Measuring Queue Capacities of IEEE 802.11 Wireless Access Points

the 4th IEEE International Conference on Broadband Communications, Networks and Systems (BROADNETS)Raleigh, NC, USA September 10-13, 2007

Measuring Queue Capacities of IEEE Measuring Queue Capacities of IEEE 802.11 Wireless Access Points802.11 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

BROADNETS September 13, 2007

IntroductionIntroduction

• 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.

OutlineOutline

• Introduction• Access Point Queue (APQ)

Method• Validation• APQ Measurements• Discussion and Conclusion

Access Point Queue Method Access Point Queue Method (APQ)(APQ)

TDDq lhp

Dh: delay measured with a full queue

Dl: delay measured with an empty queue

T: Throughput

• Compute queue capacity (qp) with equation:

, where L is Packet Length.

Sender Client

APQ SessionAPQ Session

OutlineOutline

Method• Validation• APQ Measurements• Discussion and Conclusion

APQ ValidationAPQ Validation

Demonstrated the ability to control Host AP queue capacity

APQ Validation ResultsAPQ Validation Results

APQ accurately estimates actual Host AP queue capacities with tight confidence intervals.

OutlineOutline

Method• Validation• APQ Measurements • Discussion and Conclusion

Experimental Setup 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.

Target APsTarget APs

• Selected 7 different wireless APs, three from commercial class and four from residential class.

AP StaturationAP Staturation

Maximum throughput varies across APs.

Negligible loss rate until offered load near saturation.

Throughput and Delay Changes During APQ Throughput and Delay Changes During APQ SessionSession

Approximate half second difference suggests significant difference in queue capacities.

Both throughputs are fairly consistent.

CDF of Delay during APQ CDF of Delay during APQ Session Session

• Data from Netgear Commercial AP

• Compute queue capacity based on equation: L

TDDq lhp

AP Queue Capacities AP Queue Capacities

• Packet based queue inside 7 target APs

• Queue Capacity varies from venders and classes.

Discussion 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.

ConclusionsConclusions

• 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.

Questions ? Questions ?

Measuring Queue Capacities of IEEE Measuring Queue Capacities of IEEE 802.11 Wireless Access Points802.11 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

Thank You! Thank You!

Saturates a Wireless Link

1. Starting from a sending rate(S1 ) 50 packets/sec.

2. Increasing sending rate by 50 packets/sec until packet loss rates (pi ) of 20% or more are observed for three successive sending rates (Si−2, Si-1, Si ).

3. Taking Si-1 as the saturation sending rate.

Understanding Linux Kernel Understanding Linux Kernel QueueQueue

Access Point Queue Method Access Point Queue Method (APQ)(APQ)

• Fill up “downstream” queue inside AP.

• Measure delay changes (Dh-Dl ) and throughput (T ) at saturation.

Dh: the delay measured with full queue.Dl : the delay measured with empty queue.L : Packet Length.

TDDq lhp

Measuring Queue Capacities of IEEE 802.11 Wireless Access Points

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