1. Opportunistic Use of Client Repeaters to Improve Performance of WLANs Victor Bahl 1, Ranveer Chandra 1, Patrick P. C. Lee 2, Vishal Misra 2, Jitendra Padhye 1, Dan Rubenstein 2, Yan Yu 3 1 Microsoft Research 2 Dept of Computer Science, Columbia University 3 Google Inc. Dec 12, 2008

2. 2 Outline Rate anomaly problem SoftRepeater design Fairness requirements Experimental results Conclusions

3. 3 Rate Anomaly of 802.11 Rate anomaly is well-known in WiFi 802.11 networks Low-rate stations degrade throughput of high-rate stations Why does rate anomaly exist? Stations reduce data rates when signal strength is poor (auto-rate) Low-rate stations’ packets consume more airtime 802.11 arbitrates transmissions on per-packet basis High-rate stations receive limited airtime  throughput degrades 54Mbps AP A 54Mbps B Throughput (Mbps) 10 20 30 0 A, B near AP A far from AP B A B A 18Mbps

4. 4 Limitations of Prior Solutions What’s new? Rate anomaly is well-known, with many solutions proposed. Assumptions of prior solutions: Require dedicated hardware (e.g., Cisco Aironet 1200 series APs) Change MAC layer (e.g., Lee et al., Infocom ’04; Liu et al., JSAC ’05) Construct ad-hoc mesh networks (e.g., Draves et al., Mobicom ’04) Drawbacks of prior solutions: More cost for hardware change Not compatible with widely deployed infrastructure networks Inflexible – solutions cannot be activated on demand

5. 5 Our Solution: SoftRepeater SoftRepeater: A practical, deployable system that addresses rate anomaly Main idea: High-rate station (repeater) relays traffic for low-rate station (client) Key features: Repeater is opportunistic - activated only when both repeater and client receive “beneficial” throughput No changes to 802.11 MAC and AP Deployable in infrastructure and adhoc networks AP A B client repeater traffic for A and Btraffic for A

6. 6 Design Issues How can we detect existence of rate anomaly occurring? How do we formally define “beneficial” throughput? How do we support multiple interfaces on a wireless card? We need managed mode for communication between AP and repeater We need adhoc mode for communication between repeater and client What fractions of time should we give to managed/adhoc modes to ensure “beneficial throughput”? AP A B client repeater traffic for A and Btraffic for A

7. 7 Our Contribution Propose a handshaking protocol for detecting rate anomaly and reaching consensus on using SoftRepeater Formalize a set of utility maximization problems for different fairness requirements Implement SoftRepeater on Windows XP; conduct extensive testbed experiments and QualNet simulations

8. 8 SoftRepeater Architecture Built on VirtualWifi – allowing two virtual interfaces for a wireless card: Primary Virtual Interface – communication between AP and repeater in managed mode Repeater Virtual Interface – communication between repeater and client in adhoc mode Repeater Virtual Interface activated only when beneficial to both repeater and client Alternate between primary and repeater interfaces with switching overhead < 40ms Optional Network Coding Engine that further boosts throughput, with slight modifications to AP Multiple radios can be supported (not in our current experiments)

9. 9 Detecting Rate Anomaly Goal: Determine When SoftRepeater is beneficial Key steps: Collect information from nearby stations in promiscuous mode: Number of packets transmitted Average size of packets RSSI Data transmission rate BSSID Check utilization of medium. If neighbors send about the same number of packets, but at a low rate, rate anomaly may exist.

10. 10 Repeater Utility Function Goal: capture throughput gain of both repeater and client Define α: fraction of time spent in managed mode Assumptions: Stations always have backlogged data to send (i.e., saturated case) Implying equal channel access Good approximation for file-transfer applications Zero switching overhead 1 - α = fraction of time spent in adhoc mode Can easily account for non-zero switching overhead Intuition: if utility improved for both repeater and client, activate SoftRepeater AP A B client repeater

11. 11 Repeater Utility Function Without SoftRepeater: B’s throughput: T B A’s throughput: T A With SoftRepeater: B’s Throughput: αT B / 2 A’s throughput: min(αT B / 2, (1- α)T A,B ) T A,B = inferred throughput between A and B from RSSI measurement If max-min fairness is used, repeater utility function becomes T* = max α min{αT B / 2, min(αT B / 2, (1- α)T A,B )} If T* > T A and T* > T B (better for both) activate SoftRepeater AP A B client repeater TATA TBTB T A,B

12. 12 Generalizing Repeater Utility Function For different objectives: Maximizing total throughput: starve client (bad) Max-min fairness Proportional fairness For different settings: In presence of interfering nodes In presence of multiple clients Multiple radios Multiple wireless cards Details in paper and tech report

13. 13 Repeater Initiation Protocol Goal: confirm and reach consensus on activating SoftRepeater For now: simple 4-way handshake: B broadcasts SoftRepeater offer A infers data rate from A to B (from RSSI) and unicasts response B picks clients to serve (if utility improved) and broadcasts final “Take it or leave it” offer A unicasts accept/reject AP A B client repeater 1. broadcast offer 3. broadcast new offer 2. unicast response 4. unicast accept/reject

14. 14 Testbed Experiments SoftRepeater is implemented on Windows XP Testbed experiments in office building AP located at X Repeater (node R) fixed at Y Client (node C) moved between Y, T, Z Use 802.11a, with auto-rate feature enabled Focus on Max-Min fairness

15. 15 Experiment 1: Downlink UDP UDP throughput improved by 200% with SoftRepeater when rate anomaly exists AP R C rate anomaly scenario:

16. 16 Experiment 2: Downlink TCP TCP throughput improved by 50% with SoftRepeater when rate anomaly exists, even communication alternates between managed and adhoc modes AP R C rate anomaly scenario:

17. 17 Experiment 3: UDP with 2 clients UDP throughput improved with SoftRepeater when two clients served AP RC 1 C 2 rate anomaly scenario:

18. 18 Qualnet Simulation: Effectiveness of Repeater Initiation Protocol SoftRepeater activated only when there is throughput gain AP in office 0 Client in office 9 Downlink UDP for both repeater and client

19. 19 Qualnet Simulation: Multiple Clients SoftRepeater improves the baseline throughput by more than 65%. AP in office 0 Repeater in office 3 N clients in office 9 Downlink UDP

20. 20 Summary of Experimental Results Main observation: throughput significantly improved for UDP/TCP flows when rate anomaly exists More experiments in paper/tech. report Correctness of repeater initiation protocol Extension with network coding Various traffic scenarios Qualnet simulation for more “complicated” scenarios (e.g., interfering nodes, multiple repeaters/clients)

21. 21 Conclusions Propose SoftRepeater, a practical, deployable system that addresses rate anomaly problem Formulate different utility maximization problems for SoftRepeater Implement a prototype that demonstrates the improvement of SoftRepeater

22. 22 Questions: pclee@cs.columbia.edu

23. 23 Security Issues Security concerns: Privacy End-to-end encryption (e.g., IPsec) can be used Greedy/malicious repeaters Client monitors channel; quits if performance becomes worse after SoftRepeater is used Conclusion: Security is no worse than SoftRepeater-free networks