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Divert: Fine-grained Path Selection for Wireless LAN

Divert: Fine-grained Path Selection for Wireless LAN. Allen Miu, Godfrey Tan, Hari Balakrishnan, John Apostolopoulos * MIT Computer Science and Artificial Intelligence Laboratory. * Hewlett-Packard Laboratories. Today’s wireless LAN. Backbone Network (wired).

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Divert: Fine-grained Path Selection for Wireless LAN

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  1. Divert: Fine-grained Path Selection for Wireless LAN Allen Miu, Godfrey Tan, Hari Balakrishnan, John Apostolopoulos* MIT Computer Science and Artificial Intelligence Laboratory *Hewlett-Packard Laboratories

  2. Today’s wireless LAN Backbone Network (wired) Clients communicates with one AP at a time • Clients select AP based on long term signal quality AP1 … APN-1 APN Client Client MobiSys 2004

  3. Can we use multiple transmission paths (APs)to help reduce losses in a wireless LAN? Problems • Communication suffer periods of high loss rate • Link retransmission adds delay (1-100ms) • Lowering link rate reduces bandwidth Impacts delay-sensitive, high-bandwidth apps e.g., VOIP, video-conferencing MobiSys 2004

  4. 802.11b Experiment Setup A B • Transmitters alternate broadcasts @ 11Mbps • Receiver is moving (2mx2m area) • Collect trace of two interleaved streams • Examine loss behavior • Within the same path • Across different paths Path A Path B ~15m R … Ai Bi Ai+1 Bi+1 MobiSys 2004

  5. Conditional frame loss probabilities in a mobile environment Losses are bursty within the same path Losses have low loss correlation across different paths Lag after loss frame i MobiSys 2004

  6. Explanation of loss behavior: • Transmission depends on physical path • Obstacles  Attenuation • Surfaces  Multipath • Mobility  Rapid and deep fading • Physical effects last for different durations • Losses are time-correlated (bursty) • Propagation environment is complex and dynamic • Unlikely that all paths suffer simultaneously MobiSys 2004

  7. Use fine-grained path selection to reduce transmission losses! • Goal: Switch communication quickly among nearby APs to avoid short-term channel outages (burst losses) in the current transmission path • Challenges: • Architecture for fine-grained path selection • Algorithm for fine-grained path selection AP Client AP MobiSys 2004

  8. DM AP Divert architecture (Downlink) DC • Divert Monitor (DM) • Monitors channel condition • Divert Controller (DC): • Performs path selection on a frame-by-frame basis • Performs link-layer retransmissions Wired Backbone DM DM … AP AP Client MobiSys 2004

  9. Divert architecture (Uplink) • Divert Monitor (DM) • Monitors channel condition • Divert Controller (DC): • Performs path selection on a frame-by-frame basis • Performs link-layer retransmissions • Uplink is optional Wired Backbone … AP AP AP DM DC Client MobiSys 2004

  10. Conventional WLANs have high path switch signaling overhead • Single Radio Client • Must signal client to switch frequency • Multiple Radio Client • Expands cell size M1 AP1 Cell 3 M2 AP2 Cell 1 Cell 2 MobiSys 2004

  11. Divert avoids signaling overhead by deploying secondary access points (SAP) SAP provides alternate paths • No signaling overhead • Compatible with cellular architecture M1 AP1 Cell 3 SAP1 SAP2 M2 AP2 Cell 1 SAP2 Cell 2 MobiSys 2004

  12. Practical path switching heuristic • Selecting best path for each frame transmission is difficult • Link conditions vary often and quickly • Expensive to probe all transmission paths often MobiSys 2004

  13. Practical path switching heuristic • Observation: • Losses are bursty • Losses have low loss correlation across paths • A simple heuristic is to: • Monitor channel quality in current path • Switch path if current path has fallen to a “bad state” MobiSys 2004

  14. Practical path switching heuristic • Per-client frame loss history window, H (frames) • Switch paths when T frames lost within last H transmitted frames • Can adapt H and T for • Different channel conditions • Relative loss rate differences among different transmission paths MobiSys 2004

  15. Testbed Setup A – AP B – SAP R – Receiver locations ~15 m A R1 R2 R3 B • Divert sends packet via A or B @ 11 Mbps link rate • Load: 1500 bytes @ 240pps for 5 min. (720,000 frames) • Disabled retransmissions • Experiments with stationary and moving laptop receiver at R1, R2, R3 MobiSys 2004

  16. Frame Loss Rates (at R3) Legend: { H, T } MobiSys 2004

  17. Burst Loss Length CCDF (at R3) Mobile R3 MobiSys 2004

  18. Frame Loss Rates (at R1) Mobile R1 Legend: { H, T } MobiSys 2004

  19. Burst Loss Length CCDF (at R1) Mobile R1 MobiSys 2004

  20. Related Work • Physical layer spatial diversity techniques • Diversity antenna/antenna arrays • Distributed radio bridges for wireless LAN [Leung ’96] MobiSys 2004

  21. Conclusion • Fine-grained path selection can reduceframe loss and loss burstiness, especially for moving WLAN clients • Design and implementation of a fine-grained path selection system on a wireless LAN (802.11b) Code and traces will be made available soon at http://nms.csail.mit.edu/divert MobiSys 2004

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