Fast MAC Layer Handoff in Networks

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Presentation transcript:

Fast MAC Layer Handoff in 802.11 Networks Prof. Henning Schulzrinne Andrea G. Forte Anshuman S. Rawat Sangho Shin

802.11 Networks A handoff occurs when a mobile station moves beyond the radio range of one AP and enters another BSS.

Handoff delay Mobile station All APs Probe delay New AP Probe request (broadcast) Probe response Probe delay Authentication request New AP Authentication delay Authentication response Association request Association delay Association response

Handoff Delay

Problems Solutions Handoff latency is too big for VoIP. Seamless VoIP requires less than 90ms latency. Handoff delay is from 200ms to 400ms. The biggest component of handoff latency is probing (over 90%). Solutions Selective Scanning Caching

Selective Scanning In most of the environments (802.11b & 802.11g), only channel 1, 6, 11 are used for APs. Two APs that have the same channel are not adjacent (Co-Channel interference). Scan 1, 6, 11 first !!

Selective Scanning - Algorithm Scan channels using channel mask (Full scan) Found good AP ? Yes Join to the AP No Flip channel mask Set new channel mask New channel mask = 1, 6, 11 + scanned channels – current channel

Caching Background Algorithm Spatial locality (Office, school, campus…) Algorithm After scanning, store the candidate AP info into cache (key=current AP). Use the AP info in cache for association without scanning when handoff happens. Key AP1 AP2 1 Current AP Next best AP Second best AP …. N

Cache – Simulation Coverage area Detection area Handoff !! Key Best Next 1 AP1(11) 2 Coverage area Detection area Channel mask 1, 6 AP1(11) Handoff !! AP2(6) AP3(1)

Cache – Simulation Coverage area Detection area Handoff !! Cache miss! Key AP1 AP2 1 AP1(11) 2 - Coverage area Detection area Channel mask 1, 6 AP1(11) Handoff !! AP2(6) AP3(1) Cache miss! Selective Scanning AP2, AP3

Cache – Simulation Coverage area Detection area Handoff !! Cache miss! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 11 1, 6 AP1(11) Handoff !! AP2(6) AP3(1) Cache miss! Selective Scanning AP2, AP3 Connect AP2

Cache – Simulation Coverage area Detection area Handoff !! Key AP1 AP2 - Coverage area Detection area Channel mask 1, 11 Handoff !! AP1(11) AP2(6) AP3(1)

Cache – Simulation Coverage area Detection area Handoff !! Cache miss! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 11 Handoff !! AP1(11) Cache miss! AP2(6) AP3(1)

Cache – Simulation Coverage area Detection area Handoff !! Cache miss! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 11 Handoff !! AP1(11) Cache miss! AP2(6) AP3(1) Selective Scanning AP1

Cache – Simulation Coverage area Detection area Handoff !! Cache miss! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 11 1, 6 Handoff !! AP1(11) Cache miss! AP2(6) AP3(1) Selective Scanning AP1 Connect AP1

Cache – Simulation Coverage area Detection area Handoff !! Key AP1 AP2 - Coverage area Detection area Channel mask 1, 6 AP1(11) Handoff !! AP2(6) AP3(1)

Cache – Simulation Coverage area Detection area Handoff !! Cache HIT! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 6 AP1(11) Handoff !! AP2(6) AP3(1) Cache HIT!

Cache – Simulation Coverage area Detection area Handoff !! Cache HIT! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 11 AP1(11) Handoff !! AP2(6) AP3(1) Cache HIT! Connect AP2

Cache – Simulation Coverage area Detection area Handoff !! Cache Hit!! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area AP1(11) Channel mask 1, 11 Handoff !! AP1(11) Cache Hit!! AP2(6) AP3(1)

Cache – Simulation Coverage area Detection area Handoff !! Cache Hit!! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 11 Handoff !! AP1(11) Cache Hit!! AP2(6) AP3(1) Connect AP1

Cache – Simulation Coverage area Detection area Handoff !! Cache Hit!! Key AP1 AP2 1 AP1 (11) AP2 (6) AP3 (1) 2 - Coverage area Detection area Channel mask 1, 6 Handoff !! AP1(11) Cache Hit!! AP2(6) AP3(1) Connect AP1

Experiment Result – Handoff time

Experiment Result – Packet Delay

Experiment Result – Packet Loss

Experiment Result – Packet Loss

Conclusion Fast MAC Layer handoff using selective scanning and caching Selective Scanning : 100~130 msec Caching : 2~4 msec Low Power Consumption (PDAs). Don’t need to modify AP, infrastructure, or standard. Just need to modify the wireless card driver!

Thank you! For more information: http://www.cs.columbia.edu/IRT/wireless http://www.cs.columbia.edu/~andreaf