December 2004

WiFi Telephony QoS: Can You Hear Me Now?

BY Ben Guderian

WiFi telephony is catching on in the enterprise. Enterprise adoption of WiFi wireless LAN technology has encountered several hurdles such as competing standards and security concerns, but these have been overcome through innovation and industry cooperation. WiFi telephony — using WiFi wireless networks to support mobile telephone applications — is being used in a wide variety of applications today, from healthcare to manufacturing to education. The business case for deploying WiFi telephony usually is based on improving employees’ mobility, responsiveness, and productivity. And as with any enterprise telephony application, clear and consistent voice quality is a critical requirement.

Thanks to cellular technology, people have grown accustomed to lower expectations of voice quality when using a wireless device. To be fair, cellular has come a long way from its analog beginnings. But it is still far from perfect and generally considered unacceptable for mission-critical enterprise use. WiFi technology has the advantage of being designed specifically for enterprise applications. WiFi access points are designed to be low-cost, simple to install, and deployed with seamless coverage areas. Since WiFi technology has the advantage of providing better coverage and handoff performance than cellular technology, there should be no problem guaranteeing excellent voice quality with WiFi telephony, right?

Unfortunately it’s not that simple. WiFi is a wireless LAN technology designed for packetized data applications, and therefore subject to all the issues of VoIP. Voice packets need to be prioritized on any converged network, wired or wireless, to minimize latency and jitter. To further complicate things, WiFi access points operate as Ethernet hubs — sharing bandwidth between multiple wireless client devices. So, it is clear that the need for a quality of service (QoS) mechanism for WiFi telephony is critical to ensuring good, consistent voice quality.

Where’s The Standard?
The initial 802.11 WiFi standards did not provide a workable QoS solution. Early innovators in WiFi telephony developed proprietary QoS mechanisms out of necessity. For example, our own proprietary QoS solution, SpectraLink Voice Priority (SVP), was developed as a stopgap measure. SVP has broad industry support because it is open and vendor-neutral, but was never intended to meet the broader QoS requirements of wireless multimedia applications such as streaming video. Development of an all-encompassing QoS standard has been the mission of the IEEE 802.11 standards committee.

The 802.11 Task Group E is chartered with creating a wireless QoS standard that will address the requirements for any foreseeable wireless applications, including both enterprise and consumer products. This QoS standard, called 802.11e, is expected to be ratified by early 2005. But standards like 802.11e don’t specify exactly how they should be implemented in commercial products. That’s why the WiFi Alliance, an industry association formed to provide interoperability testing of WiFi devices, has developed a QoS specification based on a profile of the 802.11e standard. The WiFi Alliance specification is called WiFi Multimedia, or WMM, and it provides the framework to make sure that QoS mechanisms are implemented consistently across various vendors’ WiFi products. WMM specifies a basic QoS implementation that is appropriate for consumer devices, and then provides additional enhancements to meet the more demanding requirements of enterprise applications such as WiFi telephony.

QoS Building Blocks
There are three primary components to a QoS solution: packet prioritization, timed delivery, and admission control. Some wireless applications might get by with a subset of these, but all three are necessary for more demanding applications such as WiFi telephony. Let’s take a closer look at each component.

Packet prioritization is simply having the ability to specify different priority levels for different applications. Applications such as WiFi telephony need the highest priority because of the sensitivity to latency and packet loss. Applications like file transfers can have a low priority because they have no time sensitivity and have higher layer protocols to deal with lost or delayed packets. One analogy for packet prioritization is a supermarket express checkout lane. Customers with a minimum number of items don’t have to wait behind those with overflowing shopping carts. But you still end up waiting in a long line if all the shoppers in the store have eight items or less. Prioritization provides a good foundation for QoS, but what happens if most or all packets are tagged with high priority?

Timed delivery of packets provides a way to improve the efficiency of the WiFi infrastructure and wireless client devices. Timed delivery gives the wireless devices a mechanism to determine when they transmit packets, either through a request-to-send approach or through negotiated parameters. A big advantage of timed delivery for battery-operated wireless devices is that it allows them to conserve power by only enabling the radio circuitry when they expect to transmit or receive packets. It also gives the client a known time to make handoff decisions, which can affect audio quality. Without timed delivery, a wireless device needs to be enabled to receive packets at any time. Timed delivery is analogous to making an appointment to minimize waiting time. Imagine how crowded a doctor’s office waiting room would be if nobody made appointments and it was just first come, first served. Having a timed delivery mechanism — assigning times for patients to arrive — improves the efficiency of the office just as timed delivery improves WiFi resource utilization.

Admission control is the third component of a comprehensive QoS solution. Admission control is necessary to prevent oversubscription of WiFi access point resources. A WiFi access point has a finite amount of bandwidth it can allocate to wireless client devices. If a device is allowed to associate with an access point that is already at capacity, or at least unable to support the bandwidth requirements of the incoming client, then it may impact the access point’s ability to meet the needs of the clients already associated with it. In the doctor’s office analogy, admission control is provided by limiting the number of patients that can be seen at one time based on the number of examination rooms and available staff. If resources are already being utilized at their capacity, adding one more client will affect the quality of service for all existing clients.

Bulletproof QoS
Enterprise WiFi telephony applications need all three components — packet prioritization, timed delivery, and admission control — to meet the users’ requirement for good voice quality in all network conditions. Prioritization is critical for delivering voice packets with minimal latency. Timed delivery is important for meeting traffic density requirements and for conserving battery life on handheld devices. And admission control is necessary to prevent overcrowding of WiFi resources. The initial implementations of WMM will provide packet prioritization, but no timed delivery or admission control. This will be sufficient for consumer applications that don’t require roaming between multiple access points or high user density.

Fortunately for enterprise WiFi telephony applications, WMM is compatible with existing proprietary QoS mechanisms, which provide the timed delivery and admission control components. Extensions of the basic WMM specification will be supported in the upcoming months. A power-saving mechanism, defined in the forthcoming 802.11e standard, which enables WiFi access points to buffer packets until the client device requests them, will provide basic timed delivery. Admission control can be managed using another 802.11e option that lets the client devices send a “Traffic Specification” to the access point to make sure sufficient bandwidth is available. The ultimate solution that incorporates all three building blocks, referred to as WMM-Scheduled Access, will finally give the industry a standards-based, enterprise-grade QoS mechanism sufficient for the demanding voice quality requirements of WiFi telephony.

Enterprises will continue to deploy WiFi telephony solutions to solve communication problems and make their employees more effective in their jobs. Today’s solutions can use proprietary QoS mechanisms that guarantee consistently good voice quality. But soon we’ll see the same level of quality available through the 802.11e standard and WMM specifications, making enterprise WiFi telephony even more viable in a wide range of applications.

Ben Guderian is director of marketing at SpectraLink Corp. For more information, please visit the company online at www.spectralink.com.

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