In July 2005, the 802.11e MAC Enhancements for Quality of Service (QoS) standard achieved the consensus needed
for formal IEEE approval. This long-awaited QoS extension will improve delivery of multimedia data, voice, audio, and video traffic over 802.11a/b/g networks. In this tip, we'll look at what 802.11e QoS provides and how you can find products that implement Wi-Fi Multimedia (WMM), a draft subset of 802.11e.
Contending for airtime
In 802.11 networks, stations connected to a given Access Point (AP) transmit and receive over the same shared channel. If more than one transmits simultaneously, a collision occurs. To avoid collisions and communicate effectively, stations take turns transmitting. The original 802.11 standard defined two methods:
- Today, all 802.11 products implement the Distributed Coordination Function (DCF). Stations using DCF monitor the channel to determine whether someone else is transmitting. If a station with traffic to send finds the channel in use, it waits a random backoff period before listening again and trying to transmit.
- A few 802.11 products also implement the Point Coordination Function (PCF) option. An AP using PCF controls channel use by polling stations for traffic. Stations with traffic to send are assigned "contention free" periods during which they can transmit.
DCF does a good job of letting stations send data while avoiding collision, particularly in WLANs where all stations can hear each other, but it doesn't provide any mechanism for asserting priority. In a lightly-used WLAN, a chatty station can easily hog more than its "fair share" of the channel. But, as load increases, all stations suffer equally by waiting longer to transmit. Although PCF enables AP control over channel utilization, it provides no mechanism for the AP to know what kind (priority) of traffic each station has to send.
This makes it hard to give latency-sensitive applications like VoIP or streaming audio/video preferential treatment over "normal" data applications. Place a VoIP call on a quiet WLAN and you'll be fine – until someone fires up a big file transfer. As contention grows, your VoIP call will experience jitter and delay, resulting in voice drop outs or disconnection.
The solution is conceptually simple: just give that VoIP call higher priority than file transfer packets. If we added streaming video to the mix, we might want three priorities: VoIP before video, video before other data. In fact, the 802.11e Enhanced Distributed Channel Access (EDCA) mechanism uses 802.1d user priority (DiffServ tags) to classify traffic into four access categories:
Voice: Priority 7 or 6 for toll-quality VoIP calls requiring low latency.
Video: Priority 5 or 4 for SDTV or HDTV video streams.
Best Effort: Priority 3 or 0 for latency-insensitive, interactive applications.
Background: Priority 2 or 1 for batch data transfer applications.
For backwards compatibility, traffic from legacy (non-QoS) stations falls is treated as though it were categorized as "Best Effort."
Stations that implement 802.11e EDCA use separate transmit queues corresponding to these four classes. Whenever the station transmits, it pulls traffic from higher priority queues before servicing lower priority queues. Furthermore, parameters that determine how long stations wait when the channel is busy and how long a given station can transmit are adjusted to reflect priority. In effect, higher access category traffic can be sent more frequently, for longer durations, than lower access category traffic.
This "prioritized QoS" EDCA mechanism is part of the new Hybrid Coordination Function (HCF) defined by 802.11e. For backwards compatibility, a QoS BSS (QoS-enabled WLAN) must have one DCF and one HCF. To promote multi-vendor interoperability, the Wi-Fi Alliance tests 802.11 products that implement EDCA; those that pass are "Wi-Fi Multimedia" (WMM) certified. To learn more, consult this Wi-Fi Alliance WMM Q&A (PDF).
WMM was based on a 2004 draft subset of the IEEE 802.11e standard. The final IEEE 802.11e standard also includes a "parameterized QoS" mechanism called HCF Controlled Channel Access (HCCA).
Like the old PCF mechanism, HCCA controls channel access through AP-directed station polling. The AP's Hybrid Controller takes QoS into consideration when scheduling station transmission times and durations, giving some traffic a bigger share of the channel. Stations using HCCA submit reservation requests; the AP then assigns transmit opportunities based on 8 possible Traffic Stream Identifiers. TSIDs are themselves based upon Transmission Specification (TSPEC) parameters like data rate, burst size, and service interval.
This parameterized QoS mechanism is arguably more complex than prioritized QoS. For example, HCCA requires stations to know what they'll want to send in advance. However, in WLANs used primarily for voice or video streams, HCCA with well-tuned QoS parameters can enable more efficient channel utilization by eliminating "wasted" backoff time.
"Quality of service means more than just prioritizing voice packets. QoS can also impact call capacity and handset battery life which are less important for consumers but critical in enterprise applications." said Ben Guderian, Spectralink's vice president of Market Strategies. "Ongoing enhancements to WMM such as power save and scheduled access mechanisms will make the standard a much better solution for enterprise Wi-Fi deployments."
The Wi-Fi Alliance did not include these mechanisms in the 2004 WMM snapshot because they were still being refined. But it is now working on a WMM Scheduled Access (WMM-SA) certification program that aligns with the final 802.11e standard. WMM-SA is currently scheduled for the first half of 2006.
Finding QoS support in products
Several vendors support proprietary QoS mechanisms, particularly for VoIP devices. For example, a number of enterprise APs implement the SpectraLink Voice Priority (SVP) mechanism employed by SpectraLink's NetLink wireless (VOIP over Wi-Fi) phones. In fact, SpectraLink's Voice Interoperability for Wireless (VIEW) certification program tests other-vendor APs that implement SVP for interoperability and performance.
Proprietary mechanisms like SVP will continue to offer good QoS for homogenous networks, but companies that want to run data and voice over a converged, heterogeneous network -- for example, using a variety of handsets, soft phones, and media servers -- should look for products that support WMM today, and/or WMM-SA in the future.
Many built-in laptop wireless adapters and APs sold to the enterprise market are adopting WMM, as are devices designed for wireless multimedia distribution within the home. The Wi-Fi Alliance has been certifying WMM products for about a year now. By September 2005, over 100 WMM-certified products were listed on the Wi-Fi Alliance website, including many Wi-Fi APs, PC cards, and miniPCI cards, and a few USB adapters, media gateways, and routers. To determine whether a given product has passed WMM testing, just search the Wi-Fi Alliance's Certified Products page.
Preliminary scheduled access implementations have been around for awhile. Finalization of 802.11e can be expected to spur products updates. Many products that support WMM now may add SA – particularly those marketed for enterprise voice/video delivery. For example, Colubris Networks hopes to include WMM-SA as a standard feature by mid-2006. "[WMM-SA] is the next in a series of enhancements targeted to optimize voice traffic over WLANs," said Roger Sands, Colubris' vice president of Engineering.
Of course, 802.11e is not the only standard that will impact wireless QoS. For example, the emerging 802.11k standard will define Radio Resource Measurement enhancements to help wireless systems get a better real-time handle on performance. The forthcoming 802.11r standard will concretely define fast roaming methods to reduce handoff delay, improving support for mobile applications like (secure) VOIP.
Nonetheless, finalization of 802.11e is a big step in the right direction. Getting over this hump should increase availability of interoperable WMM (and next year, WMM-SA) certified products. Standard QoS mechanisms will help today's "best effort" WLANs mature, letting them deliver the manageable performance required by tomorrow's wireless multimedia applications.
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About the author: Lisa Phifer is vice president of Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of data communications, internetworking, security, and network management products for nearly 20 years. She teaches about wireless LANs and virtual private networking at industry conferences and has written extensively about network infrastructure and security technologies for numerous publications. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.