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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 ACM MobiCom Accurate Measurement of Link Quality Routing protocols Quality-of-Service Fault diagnosis Channel assignment Internet Gateway 1 Gateway 2 Accuracy Efficiency Asymmetry Feasibility Focus of this work Present a novel link-quality measurement framework Show potential benefits of the framework ` 2 Outline Limitations Approach Evaluation Conclusion 3 Limitations Broadcast-based Active Probing (BAP) ▪ Based on inexpensive broadcast ▪ Easy to implement at all layers LAB S AB SBA A B Different PHY settings [Aguayo04] Bidirectional measurements BAP A Data B SBA=0.6 LAB = LBA = 0.54 A SAB=0.9 B ACK LAB= 0.9 4 Limitations Unicast-based Active Probing ▪ Same PHY settings as data transmissions ▪ Unidirectional measurement (LAB≠ LBA) A B 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 5 Outline Limitations Approach Evaluation Conclusion 6 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 Mesh Router Inner EAR or iEAR IP EAR Device driver Outer EAR or oEAR MAC / PHY 7 EAR Design and Operations Tegg ≥ Pthresh MeasureMeasureTcrss ≥ Cthresh Cooperative period (i) Cycle (i) Tcrss < Pthresh Tcrss ≥ Cthresh UpdateActiveperiod (i) Time Tcrss ≤ Cthresh Techniques Passive Cooperative Active Routing-table Manager Link State Table Passive Tegg < Pthresh Task Processor Outgoing traffic Incoming traffic oEAR MAC Tegg ≥ Pthresh Task Timers iEAR Distributed measurement Hybrid techniques Unicast-based results Cross-layer interaction Management Information Base at MAC Data frame transmission results Link quality of interest Link capacity: Data transmission rate Delivery ratio: d = NS/NT 8 Measurement Techniques (1) Passive scheme A B C Monitoring at a device driver Interaction with MAC’s MIB Obtaining transmission results Link-state table at B Time Links Scheme Ratio Data rate BA Passive 0.9 11 Mbps 9 Measurement Techniques (2) Cooperative scheme A B C Selective overhearing Overhearing cross traffic Reporting overhearing results Time Link-state table at B Links Scheme Ratio Data rate BA Passive 0.9 11 Mbps BC Coop 0.9 11 Mbps 10 Measurement Techniques (3) Active scheme A B C Minimizing probe overheads Adaptive active probing timer (ET) Using a cooperation technique Link-state table at B Time Links Scheme Ratio Data rate BA Active 0.9 11 Mbps BC Active-Co 0.9 11 Mbps ET=rand[0,W] P W=2 W=4 W=1 P P P P Cycle 11 Outline Limitations Approach Evaluation Conclusion 12 Performance Evaluation Implementation ▪ Linux kernel-2.4.20 (Netfilter and Orinoco device driver) ▪ ETX and ETT routing metrics N W E ▪ BAP for comparison N4 N8 S Testbed ▪ ▪ ▪ ▪ Corridor 2nd floor of EECS Building N10 10 mesh nodes IEEE 802.11b PCMCIA N5 N2 Other public networks (802.11b/g) N9 N7 N1 Offices N6 N3 Evaluation Metrics ▪ Accuracy, asymmetry-awareness, and efficiency 13 Characteristics of Link Asymmetry Link asymmetry is common diff =| SF– SB | duration Wireless link-quality has different degrees of quality asymmetry with different amounts of asymmetry duration 14 Accuracy Comparison between BAP and EAR ▪ BAP: 10.2% error ▪ EAR: 1.6% error SN1N2 N1 N2 LN1N2 EAR reduces measurement error from 4 to 20 times, compared to BAP, and provides unidirectional results 15 Asymmetry Awareness EAR improves end-to-end throughput N W E N4 S N8 Corridor BAP N5 N10 N9 EAR N7 N1 N2 Offices N6 N3 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 16 Efficiency Probing overheads ▪ Large number of neighboring ▪ ▪ nodes in 200m x 200m No egress/cross traffic Thanks to cooperation and exponential back-off timers Use of data traffic for measurements 13 times more measurement traffic than BAP owing to hybrid approach 17 Outline Limitations Approach Evaluation Conclusion 18 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 19 Any questions? Thank You ! Contact: Kyu-Han Kim ([email protected]) Real-Time Computing Laboratory (http://kabru.eecs.umich.edu) 20