Faster Handoff Between Wi-Fi Networks Promises Near-Seamless 802.11 Roaming
San Diego, CA, April 13, 2004 -- Road warriors may no longer have to stay put in an airport lounge or Starbucks to access the high-speed Internet via an 802.11 Wi-Fi network. Thanks to software developed by two computer scientists at the University of California, San Diego, the time it takes to hand off from one Wi-Fi wireless network to the next can be dramatically shortened -- overcoming a major obstacle in Wi-Fi roaming.
Jacobs School of Engineering professor Stefan Savage and graduate student Ishwar Ramani have a patent pending on the basic invention behind SyncScan, a process to achieve practical, fast handoff for 802.11 infrastructure networks. Their study will be published in the Proceedings of the IEEE InfoCom 2005.
"Wi-Fi offers tremendous speeds if you stay in one place or at least within 100 meters of the same access point," said Savage, an assistant professor in the Computer Science and Engineering department and academic participant in the California Institute for
At present, Wi-Fi handoffs are cumbersome and time-consuming. Not until the access-point signal weakens substantially and begins losing packets of data does a Wi-Fi-enabled laptop, PDA or mobile phone begin scanning for a stronger signal. At that point, it broadcasts requests on all channels to find nearby access points. The result: a delay of up to one second, during which any packets may be lost. That may not seem inordinate when downloading data, but it can be unacceptable if the user is trying to listen to Internet radio, watch a streaming movie trailer or talk on a Wi-Fi phone.
"Today most Wi-Fi users accept being tethered to a single location in exchange for the broadband speeds that Wi-Fi offers," said Ph.D. candidate Ramani. "But increasingly they want to be able to make Voice of IP (VoIP)phone calls or stream multimedia while commuting or on the move, and a one-second disruption can seem like an eternity."
To test their SyncScan algorithm, the researchers used a laptop running a voice application while walking between two areas of the UCSD campus served by neighboring Wi-Fi access points. "We used a popular VoIP called Skype which uses UDP [user datagram protocol] packets exchanged between two clients for voice communication," explained Savage. "Using SyncScan with a measurement interval of 500 millseconds, handoff delay was virtually imperceptible - roughly 5 milliseconds. Repeating the tests without SyncScan, the average handoff time was 450 milliseconds, but ranging up to a full second in some cases."
Just over 110,000 VoWi-Fi handsets were sold in 2004, mostly in Japan. Vonage is set to roll it out commercially in the U.S. later this spring as an add-on to its popular VoIP service, and sales of dual-use phones incorporating both cellular and VoWi-Fi could reach $3 billion by 2009, according to a study by Infonetics Research.
Stefan Savage on the rationale behind the new Wi-Fi handoff algorithm Length: 2:18 Professor Savage on potential market applications of the Wi-Fi innovation Length: 2:18 Grad student Ishwar Ramani explains how he tested the SyncScan algorithm Length: 2:30