D EPT. OF I NFO. & C OMM., GIST On Accurate and Asymmetry-aware Measurement of Link Quality in Wireless Mesh Networks Author : Kyun-Han Kim Conference : MobiCom Presenter : Koh Choi Networked Media Laboratory Dept. of Information & Communications Gwangju Institute of Science & Technology (GIST)

D EPT. OF I NFO. & C OMM., GIST 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 based 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 Introduction

D EPT. OF I NFO. & C OMM., 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 S AB =0.9S BA =0.6 AB BAP AB Data L AB = 0.9L AB = L BA = 0.54 ACK Bidirectional measurement Real data transmission AB

D EPT. OF I NFO. & C OMM., GIST Unicast-based Active Probing Same PHY settings as data transmissions Unidirectional measurement(L AB ≠ L BA ) 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 Limitation of related work(2/2) AB

D EPT. OF I NFO. & C OMM., GIST 5 EAR Design and Operations(1/1)  Distributed measurement  Hybrid techniques  Distributed measurement  Hybrid techniques  Unicast-based results  Distributed measurement  Hybrid techniques  Unicast-based results  Cross-layer interaction Outgoing trafficIncoming traffic o EAR Techniques Task Processor Routing-table Manager Link State Table Task Timers Cooperative Passive Active i EAR MAC Measure- period (i) Update- period (i)       Link quality of interest  Link capacity: Data transmission rate  Delivery ratio: d = N S /N T Management Information Base at MAC  Data frame transmission results Time Measure- Cycle (i) Cooperative Active T egg ≥ P thresh T crss ≥ C thresh T crss ≤ C thresh T egg < P thresh T crss < P thresh T crss ≥ C thresh T egg ≥ P thresh Passive 

D EPT. OF I NFO. & C OMM., GIST 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 fully- distributed 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 Approach of EAR(1/2) Inner EAR or i EAR Outer EAR or o EAR MAC / PHY Device driver IP EAR Mesh Router

D EPT. OF I NFO. & C OMM., GIST Cooperative Active T egg ≥ P thresh T crss ≥ C thresh T crss ≤ C thresh T egg < P thresh T crss < P thresh T crss ≥ C thresh T egg ≥ P thresh Passive Approach of EAR(2/2) 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. Egress traffic(T egg ) Certain threshold(P thresh ) Actual traffic level(C thresh )

D EPT. OF I NFO. & C OMM., GIST Measurement techniques(1/3) A B Time C Passive scheme  Monitoring at a device driver  Interaction with MAC’s MIB  Obtaining transmission results MbpsPassive LinksSchemeRatioData rate Link-state table at B BA

D EPT. OF I NFO. & C OMM., GIST Measurement techniques(2/3) CoopREP(N S ) CoopREQ(A) A B Time C Cooperative scheme  Selective overhearing  Overhearing cross traffic  Reporting overhearing results MbpsPassive LinksSchemeRatioData rate Link-state table at B BA BC 11 MbpsCoop 0.9

D EPT. OF I NFO. & C OMM., GIST Measurement techniques(3/3) 10 A B Time C CoopREP(N S ) CoopREQ(A) Active scheme  Minimizing probe overheads  Adaptive active probing timer (E T )  Using a cooperation technique PPP W=2 W=4 W=1 PP Cycle E T =rand[0,W] MbpsActive LinksSchemeRatioData rate Link-state table at B BA BC 11 Mbps Active-Co 0.9

D EPT. OF I NFO. & C OMM., GIST Performance Evaluation(1/1) Implementation Linux kernel ETX and ETT routing metrics BAP for comparison Testbed 2 nd floor of EECS Building 10 mesh nodes IEEE b PCMCIA Evaluation Metrics Accuracy, asymmetry-awareness, and efficiency

D EPT. OF I NFO. & C OMM., GIST Performance Evaluation(1/3)-Accuracy L N1N2  Comparison between BAP and EAR ▪ BAP: 10.2% error ▪ EAR: 1.6% error EAR reduces measurement error from 4 to 20 times, compared to BAP, and provides unidirectional results S N1N2 N1N2

D EPT. OF I NFO. & C OMM., GIST Performance Evaluation(2/3)-Link asymmetry  Link asymmetry is common duration diff =| S F – S B | Wireless link-quality has different degrees of quality asymmetry with different amounts of asymmetry duration

D EPT. OF I NFO. & C OMM., GIST Performance Evaluation(3/3)-Efficiency 14  Use of data traffic for measurements  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

D EPT. OF I NFO. & C OMM., GIST 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.