Standard set to boost wireless QoS

By Tim Greene
Network World, 04/18/05

A standard to define QoS in Wi-Fi networks is coming soon, but even before it is finalized users can expect QoS improvements from vendors that have implemented their own performance-enhancing technologies.

Wireless hardware vendor Colubris Networks says a pending software upgrade will improve its monitoring of call quality, so perceived bad quality can trigger automatic adjustments to a wireless network or alert administrators to deal with problems it has identified elsewhere on a connection.

Aruba Wireless Networks will support an informal QoS standard created by the Wi-Fi Alliance that is already supported by more than a dozen other wireless vendors. Similarly, Trapeze Networks will support the same Wireless Multimedia (WMM ) specification with its next software release, due within 90 days.

The flurry of activity surrounding Wi-Fi QoS is mainly due to the growing popularity of voice over Wi-Fi (VoWi-Fi ) and the demand that phone calls be reliable and intelligible, says Ellen Daley, an analyst with Forrester Research. A host of vendors, including 3Com, Broadcom, Cisco, Linksys, Conexant, D-Link Systems, HP, IBM, Intel, NEC and Netgear, already have certified their gear is WMM-compliant.

WMM is a subset of the IEEE RFC known as 802.11e , which vendors expect will be approved this year. WMM was created to promote use of QoS that would be interoperable among multivendor Wi-Fi gear, she says. "That means businesses can do voice over wireless pretty respectably today. They may need to upgrade when the standard comes out if they want to be standards-compliant," Daley says.

But for most users, the life cycle of wireless gear is short enough that just about the time wireless gear bought today is ready for replacement, the 802.11e gear should be ready to buy, says Craig Mathias, a principal at Farpoint Group. In the meantime, most Wi-Fi customers are getting by with single-vendor deployments of QoS-enabled devices or deployments of multiple vendors' gear whose QoS schemes have proven interoperable, he says.

The overriding challenge for QoS is that Wi-Fi is a shared medium, much as Ethernet was in the days before switching. There is just so much bandwidth and client devices have to share.

Three years ago, Bob Longhini was evaluating BreezeCom VoWi-Fi gear for door and window maker Kolbe & Kolbe, but pulled the plug on the project because of QoS issues. "We had echo and breakup in the calls, especially if there was activity from handhelds and laptop computers," says Longhini, who now is evaluating VoWi-Fi for his new employer, Jennie-O Turkey Store, a billion dollar subsidiary of Hormel.

Only workers with desperate need for mobile phones liked the early equipment, he says. "They really saw the benefit of having the phone on their hip even if they ran into quality problems," Longhini says. But things have greatly improved with a clear road map being set for QoS and many vendors already implementing early versions of the standards-bound technology.

Ideally, client devices - in the case of VoWi-Fi that means phones - would announce their bandwidth requirements and the wireless network would take steps to accommodate them, if possible. In its WMM implementation, Cisco's wireless gear (formerly Airespace) checks whether an access point in range of the phone has enough free bandwidth to accommodate the call, says Kathy Small, Cisco's marketing manager for wireless and mobility. WMM then can offer four levels of service.

WMM addresses how clients and access points communicate what they need and what they can provide, respectively, but not how devices decide whether to accept an available connection, says Partha Narasimhan, wireless architect for Aruba. Even with WMM, that is left up to individual vendors to implement, he says.

Once a phone is accepted by an access point, algorithms determine when each device connected to a single access point gets to send, with top priority voice traffic getting to send more often, says Roger Sands, vice president of enterprise development for Colubris. These algorithms were created to deal with collisions and retransmissions on Wi-Fi networks but have been fine-tuned to give voice the edge over other applications. Properly adjusting these algorithms in Colubris gear shaves at least 20 microsec off a packet's wait time, according to Colubris engineers.

WMM also calls for phones to tag voice packets so access points and wireless switches can treat inbound packets with priority and drop them onto the appropriate virtual LAN in wired networks to which access points are connected. Many companies create separate VLANs just for voice to ensure QoS and boost security.

Similarly, wireless switches mark outbound voice packets for top priority "to make sure they don't sit in the access point waiting for data packets," Sands says.

Once VoWi-Fi users make calls, they likely will move around, forcing the wireless network to hand off the calls from access point to access point. "Roaming is critical," Cisco's Small says.

"But with QoS plus mobility, you have an even bigger problem," Aruba's Narasimhan says. Both the handsets and the access points have to seek the next access point for the caller to connect to and figure out if it has the bandwidth to accept the call, he says.

A separate proposal called 802.11r is in the works to deal with roaming, where the key problems are maintaining the security state and the QoS context for the call without forcing the handset to carry out a full negotiation with the next access point, he says.

Enabling handoffs with QoS might call for each access point to reserve some bandwidth to deal with handoffs of ongoing calls. This bandwidth buffer would be adjustable and set by network executives depending on how much their users roam.

Handoffs are unnecessary in the office segment of the Aruba wireless LAN at Commercial Alcohols, an alcohol distributor in Brampton, Ontario, says Chris Thomas, the company's IT director. But in the warehouse, where managers move around quite a bit, smooth handoffs are a requirement. So a small buffer or none at all might be sufficient for the office segment, while a significant percentage of total bandwidth might be required in the warehouse.

As 802.11e and 802.11r near completion, Spectralink, the vendor that created the predominant non-standard wireless QoS mechanism, is planning to abandon its earlier technology and adopt standards. Spectralink Voice Priority (SVP) was adopted by the major VoWi-Fi vendors and now is being replaced by WMM. "The SVP approach was great and got us where we are today and is the reason we have voice over Wi-Fi at all today," says Ben Guderian, director of marketing strategy for the company. "But it's served its purpose."

The question remains whether customers will want gear that is fully compliant with 802.11e or whether WMM supports stringent enough QoS to meet business needs. "The goal may become a balance between QoS and complexity," says Bruce Van Nice, vice president of marketing for Trapeze. "Users tend to balance toward the pragmatic."

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