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Fine-Grained Path Selection to Enhance Wireless LAN Performance

This paper explores fine-grained path selection in wireless LANs to improve communication quality and reduce frame loss. The authors investigate how clients communicate with multiple access points (APs) and propose strategies to swiftly switch between APs to avoid burst losses and minimize high loss rates, which can impede delay-sensitive applications like VOIP and video conferencing. The study presents an innovative architecture for this fine-grained selection process, focusing on algorithm development that monitors channel conditions in real-time, ensuring swift and efficient path transitions.

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Fine-Grained Path Selection to Enhance Wireless LAN Performance

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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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