MAC Layer Handoff

1. 6 6 11 11 Handoff Time Abstract Caching Mechanism Background: Spatial Locality - Users use a phone in the same place (Office, Home, School), and move around the same environment. Idea: Store the AP info into a cache, and reuse it without scanning when handoff happens Mobile station performs a handoff whenever it moves out of the range of one access point (AP). This takes a few hundred milliseconds, causing interruptions in VoIP sessions. We developed a new handoff procedure which reduces the MAC layer handoff latency, in most cases, to a level where VoIP communication becomes seamless, using a selective scanning algorithm and a caching mechanism. Also, we implemented the new algorithm with hostap driver and we confirmed its performance. • Handoff takes 100 ms to 400 ms • Seamless VoIP requires less than 100 ms mouth-to-ear delay • The Biggest component of the delay is the scanning time Fast MAC Layer Handoff in Wireless NetworksSangho Shin, Andrea Forte, Anshuman Rawat and Henning Schulzrinne Columbia University http://www.cs.columbia.edu/IRT/wireless Cache Miss Case Signal Low Selective Scanning Store AP Info to Cache 6 6 1 11 11 1 1 B B SelectiveScanning MAC Layer Handoff C C A A A Coverage Area In 802.11b/g, there are only three non-overlapping channels (1, 6, 11) . Cache Cache Key Best Next Key Best Next A A B C B After selective scanning, a mobile node stores the AP info into a cache table • Scan channel 1, 6, 11 first. • Don’t scan the current channel. • Keep the previous scanned channels. • If no available AP found, flip the channel mask and scan the other channels. Scan APs (ch 6,11 only) AP AP Cache Hit Case 6 1 11 Signal Low Cache Hit Connect to the Best AP Mobile Node 6 6 Handoff occurs when a mobile station moves beyond the radio range of one AP and enters another BSS 1 11 11 1 1 B B C C A A A New channel mask = 1, 6, 11 + scanned channels – current channel Cache Cache Handoff Process Key Best Next Key Best Next A B C A B C B C A B C A A mobile node can use the AP info in the cache when handoff happens. Scan APs Authenticate Associate Experiments and Results 6 6 6 1 11 6 5 7 4 8 3 9 2 10 1 Channels 11 APs New AP • Handoff time is reduced to a half by using Selective Scanning, and reduced to 2 ~ 4 ms by using Caching Mechanism. • Packet delay is eliminated in a sender side using Caching • But, there are some packet loss in receiver side even if the handoff time is reduced drastically. This is because of the bridging delay is still present and big. Association Request Authentication Request Probe Request Authentication Response Association Response Probe Responses ……