DEPT. OF INFO. & COMM., GIST

On Accurate and Asymmetry-aware Measurement of Link Quality in Wireless

Mesh Networks

Author : Kyun-Han KimConference : MobiCom 06

2009. 5. 18Presenter : Koh Choi

Networked Media LaboratoryDept. of Information & Communications

Gwangju Institute of Science & Technology (GIST)

DEPT. OF INFO. & COMM., GIST

Important thing of link-quality informationAvailability of accurate link-quality information

To select the best relay nodes.

Application, such as video streaming and VoIP, need link-quality informationDiagnosing 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 asymmetryBy measuring the quality of each link in both directions of the link

Cross-layer architecture across both the network layer and the IEEE 802.11-based device driver

Makes EAR easily deployable in existing multi-hop wireless mesh networks

Main focusPresent a novel link-quality measurement frameworkShow potential benefits of the framework

Introduction

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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 broadcastEasy to implement at all layersDifferent PHY settingsBidirectional measurements

BAABAB SSL

SAB=0.9SBA=0.6

A B

BAP

A B

Data

LAB= 0.9LAB = LBA = 0.54

ACK

Bidirectional measurement Real data transmission

A B

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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

Limitation of related work(2/2)

A B

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EAR Design and Operations(1/1)

Distributed measurement Distributed measurement Hybrid techniques Distributed measurement Hybrid techniques Unicast-based results

Distributed measurement Hybrid techniques Unicast-based results Cross-layer interaction

Outgoing traffic Incoming traffic

oEA

R

Techniques

TaskProcessor

Routing-tableManager

Link StateTable

Task Timers

Cooperative

Passive

Active

iEA

RM

AC

Measure-period (i)

Update-period (i)

Link quality of interest

Link capacity: Data transmission rate Delivery ratio: d = NS/NT

Management Information Base at MAC Data frame transmission results

Tim

e

Measure- Cycle (i)

Cooperative

Active

Tegg ≥ Pthresh

Tcrss ≥ Cthresh

Tcrss ≤ Cthresh

Tegg < Pthresh

Tcrss < Pthresh Tcrss ≥ Cthresh

Tegg ≥ Pthresh

Passive

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Efficient and Accurate link-quality monitoRExploits existing traffic by adaptive selection of passive, active or cooperative measurement schemeUses unicast packets and derives unidirectional resultsDistributed and periodic measurement

Independently measures the quality of link from a node to its neighbor in a fully-distributed way

Cross-layer interactionInner 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 iEAR

Outer EAR or oEARMAC / PHY

Device driver

IPEAR

Mesh Router

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Cooperative

Active

Tegg ≥ Pthresh

Tcrss ≥ Cthresh

Tcrss ≤ Cthresh

Tegg < Pthresh

Tcrss < Pthresh Tcrss ≥ Cthresh

Tegg ≥ Pthresh

Passive

Approach of EAR(2/2)

Operation of EARMeasuring node has enough egress traffic, EAR favors passive monitoring

Passive scheme : collect accurate and stable link-quality information from a largevolume of existing data traffic.

When measuring node has no egress traffic to a neighbor node, but hascross-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 otherneighbors(A) – (cross traffic)

No egress/cross traffic, use active schemeSend unicast probe packets to neighbor nodes.

• Egress traffic(Tegg)

• Certain threshold(Pthresh)

• Actual traffic level(Cthresh)

(A) (B) (C)

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Measurement techniques(1/3)

A B

Tim

e

CPassive scheme Monitoring at a device driver Interaction with MAC’s MIB Obtaining transmission results

0.9 11 MbpsPassive

Links Scheme Ratio Data rate

Link-state table at B

BA

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Measurement techniques(2/3)

CoopREP(NS)

CoopREQ(A)

A B

Tim

e

CCooperative scheme Selective overhearing Overhearing cross traffic Reporting overhearing results

0.9 11 MbpsPassive

Links Scheme Ratio Data rate

Link-state table at B

BABC 11 MbpsCoop 0.9

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Measurement techniques(3/3)

10

A B

Tim

e

C

CoopREP(NS)

CoopREQ(A)

Active scheme Minimizing probe overheads Adaptive active probing timer (ET) Using a cooperation technique

P P P

W=2 W=4 W=1

PP Cycle

ET=rand[0,W]

0.9 11 MbpsActive

Links Scheme Ratio Data rate

Link-state table at B

BA

BC 11 MbpsActive-Co 0.9

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Performance Evaluation(1/1)

N

SW E

Corridor

Offices

N4

N9N10

N3

N8

N1N2

N6

N7

N5

Implementation

Linux kernel-2.4.20

ETX and ETT routing metrics

BAP for comparison

Testbed

2nd floor of EECS Building

10 mesh nodes

IEEE 802.11b PCMCIA

Evaluation Metrics

Accuracy, asymmetry-awareness, and

efficiency

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Performance Evaluation(1/3)-Accuracy

LN1N2

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

SN1N2

N1 N2

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Performance Evaluation(2/3)-Link asymmetry

Link asymmetry is common

duration

diff =| SF– SB |

Wireless link-quality has different degrees of qualityasymmetry with different amounts of asymmetry duration

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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

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Conclusion

EAR(Efficient and Accurate link-quality monitoR)Solves problems of varying and asymmetric wireless link-quality in wireless mesh networkA hybrid measurement framework

Efficiently and accurately measures wireless link quality

Useful for wireless network protocolsRouting, QoS support and networks diagnosis

Remaining IssuesMeasurement 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.