1. the 4th IEEE International Conference on Broadband Communications, Networks and Systems (BROADNETS) Raleigh, NC, USA September 10-13, 2007 Measuring Queue Capacities of IEEE 802.11 Wireless Access Points Feng Li, Mingzhe Li, Rui Lu, Huahui Wu, Mark Claypool, and Robert Kinicki {lif, lmz, kkboy, flashine, claypool, rek}@cs.wpi.edu Computer Science Department at Worcester Polytechnic Institute Worcester, MA, 01609, USA

2. BROADNETS September 13, 2007 2 Introduction Performance bottleneck for many Internet applications exists at last hop from wireless access point (AP) to wireless end host. –Corporate/Campus networks. –Wireless clients have weak wireless connection. Queue capacity of APs impacts performance of wireless applications. –Too small  may decrease throughput ! –Too large  increase latency ! Little known on queue capacity choice for APs. –Blackbox devices.

3. BROADNETS September 13, 2007 3 Outline Introduction Access Point Queue (APQ) Method Validation APQ Measurements Discussion and Conclusion

4. BROADNETS September 13, 2007 4 Access Point Queue Method (APQ) D h : delay measured with a full queue D l : delay measured with an empty queue T: Throughput AP Compute queue capacity (q p ) with equation:, where L is Packet Length. SenderClient

5. BROADNETS September 13, 2007 5 APQ Session sec

6. BROADNETS September 13, 2007 6 APQ Session sec

7. BROADNETS September 13, 2007 7 Outline Introduction Access Point Queue (APQ) Method Validation APQ Measurements Discussion and Conclusion

8. BROADNETS September 13, 2007 8 APQ Validation Demonstrated the ability to control Host AP queue capacity

9. BROADNETS September 13, 2007 9 APQ Validation Results APQ accurately estimates actual Host AP queue capacities with tight confidence intervals.

10. BROADNETS September 13, 2007 10 Outline Introduction Access Point Queue (APQ) Method Validation APQ Measurements Discussion and Conclusion

11. BROADNETS September 13, 2007 11 Experimental Setup Test bed similar to validation by replacing Host AP with target AP. For all APQ measurement runs: –Data rate fixed at 11 Mbps. –RTS/CTS disabled. –Antenna positioned carefully to reach better performance. MGEN generates UDP traffic for all experiments. Two packet sizes (1500 bytes/packet and 750 bytes/packet) used to determine packet based queue or byte based queue.

12. BROADNETS September 13, 2007 12 Target APs Selected 7 different wireless APs, three from commercial class and four from residential class.

13. BROADNETS September 13, 2007 13 AP Staturation Maximum throughput varies across APs. Negligible loss rate until offered load near saturation.

14. BROADNETS September 13, 2007 14 Throughput and Delay Changes During APQ Session Approximate half second difference suggests significant difference in queue capacities. Both throughputs are fairly consistent.

15. BROADNETS September 13, 2007 15 CDF of Delay during APQ Session Dh-DlDh-Dl Data from Netgear Commercial AP Compute queue capacity based on equation:

16. BROADNETS September 13, 2007 16 AP Queue Capacities Packet based queue inside 7 target APs Queue Capacity varies from venders and classes.

17. BROADNETS September 13, 2007 17 Discussion Difficult to decide on “best” queue capacity. Queue capacity guideline (bandwidth-delay product) not followed. More research needed to decide “best” queue. Research on adaptive queue.

18. BROADNETS September 13, 2007 18 Conclusions APQ - two step black box method which accurately measures AP queue capacity. Measured seven off-shelf APs. AP queue capacities vary across vendors and classes. AP queue capacity measurements provide: –Realistic queue capacity parameters for WLAN simulations. –Starting point for future AP design strategies.

19. BROADNETS September 13, 2007 19 Questions ? Measuring Queue Capacities of IEEE 802.11 Wireless Access Points Feng Li, Mingzhe Li, Rui Lu, Huahui Wu, Mark Claypool, and Robert Kinicki {lif, lmz, kkboy, flashine, claypool, rek}@cs.wpi.edu Computer Science Department @Worcester Polytechnic Institute Worcester, MA, 01609, USA

20. BROADNETS September 13, 2007 20 Thank You!

21. BROADNETS September 13, 2007 21 Saturates a Wireless Link 1.Starting from a sending rate(S 1 ) 50 packets/sec. 2.Increasing sending rate by 50 packets/sec until packet loss rates (p i ) of 20% or more are observed for three successive sending rates (S i−2, S i-1, S i ). 3.Taking S i-1 as the saturation sending rate.

22. BROADNETS September 13, 2007 22 Understanding Linux Kernel Queue

23. BROADNETS September 13, 2007 23 Access Point Queue Method (APQ) Fill up “downstream” queue inside AP. Measure delay changes (D h -D l ) and throughput (T ) at saturation. D h : the delay measured with full queue. D l : the delay measured with empty queue. L : Packet Length.