department of computer science and engineering uestc 1 rxlayer: adaptive retransmission layer for...

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Department of Computer Science and Engineering UESTC

RxLayer: Adaptive Retransmission Layer for Low Power Wireless

Daibo Liu1, Zhichao Cao2, Jiliang Wang2

Mengshu Hou1 and Yunjun Li1

1 University of Electronic Science and Technology of China2 Tsinghua University

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Wireless data transmissionWireless data transmission

Data

ACKData

Packet loss

• Countermeasures:– Retransmission– Link quality update– Change next-hop

Data

Retransmission & update link quality

Data

Data

ACK

Change next-hop

• Communication over unreliable wireless links

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Link burstinessLink burstinessHowever..

Receivers' PRRReceivers' PRR

The change of RSSI The change of RSSI at receiverat receiver

Appearance of ObstaclesAppearance of Obstacles

• Many factors have effect on link burstiness

Link burstiness brings about consecutive retransmission failures

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Link correlationLink correlationHowever..

Three receivers' PRRThree receivers' PRR

Different concurrent interferenceDifferent concurrent interference

R1

R2

R3

Link correlation brings about ill-advisednext-hop change, e.g., replacing R1 with R2.

Inte

rfer

ence

(dB

m)

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Serious situationSerious situation

Complicated external circumstance aggravates link burstiness and correlation

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IIllll-advised retransmission-advised retransmissionConsequence..

SinkS

A

B

C

DEF

H

I

JK

L

M

Bursty area Reliable link

Bursty linkInterference sources

Quick decrease of <S, A>' link quality

Quick decrease of <S, B>' link quality

<S, D> is relatively stable

<S, D> is not the optimal link now

S is source node.A is S’ parent node.B, C, and D are S’ candidate next-hop.

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IIllll-advised retransmission-advised retransmission

SinkS

A

B

C

DEF

H

I

JK

L

M

Bursty area Reliable link

Bursty linkInterference sources

S is source node.A is S’ parent node.B, C, and D are S’ candidate next hop node.

Consecutive retransmission strategy is inefficient when link is Consecutive retransmission strategy is inefficient when link is severely degraded.severely degraded.

It is ineffectual to change next-hop node only according to link It is ineffectual to change next-hop node only according to link quality.quality.

Consecutive retransmission strategy misleads link estimator in the Consecutive retransmission strategy misleads link estimator in the presence of link burstiness and correlation.presence of link burstiness and correlation.

How to accurately perceive the link burstiness?

How to select the optimal candidate receiver?

Consequence..

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Efficiency of retransmissionEfficiency of retransmission

• Quantify the conditional probability of immediate retransmission– Conditional packet delivery functions (CPDF)

i

ii

NNNiCPDF 1)(

CPDF(i) is the probability that the ith retransmission successes after i-1 consecutive failures.

Ni is the cumulative count that packets are retransmitted no less than i times.

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Efficiency of retransmissionEfficiency of retransmission

CPDF quickly slips down to CPDF quickly slips down to 0.2.0.2.

Consecutive retransmission Consecutive retransmission (CR) is inefficient.(CR) is inefficient.

A interrupt point of CR is A interrupt point of CR is needed .needed .

Pause consecutive retransmissions

i

ii

NNNiCPDF 1)(

• Quantifying the conditional probability of immediate retransmission– Conditional packet delivery functions (CPDF)

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Dynamic feature of CPDFDynamic feature of CPDF

Outdoor IndoorTwo hours later

• Link CPDF– Different scenarios– Different time

However..

Link burstiness is time-varying and spatial-varyingLink burstiness is time-varying and spatial-varying..

Online capturing link burstiness is needed.Online capturing link burstiness is needed.

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Online model for link burstinessOnline model for link burstinessSolution

failifCPDF

successifCPDFCPDF

oldi

oldinew

i)1(

)1(

• Update link burstiness by moving average

is the cumulated CPDFi.oldiCPDF

• Value of α:

newiCPDF is the updated CPDFi.

Making a tradeoff between the adaptability to network dynamics and accuracy.

Long-term trace data could learn an appropriate value.

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Correlation of link pairCorrelation of link pair

High PRR and low P(0,6) indicating link 6 is not good when link 0 fails.

