1. Kyu-Han Kim and Kang G. Shin
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. Accurate Measurement of Link Quality
Focus of this work
Present a novel link-quality measurement framework
Show potential benefits of the framework
Asymmetry
Accuracy
Efficiency
Accuracy
Efficiency
Asymmetry
Feasibility
Accuracy
Efficiency
Routing protocols
Quality-of-Service
Fault diagnosis
Channel assignment
Routing protocols
Routing protocols
Quality-of-Service
Routing protocols
Quality-of-Service
Fault diagnosis

3. Outline
Limitations
Approach
Evaluation
Conclusion

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

5. 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

6. Outline
Limitations
Approach
Evaluation
Conclusion

7. EAR: 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
is easily deployable and places itself at a network layer and a device driver for cross-layer interactions
MAC / PHY
Device driver IP
EAR
Mesh Router
Inner EAR or iEAR
Outer EAR or oEAR

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

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

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

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

12. Outline
Limitations
Approach
Evaluation
Conclusion

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

14. 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

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

16. EAR helps routing protocols identify/use asymmetric links
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. Efficiency Probing overheads Use of data traffic for measurements
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

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
Real-Time Computing Laboratory (