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On Accurate and Asymmetry-aware Measurement of Link Quality in Wireless Mesh Networks Author : Kyun-Han Kim Conference : MobiCom 06 2009. 5. 18 Presenter : Koh Choi Networked Media Laboratory Dept. of Information & Communications Gwangju Institute of Science & Technology (GIST) DEPT. OF INFO. & COMM., GIST Introduction Important thing of link-quality information Availability of accurate link-quality information To select the best relay nodes. Application, such as video streaming and VoIP, need link-quality information Diagnosing a network Large scale WMN requires accurate long-term statistics of link-quality information Proposed EAR(Efficient and Accurate link-quality monitoR) Three complementary measurement schemes Passive, Cooperative, and Active monitoring Identify the existence of wireless link asymmetry By measuring the quality of each link in both directions of the link Cross-layer architecture across both the network layer and the IEEE 802.11based device driver Makes EAR easily deployable in existing multi-hop wireless mesh networks Main focus Present a novel link-quality measurement framework Show potential benefits of the framework DEPT. OF INFO. & COMM., GIST Limitation of related work(1/2) BAP(Broadcast-based Active Probing) Widely used for adopting link-quality-aware routing metrics Such as ETX(Expected Transmission Count) and ETT(Expected Transmission Time) Based on inexpensive broadcast Easy to implement at all layers Different PHY settings Bidirectional measurements A B LAB S AB SBA BAP A Data B SBA=0.6 A SAB=0.9 LAB = LBA = 0.54 Bidirectional measurement DEPT. OF INFO. & COMM., GIST B ACK LAB= 0.9 Real data transmission Limitation of related work(2/2) Unicast-based Active Probing Same PHY settings as data transmissions Unidirectional measurement(LAB≠ LBA) 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 DEPT. OF INFO. & COMM., GIST A B EAR Design and Operations(1/1) Tegg ≥ Pthresh MeasureMeasureTcrss ≥ Cthresh Cooperative period (i) Cycle (i) Tcrss < Pthresh Tcrss ≥ Cthresh UpdateActiveperiod (i) Time Tcrss ≤ Cthresh DEPT. OF INFO. & COMM., GIST Task Processor Techniques Passive Cooperative Active Routing-table Manager Link State Table Passive Tegg < Pthresh Task Timers Outgoing traffic Incoming traffic oEAR MAC Tegg ≥ Pthresh 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 5 Approach of EAR(1/2) Efficient and Accurate link-quality monitoR Exploits existing traffic by adaptive selection of passive, active or cooperative measurement scheme Uses unicast packets and derives unidirectional results Distributed and periodic measurement Independently measures the quality of link from a node to its neighbor in a fullydistributed way Cross-layer interaction Inner EAR : periodically collects and derives link-quality information in the network layer Outer EAR : monitors egress/cross traffic at the device driver Mesh Router Inner EAR or iEAR IP EAR Device driver MAC / PHY DEPT. OF INFO. & COMM., GIST Outer EAR or oEAR Approach of EAR(2/2) Tegg ≥ Pthresh Passive Tegg < Pthresh Tegg ≥ Pthresh Tcrss ≥ Cthresh Cooperative (A) Tcrss < PthreshTcrss ≥ Cthresh Tcrss ≤ Cthresh Active (B) (C) • Egress traffic(Tegg) • Certain threshold(Pthresh) • Actual traffic level(Cthresh) Operation of EAR Measuring node has enough egress traffic, EAR favors passive monitoring Passive scheme : collect accurate and stable link-quality information from a large volume of existing data traffic. When measuring node has no egress traffic to a neighbor node, but has cross-traffic, use cooperative scheme This scheme use broadcast nature of wireless media. Cooperative node(C) overhear the traffic from the measuring node (B) to the other neighbors(A) – (cross traffic) No egress/cross traffic, use active scheme Send unicast probe packets to neighbor nodes. DEPT. OF INFO. & COMM., GIST Measurement techniques(1/3) 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 DEPT. OF INFO. & COMM., GIST Measurement techniques(2/3) Cooperative scheme A B C Selective overhearing Overhearing cross traffic Reporting overhearing results Link-state table at B Time Links Scheme Ratio Data rate BA Passive 0.9 11 Mbps BC Coop 0.9 11 Mbps DEPT. OF INFO. & COMM., GIST Measurement techniques(3/3) 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 DEPT. OF INFO. & COMM., GIST W=2 W=4 W=1 P P P P Cycle 10 Performance Evaluation(1/1) Implementation Linux kernel-2.4.20 N W ETX and ETT routing metrics BAP for comparison E N4 S N8 Corridor Testbed N10 N9 2nd floor of EECS Building 10 mesh nodes N7 N5 N1 N2 IEEE 802.11b PCMCIA Evaluation Metrics Accuracy, asymmetry-awareness, and efficiency DEPT. OF INFO. & COMM., GIST Offices N6 N3 Performance Evaluation(1/3)-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 DEPT. OF INFO. & COMM., GIST Performance Evaluation(2/3)-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 DEPT. OF INFO. & COMM., GIST Performance Evaluation(3/3)-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 DEPT. OF INFO. & COMM., GIST 14 Conclusion EAR(Efficient and Accurate link-quality monitoR) Solves problems of varying and asymmetric wireless link-quality in wireless mesh network A hybrid measurement framework Efficiently and accurately measures wireless link quality Useful for wireless network protocols Routing, QoS support and networks diagnosis Remaining Issues Measurement of other QoS parameters(e.g, latency) Extension for MANETs What is link quality? Why does not consider available bandwidth of link? They only consider data transmission rate and delivery ratio. It need available bandwidth and latency value. DEPT. OF INFO. & COMM., GIST