24st Oct 2013 1

Correlated Coding: Efficient Network Coding under

Unreliable Wireless Links

Shuai Wang, Song Min Kim, Zhimeng Yin, and Tian He

University of Minnesota

ICNP 2014

Network Coding (NC)

Network coding has the potential to Improve the network performance.

Opportunistic Coding Linear Network Coding

Widely used coding technologies:

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Existing Problems & This Paper

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Use network coding or not?

Existing Problems - Community’s concern:

Our Solutions focus on the key factor – Link Correlation:

Help network designers decide whether to apply NC.

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Does the assumption reflect the real situation?

Assumptions in existing protocol designs, e.g., 1/LQ in COPE and MORE:Wireless transmissions are independent.

Unrealistic Assumption and Modeling

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Synthetic Independent TraceEmpirical Trace

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Wireless Links are Correlated!

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802.11 Testbed:1 AP6 Laptops100 Packets

Synthetic Independent TraceEmpirical Trace

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Wireless Links are Correlated!

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802.15.4 Testbed:1 Source node6 Receivers100 Packets

How Link Correlation Impacts NC?

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Network Coding

Step1: Coding Step2: Transmit

e.g., Opportunistic listening, encoding

Send out with coded packets

How LC Impacts Step 1 - “Coding”?

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Impact of link correlation on the “Coding” procedure:

Non-Coding ScenarioCoding Scenario

Sub-Conclusion 1: the “Coding” procedure prefers low link correlation (i.e., high diversity).

How LC Impacts Step 2 - “Transmit”？

(a) Low Correlated:

(b) High Correlated:

Sub-Conclusion 2: the “Transmit” procedure prefers high link correlation (i.e., low diversity). .

Link quality: 0.8# of coded pkts need to be retransmitted: 4

Link quality: 0.7# of coded pkts need to be retransmitted: 3

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Impact of link correlation on the “Transmit” procedure:

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Put Together: How LC Impacts NC?

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Network Coding

Step1: Coding Step2:Transmit

e.g., Opportunistic listening, encoding

Send out with one coded pkt

Low Link Correlation

High Link Correlation

Conclusion: there exists a tradeoff between the coding opportunity and transmit efficiency.

Our Solution – Key Idea

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1. Decompose Network Coding into two steps: (i) Coding, and (ii) Transmit.

2. Use the link correlation model to quantify the potential cost of Coding and Transmit separately.

3. Provide unified Correlated Coding metrics.

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The Solution

2( 1) ( )

K

i iil k k

Physical Meaning: The number of packets needs to be sent in the output queue after Network Coding.

Quantify the potential cost of Step 1 - “Coding”:

Wo/ NC:

W/ NC:

2

1

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The Solution

1 21

(S (u))1 1

(e ) (e ) (S (u))

K K ii i

i i i

p

p p p

Physical Meaning: is the expected transmissions to reliably broadcast one packet under link correlation.

Quantify the potential cost of Step 2 - “Transmit”:

1.56

… details in the paper!

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Physical Meaning: the per-receiver transmission cost to reliably broadcast one packet to K receivers

The Solution

The unified Correlated Coding Metric:

No. of Coded Pkts Per Pkt Transmit Cost

ETX / lK

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Applications – Metric Embedding

Network Coding Protocol

Packet Reception Information

Link Independence

Transmission Cost Estimation

1/LQ

Link Correlation

Correlated Coding Metric

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Applications – An Example

Switch/Hub

AP AP

Correlated Coding Metric

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Supported Protocols

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• Integrated Protocols:I. Unicast:

1). ZigBee

2). OLSR

3). ETX

II. Broadcast:

4). Spanning Tree

5). Forwarding Node Cluster

6). Partial Dominating Pruning

III. Multicast:

7). Flexible Multicast Service

Compared Protocols and Performance Metric

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• Compared Protocols:• Protocols W/O NC• Protocols W/ Coding Aware Design • Protocols W/ Correlated Coding Metric

• Performance Metric:• Number of Transmissions• Number of Coding Operations

Testbed Environment 19

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802.15.4: Lab 802.15.4: Open Office

802.15.4: Outdoor 802.11: University Building

Evaluation

• Number of Transmissions

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Compared with the protocol wo/ NC, w/ NC, w/ Coding aware design, the correlated coding design saves about 60%, 40%, and 25% of the transmissions.

802.11 testbed 802.15.4 testbed

Evaluation• Number of Coding Operations

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Compared to coding aware protocols, the number of coding operations is reduced while the transmission efficiency is improved!

Conclusion

1. We introduce link correlation to NC, and find that the previous link independence assumption overestimates the true diversity benefit.

2. We propose a correlated coding metric to help network designers decide when to use network coding.

3. The experiments results on one 802.11 testbed, and three 802.15.4 testbeds show that with our design, coding operations are reduced while the transmission efficiency is improved by 30% ~50%.

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Thank you!

Q&A

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