For wireless telephony, clients such as mobile phones must be able to rapidly disassociate from one access point and connect to another. The delay that occurs during handoff cannot exceed about 50ms, the interval that is detectable by the human ear. However, current roaming delays in 802.11 networks average in the hundreds of milliseconds. This can lead to transmission "hiccups," loss of connectivity and degradation of voice quality. Faster handoffs are essential for 802.11-based voice to become widely deployed.

The 802.11r working group of the IEEE is drafting a protocol that will facilitate the deployment of IP-based telephony over 802.11-enabled phones. The 802.11r standard is designed to speed handoffs between access points or cells in a wireless LAN. The working group is drafting the final protocol, which should be approved toward the end of 2006.

Another problem with current 802.11 wireless gear is that a mobile device cannot know if necessary QoS resources are available at a new access point until after a transition. Thus, it is not possible to know whether a transition will lead to satisfactory application performance.

Mobile refinement
802.11r refines the transition process of a mobile client as it moves between access points. The protocol allows a wireless client to establish a security and QoS state at a new access point before making a transition, which leads to minimal connectivity loss and application disruption. The overall changes to the protocol do not introduce any new security vulnerabilities. This preserves the behavior of current stations and access points.

When approved, 802.11r will govern the way roaming mobile clients communicate with candidate access points, establish security associations and reserve QoS resources. Under 802.11r, clients can use the current access point as a conduit to other access points, allowing clients to minimize disruptions caused by changing channels.

There are trade-offs among the speed of a handoff, the certainty of communication with an access point and disruption of current communications. A client can stay on its current channel and use its current access point to communicate with other candidate access points. This minimises disruption to the client's datastream but does not allow the client to determine anything about its ability to communicate with other access points over the air. The client also can change to the channel of another access point.

This allows the client to be certain of the quality of its communication with the other access point over the air but causes some disruption to communication with its current access point.

Increased density means more hand-offs
As usage of wireless networks increases, the density of access points may increase to satisfy increased capacity, which will lead to more frequent handoffs. Such environments would benefit from the capabilities provided by 802.11r.

The 802.11r protocol will be a major milestone for millions of wireless users. It is expected to spur convergence of voice, data and video, which will lead to improved functionality, performance and usability for mobile devices. It will bring about faster deployment of 802.11-based technologies and lead to less expensive, more comprehensive security.

Calhoun is CTO and O'Hara is technical leader for Cisco Systems' Wireless Business Unit. They can be reached at [email protected] and [email protected], respectively. This article appeared in Network World.