On Accurate Measurement of Link Quality in Multi-hop Wireless Mesh Networks

1. On Accurate Measurement of Link Quality in Multi-hop Wireless Mesh Networks Kyu-Han Kim and Kang G. Shin Real-Time Computing Laboratory Department of EECS, The University of Michigan September 25, 2006

2. Accuracy Efficiency Accuracy Efficiency Accuracy Asymmetry Efficiency Asymmetry Feasibility Fault diagnosis Routing protocols Routing protocols Quality-of-Service Routing protocols Routing protocols Fault diagnosis Quality-of-Service Channel assignment Quality-of-Service Focus of this work • Present a novel link-quality measurement framework • Show potential benefits of the framework Accurate Measurement of Link Quality

3. Outline • Limitations • Approach • Evaluation • Conclusion

4. A B BAP Data A A B B Limitations • Broadcast-based Active Probing (BAP) • Based on inexpensive broadcast • Easy to implement at all layers • Different PHY settings [Aguayo04] • Bidirectional measurements ACK SBA=0.6 SAB=0.9 LAB = LBA = 0.54 LAB= 0.9

5. A B Limitations • Unicast-based Active Probing • Same PHY settings as data transmissions • Unidirectional measurement (LAB≠ LBA) • Capacity overhead (i.e., O(n) vs. O(1) ) • Blind to underlying retransmission at MAC • Self-monitoring data frame transmission • Reduce probing overheads • Use unicast and unidirectional results • Require active probing for probing idle links • Blind to underlying retransmission at MAC

6. Outline • Limitations • Approach • Evaluation • Conclusion

7. Mesh Router IP EAR Outer EAR or oEAR Device driver MAC / PHY EAR: Efficient and Accurate link-quality monitoR • EAR • exploits existing traffic by adaptive selection of passive, active or cooperative measurement scheme • uses unicast packets and derives unidirectional results • is easily deployable and places itself at a network layer and a device driver for cross-layer interactions Inner EAR or iEAR

8.     iEAR Techniques Routing-table Manager Task Timers Passive Cooperative Task Processor Active oEAR     Passive Tegg ≥ Pthresh Tegg < Pthresh Outgoing traffic Incoming traffic Tegg ≥ Pthresh Measure-period (i) Measure- Cycle (i) Cooperative Tcrss ≥ Cthresh Link State Table Tcrss < Pthresh Tcrss ≥ Cthresh Update-period (i) Active Tcrss ≤ Cthresh Time EAR Design and Operations Distributed measurement Distributed measurement  Hybrid techniques  Unicast-based results Distributed measurement  Hybrid techniques Distributed measurement  Hybrid techniques  Unicast-based results  Cross-layer interaction MAC • Management Information Base at MAC • Data frame transmission results • Link quality of interest • Link capacity: Data transmission rate • Delivery ratio: d = NS/NT

9. B C A Link-state table at B Links Scheme Ratio Data rate BA Measurement Techniques (1) Passive scheme • Monitoring at a device driver • Interaction with MAC’s MIB • Obtaining transmission results Passive 0.9 11 Mbps Time

10. B C CoopREQ(A) A Link-state table at B Links Scheme Ratio Data rate BA Passive 11 Mbps CoopREP(NS) BC Coop 11 Mbps Measurement Techniques (2) Cooperative scheme • Selective overhearing • Overhearing cross traffic • Reporting overhearing results 0.9 0.9 Time

11. B C CoopREQ(A) A Link-state table at B Cycle Links Scheme Ratio Data rate BA Active 0.9 11 Mbps CoopREP(NS) BC Active-Co 11 Mbps 0.9 Measurement Techniques (3) Active scheme • Minimizing probe overheads • Adaptive active probing timer (ET) • Using a cooperation technique Time ET=rand[0,W] W=4 W=2 W=1 P P P P P

12. Outline • Limitations • Approach • Evaluation • Conclusion

13. Performance Evaluation • Implementation • Linux kernel-2.4.20 (Netfilter and Orinoco device driver) • ETX and ETT routing metrics • BAP for comparison • Testbed • 2nd floor of EECS Building • 10 mesh nodes • IEEE 802.11b PCMCIA • Other public networks (802.11b/g) • Evaluation Metrics • Accuracy, asymmetry-awareness, and efficiency

14. duration Characteristics of Link Asymmetry • Link asymmetry is common diff =| SF– SB | Wireless link-quality has different degrees of quality asymmetry with different amounts of asymmetry duration

15. LN1N2 Accuracy • Comparison between BAP and EAR • BAP: 10.2% error • EAR: 1.6% error SN1N2 N1 N2 EAR reduces measurement error from 4 to 20 times, compared to BAP, and provides unidirectional results

16. Asymmetry Awareness • EAR improves end-to-end throughput BAP EAR • Benefits • Goodput improvement • 12.9~35.2% (1-hop), 114% (3-hop) • Thanks mainly to unidirectional measurements of EAR EAR helps routing protocols identify/use asymmetric links

17. Use of data traffic for measurements Efficiency • Probing overheads • Large number of neighboring nodes in 200m x 200m • No egress/cross traffic • Thanks to cooperation and exponential back-off timers 13 times more measurement traffic than BAP owing to hybrid approach

18. Outline • Limitations • Approach • Evaluation • Conclusion

19. Conclusion • EAR solves problems of varying and asymmetric wireless link-quality in wireless mesh networks • EAR is a hybrid measurement framework that efficiently and accurately measures wireless link quality • EAR’s link-asymmetry-awareness improves end-to-end throughput by up to two times • EAR is useful for wireless network protocols, such as routing, QoS support and network diagnosis • Remaining Issues • Measurement of other QoS parameters (e.g., latency) • Extension for MANETs

20. Any questions? Thank You ! Contact: Kyu-Han Kim (kyuhkim@eecs.umich.edu) Real-Time Computing Laboratory (http://kabru.eecs.umich.edu)