Low PRR and high P(0,9) indicating link 9 is a good substitution for link 0

1 Source node16 Receivers1000 Packets

P(i, j)P(i, j) is the probability that a packet transmission will success in link is the probability that a packet transmission will success in link jj while failed in link while failed in link ii..

• Correlation between link pair: P(i, j)

A high quality link is not always an optimal candidate A high quality link is not always an optimal candidate for a severely degraded link. for a severely degraded link.

The correlation between each pair of links should beThe correlation between each pair of links should becaptured with low overhead. captured with low overhead.

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Capturing link correlationCapturing link correlation

P(B,A) P(B,C)

P(A,B)

P(C,A)

P(A,C)

P(C,B)

• Bitmap and uniform broadcast sequence Bitmap and uniform broadcast sequence number(BSN)number(BSN)

5.0),(

5.0),(

21

21

BCP

ACP

75.0),(

1),(

43

44

CBP

ABP

However, bitmap size is limited, e.g., 2 bytes.

Solution

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Online model for link correlationOnline model for link correlationSolution

0)0()(

0)0()1/0(),,(

)(

)()(/

Si

Si

Sji

PjPRR

PPjiS

• Model for capturing link correlation: ω

• Update by using moving average

: the probability, when S transmits, that a packet succeed on link Sj given that it failed on link Si. : is the probability of packets failed on link Si.

)1/0()(/SjiP

)0()(SiP

),,(),,()1(),,( jiSjiSjiS newold

is the accumulated correlation between link i and j.),,( jiSold

is the computed correlation using the latest BSN set.),,( jiSnew

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Online model for link correlationOnline model for link correlationSolution

• Correlation update

),,(),,()1(),,( jiSjiSjiS newold

• Value of θ

n

M iigap 2)(

ωi is the correlation calculated by moving average by hearing the ith routing beacon.

Mi is the computed correlation using collected BSNs from 0 to i..

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RxLayer: Decision maker rulesRxLayer: Decision maker rules

Transmissionmodel

Link burstinessmodel

Link correlationmodel

1. Transmission failure/success

2. Immediate retransmission

3. Pause consecutive retransmissions4. Change next-hop

node

Solution

• Exploit link burstiness and correlation

Link burstiness model: update CPDF, pause consecutive retransmission.Link correlation model: select the optimal candidate receiver.Transmission model: transmit packet and report result to network layer.

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RxLayer in protocol stackRxLayer in protocol stack

• Integrate RxLayer into protocol stack

Solution

MAC Layer

Network Communication Layer

Link Estimator

RxLayer

Decision Maker

Link burstiness Model

Link correlation

Model

Beneath network communication layer;Above MAC layer;Connecting with link estimator.

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• Implementation:

-Integrating with CTP built upon LPL in TinyOS 2.1.1

• Goals

- High energy efficiency

- Improvement on forwarding delay, network reliability

• Scenarios

- Indoor Testbed: 22 Telosb nodes

- Outdoor Scenario: 30 Telosb nodes

EvaluationEvaluation

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Network reliabilityNetwork reliability

Indoor Outdoor

Indoor, the average PRRCTP+LPL+RxLayer with 1.53% improvement Than CTP+LPL

Outdoor, the average PRRCTP+LPL+RxLayer with 7.82%

improvement than CTP+LPL

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Transmission efficiencyTransmission efficiency

Indoor Outdoor

Indoor, the avg. tx countCTP+LPL+RxLayer with 24.7%


Outdoor, the avg. tx countCTP+LPL+RxLayer with 36.3%


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Energy consumptionEnergy consumption

Indoor Outdoor

Indoor, the avg. radio duty cycleUsing RxLayer, radio duty cycle

is reduced by about 3.5%.

Outdoor, the avg. duty cycleUsing RxLayer, duty cycle is

decreased from 19.3% to 10.4%

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- Online link burstiness model - Online link correlation model

Key design

- Indoor and outdoor experiments - Improvements on network efficiency

Evaluation

- Large testbed - Dyanmic forwarding

Future works

ConclusionsConclusions

• RxLayer is a ready-to-use module for existing protocol stack

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Thank you!Thank you!

Q&AQ&A

department of computer science and engineering uestc 1 rxlayer: adaptive retransmission layer for...

Documents

department of computer

engineering uestc

optimal link

link estimator

presence of link burstiness

retransmission consequence

correlation slide

consecutive retransmission