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

6/9/2004 MobiSys Clients communicates with one AP at a time Clients select AP based on long term signal quality Today’s wireless LAN … AP 1 Client AP N-1 AP N Backbone Network (wired) Client

6/9/2004 MobiSys 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 Can we use multiple transmission paths (APs) to help reduce losses in a wireless LAN?

6/9/2004 MobiSys b Experiment Setup Transmitters alternate 11Mbps Receiver is moving (2mx2m area) Collect trace of two interleaved streams Examine loss behavior  Within the same path  Across different paths A B R Path A Path B ~15m … AiAi BiBi A i+1 B i+1

6/9/2004 MobiSys 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

6/9/2004 MobiSys 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

6/9/2004 MobiSys 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

6/9/2004 MobiSys Divert architecture (Downlink) DM AP DC AP … DM AP DM Wired Backbone Divert Monitor (DM)  Monitors channel condition Divert Controller (DC):  Performs path selection on a frame-by-frame basis  Performs link-layer retransmissions Client

6/9/2004 MobiSys Divert architecture (Uplink) AP … Wired Backbone DC DM Client 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

6/9/2004 MobiSys Conventional WLANs have high path switch signaling overhead AP 1 AP 2 M1M1 M2M2 Cell 1 Cell 2 Cell 3 Single Radio Client  Must signal client to switch frequency Multiple Radio Client  Expands cell size

6/9/2004 MobiSys AP 2 M2M2 SAP 1 SAP 2 AP 1 M1M1 Divert avoids signaling overhead by deploying secondary access points (SAP) Cell 1 Cell 2 Cell 3 SAP provides alternate paths No signaling overhead Compatible with cellular architecture

6/9/2004 MobiSys 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

6/9/2004 MobiSys 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”

6/9/2004 MobiSys 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

6/9/2004 MobiSys Testbed Setup Divert sends packet via A or 11 Mbps link rate Load: pps for 5 min. (720,000 frames) Disabled retransmissions Experiments with stationary and moving laptop receiver at R1, R2, R3 A – APB – SAP R – Receiver locations A B R1 R2R3 ~15 m

6/9/2004 MobiSys Frame Loss Rates (at R3) Legend: { H, T }

6/9/2004 MobiSys Burst Loss Length CCDF (at R3) Mobile R3

6/9/2004 MobiSys Frame Loss Rates (at R1) Legend: { H, T } Mobile R1

6/9/2004 MobiSys Burst Loss Length CCDF (at R1) Mobile R1

6/9/2004 MobiSys Related Work Physical layer spatial diversity techniques  Diversity antenna/antenna arrays Distributed radio bridges for wireless LAN [Leung ’96]

6/9/2004 MobiSys Conclusion Fine-grained path selection can reduce frame 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