for Quality of Service (QoS). This amendment specifies 802.11 extensions to optimize application performance in WLANs that carry a mixture of voice, audio and video. Specifically, 802.11e defines two ways to control airtime use: prioritization using Enhanced Distribution Channel Access (EDCA) and scheduling using the Hybrid Coordination Function (HCF).

802.11e also specifies a Power Save (PS) extension to help conserve battery life. PS lets clients sleep between transmissions to reduce power consumption. 802.11e improves PS by giving clients (and their applications) better control over when they can ask for data received while asleep. In late 2005, the Wi-Fi Alliance established another program -- WMM Power Save -- to promote interoperable implementation of this enhancement.

Who can benefit from Wi-Fi Multimedia

In situations where all applications and users deserve the same treatment and access is not crucial, priority is irrelevant. But RF channels are shared media. 802.11 clients use a Distributed Coordination Function (DCF) to play nicely with others contending for the same channel. Whenever a client has data to send, it first listens to determine whether someone else is transmitting. If the channel is busy, the client must wait a random back-off period before trying again.

DCF is fine when airtime is plentiful. But when diverse applications share the same WLAN, bandwidth hogs -- like media players -- can starve other users. Furthermore, as load increases, everyone suffers by waiting longer to transmit. Brief delays may go unnoticed when Web surfing, but a gap of just 100 milliseconds can cause voice call drop outs. Buffering makes streaming less vulnerable, but audio and video applications still need consistent chunks of airtime to perform well.

WMM uses sender-specified priority to provide every application with its "fair share" of the channel. Note that WMM cannot guarantee airtime -- a device that refuses to play fair can still transmit for long periods and block everyone else out. Nonetheless, WMM is an important building block for keeping everyone happy in multi-media WLANs.

How WMM works

To better manage airtime, WMM classifies Wi-Fi "QoS Data" frames into four Access Categories (ACs):



  Type of Application



  Toll-quality VoIP calls requiring low latency.



  SDTV or HDTV video streams.

  Best Effort


  Applications affected by lengthy delays and legacy (non-
  QoS) users/applications.



  Latency-insensitive batch transfer applications.

When IP packets are submitted to the MAC layer for transmission, User Priority (UP) can be specified by 802.1d tags or Differentiated Services Code Point (DSCP) headers. WMM maps that UP (if any) to the corresponding AC -- for example, top-priority packets are sent as Voice frames. QoS Data frames are then inserted into one of four separate transmit queues. (Some APs even implement four transmit queues per client or SSID.)

Parameters determine how often QoS Data frames from each category can be transmitted:

  • Arbitrary Inter-Frame Space Number (AIFSN) controls how long the client waits after each transmission. AIFSN defaults range from 2 timeslots for voice to 7 timeslots for the background traffic.
  • Contention Window (CW) controls the random back-off interval used when channel is busy. By default, Background and Best Effort min and max CW are twice as long as those used for Video and four times those used for Voice.

The net result is that higher-priority frames receive more "transmit opportunities" than lower-priority frames. When several queues have data to send, high priority queues also get serviced more often than lower priority queues.

Without prioritization, airtime competition can significantly degrade voice quality. On the flip side, because VoIP sends many short packets, even one phone call keeps a channel pretty busy. WMM prioritization stops latency-sensitive applications like VoIP from being crippled by delay and jitter, while preventing low-priority applications from being shut out altogether. (To hear the difference, listen to this Interop 2006 demo.) WMM Power Save further improves life for small devices like VoIP handsets and media players that are latency-sensitive by letting them "doze" longer without degrading quality.

Putting WMM into action

If your WLAN is composed of recent devices -- especially business APs and VoIP handsets -- you probably have the equipment to take advantage of WMM. To determine whether your gear supports WMM, search the Wi-Fi Alliance's certified products list.

To prioritize traffic with WMM, both the AP and client must support EDCA. WMM is backwards-compatible, meaning that an AP can prioritize traffic from QoS-aware clients while also servicing traffic from legacy clients. This is accomplished by including WMM support -- and the parameters for each AC -- in AP beacons. For example, this beacon was sent by an Actiontec AP:

beacon sent by an Actiontec AP

Legacy clients ignore this unknown element, while QoS-aware clients agree whether to use WMM when they associate to the AP. Some clients must be explicitly configured to enable WMM. For example, see this Dell laptop Wi-Fi card:

Dell laptop Wi-Fi card

Many other clients do not expose WMM settings -- if they associate to an AP that advertises WMM, they will use it. But negotiation results may still be visible -- for example, see QoS and Power Save Mode values in this Cisco client's Status:

QoS and Power Save Mode values in Cisco client's Status

However, agreeing to use WMM is not enough. To get anything other than Best Effort, applications must supply User Priority values (802.1d tags or DSCP flags) in the IP packet they generate. For example, compare the following pair of frames:

Frame 1   Frame 2
untagged frame   encrypted frame with User Priority 6

Frame 1, at left, is an untagged frame (IP Type of Service 0). Frame 2, at right is an encrypted frame with User Priority 6. Although that IP packet is obscured by encryption, we know that it carries Type of Service=6, which WMM mapped to the Voice category. Indeed, the packet on the right was generated by a QoS-aware softphone program.

In fact, tagging is a critical step. Purpose-built Wi-Fi devices that are WMM-capable often run software that tags outbound packets. Examples include Wi-Fi SIP Phones like SMC Tigervoip and Cisco Unified Wireless IP phones, dual-mode Smartphones like RIM BlackBerry 8820 and Sony Ericsson W960i, media centers like Linksys DMA2100 and Philips MCP9360i, and Set Top Box/DVRs like HP MediaSmart Connect.

On general-purpose devices like laptops, start with a WMM-capable adapter like the Dell and Cisco cards mentioned above. However, applications must also be developed to use WMM. To learn more about Windows applications, see Microsoft QoS Support and Vista WMM Support articles.

Finally, QoS algorithms must be applied at the AP and integrated with wired network traffic management. Some APs just use defaults specified by 802.11e. Other APs provide configurable WMM parameters per radio or SSID, like this Motorola AP-5131:

Motorola AP-5131

However, enabling WMM only prioritizes airtime. Additional AP and/or Controller parameters determine how QoS operates within the AP and how packets are tagged at ingress/egress interfaces.

For example, your AP may let you specify WMM ACs applied to traffic sent to individual clients (MAC addresses) or applications (TCP/UDP ports). Or you have control over how inbound DSCP or 802.1d tags are mapped to outbound packets, regardless of application payload. Many business-grade APs offer further bandwidth management features, like weighted queuing when a radio/channel supports several virtual APs, or traffic shaping based on IP Type of Service.

These capabilities vary quite a bit, so examine your own gear and how it fits WMM into "big picture" QoS. To learn more about WMM and end-to-end QoS management for voice over Wi-Fi, see the Wi-Fi Alliance's Voice-Personal and upcoming Voice-Enterprise programs.

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