Wi-Fi client devices, such as a laptop computer, will connect to the strongest available signal. But when the signal degrades and data packets begin dropping, the client device sends out a SOS on all channels looking for another access point, a process that could take as long as one second.
In other words: sometimes the solid Wi-Fi connection linking you to the Internet turns to mist, and that broadband pipe you were relying upon becomes a soda straw. It is the times like these that two University of California computer scientists hope to banish.
The new SyncScan software developed at Cal State, San Diego reduces that second for handoff to around 5 milliseconds, according to the authors, Stefan Savage, computer science professor at the university’s Jacobs School of Engineering, and Ishwar Ramani, a graduate student.
“SyncScan is a hand-off algorithm which can cut the time it takes to switch from one Wi-Fi access point to another by a factor of a hundred over existing solutions,” says Savage.
A speedy handoff is a requirement for applications, such as voice, “where even short interruptions can disrupt the illusion of continuous connectivity,” Savage says.
The academic paper describing SyncScan is slated to be published in the Proceedings of the IEEE InfoCom 2005. While the university maintains a 100 percent patent on all intellectual property created by staff and students, the software—for now—can also be downloaded free.
While the Cal State scientists believe SyncScan is the answer to more demanding wireless applications such as Voice-over-Wi-Fi or streaming video, several roadblocks could prevent the software reaching beyond the university campus.
Tethered to Broadband?
“Today, most Wi-Fi users accept being tethered to a single location in exchange for the broadband speeds that Wi-Fi offers,” says Ramani. When Wi-Fi users do decide to move, “then there is a mad scramble” looking for a better signal, Savage tells Wi-Fi Planet.
“But increasingly, [users] want to be able to make Voice over IP (VoIP) phone calls or stream multimedia while commuting or on the move, and a one-second disruption can seem like an eternity,” says Ramani.
“The scanning phase completely dominates the cost of handoff, usually contributing more than 90 percent of the overhead,” according to the SyncScan paper.
Delays created when handing off from a failing AP have “crippled the deployment of mission-critical applications over WLANs and hampered the realization of any ROI from that WLAN,” wrote Dr. Vanduvar Bharghavan, CTO of Meru Networks, in 2004. Bharghavan made the statement to support Meru’s virtual AP solution.
Timing is Key
The key to SyncScan is timing. Rather than waiting until signal strength drops low, SyncScan coincides its sweeps with the regularly-scheduled “hello” beacon messages transmitted by access points.
“The process eliminates the current need to start from scratch when looking for a stronger signal, and replaces the long scanning delay with many small delays that are imperceptible to the user,” according to a university statement.
“Better handoff decisions can happen if you don’t wait until the last second,” Savage says.
To test how SyncScan would impact demanding Wi-Fi applications such as voice, the developers used Skype, the popular VoIP application. Walking across the campus serviced by two APs, Savage and Ramani found that “hand-off delay was virtually imperceptible—roughly five milliseconds,” says Savage. Without SyncScan, “the average handoff time was 450 milliseconds, but ranging up to a full second in some cases.”
Besides a benefit of faster hand-offs between access points, the researchers discovered no packets were dropped, compared to “substantial packet loss” with technology now in use, according to a statement. A routine delay of 250 milliseconds caused dropped packets, according to the university.
The UC San Diego scientists believe “the same improvements can be achieved on most Wi-Fi devices and using most applications, not just voice.”
No Changes Required
Despite the performance boost, SyncScan carries very little overhead demands, according to Savage. Although shorter hand-off times and fewer lost packets would be experienced if both the client device and AP supported the new system, users could still benefit if only one side of a Wi-Fi link included SyncScan, Savage says.
No firmware changes are required, says Savage. Most APs support the beacon timing employed by SyncScan. Savage did point to AP chipsets from Conexant and Atheros as compatible.
SyncScan would require no changes in the 802.11 standard, Savage assures. “This is very simple technology,” he says.
Some Drawbacks
While Savage believes in SyncScan, he admits there are some drawbacks at the moment. “There isn’t a Windows driver yet,” he says, pointing out one of the drawbacks to working in an educational setting.
Although SyncScan will “definitely be deployed” in the on-campus building housing the Computer Science and Engineering department, the researchers are still trying to convince Avaya, which is the hardware vendor.
Then there are the patents surrounding SyncScan. “The University of California always owns 100 percent of the intellectual property generated by faculty and students on UC campuses — we never budge on that,” says Doug Ramsey, university spokesman.
Ramsey does say there will be a variety of licensing options, the most common being a private company which is interested in the technology and willing to pay a licensing fee.
Savage says there are three routes for SyncScan to be adopted by the Wi-Fi community. A vendor could become interested in SyncScan, the software could become part of the 802.11 standard, or fans of VoIP could take up the flag. One thing is sure: “I’m not about to run off and start a company,” assures Savage.
AP Companies Respond
“The SyncScan concept has been around for some time,” Meru’s Bharghavan tells Wi-Fi Planet. “In fact, a company called SIPquest utilizes this protocol.” Ottawa-based SIPquest is a VoIP developer concentrating on Session Initiation Protocol. SIPquest offers a feature it calls VoWiFi Fast Roaming.
“The basic idea is to scan for APs in advance, so the client can make a good choice rather than be forced into one,” Bharghavan says. Because it requires client software, Meru hasn’t implemented it.
“All Atheros products have always supported what we call ‘background scanning’,” says Bill McFarland, Atheros‘ CTO. The process checks the environment, and maps the location and signal strength of nearby APs. When the signal strength of the current AP drops, the chipset uses the map to shift quickly to another AP.
IEEE 802.11r
The 802.11 Working Group has the 11r amendment attached to handle client devices quickly associating with a single AP. The 802.11r Task Group seeks to do the same thing for handoffs between more than one AP — the group seeks to lower the handoff delay to one millisecond.
