1st Oct 2013 1

CorLayer: A Transparent Link Correlation Layer for Energy

Efficient Broadcast

Shuai Wang, Song Min Kim, Yunhuai Liu, Guang Tan, and Tian He

University of Minnesota

MobiCom 2013

The Need for Broadcast Operation

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Code Dissemination Global Time Sync

Routing Discovery Data Collection

Wireless communication essentially occurs in a broadcast medium.

Multi-path Routing

Opportunistic Forwarding Network Coding

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The Need for Broadcast Operation

Advanced designs exploit the benefit from broadcast nature.

Motivation

Despite the fact that wireless communication essentially occurs in a broadcast medium with concurrent receptions

Existing research predominately examine separate statistics for individual links (channel) or path: ETX, PPR, LQI, RSSI

Little research has been done to investigate the joint statistics involving concurrent wireless links (e.g. broadcast)

Because of the legacy assumption of

link independence

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Legacy Assumption

It is assumed that wireless reception among concurrent links are independent due to multipath induced fading.

5

N1

N2

S

i.e., Packet loss at N2 is independent of packet loss at N1.

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Unfortunately….

Legacy assumption no longer holds well because packet loss due to the coexistence of wireless networks

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The co-existence of ZigBee and Wi-Fi

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The co-existence of ZigBee and Wi-Fi

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Wireless spectrum becomes crowded: 802.11b, 802.11g, and 802.15.4 all use the 2.4 GHz ISM band.

Interference becomes the major cause of pack loss instead of fading

25dB difference

University of Minnesota

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Explosive Growth of Wi-Fi

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1100%

Wi-Fi Hotspots in U.S.

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Increasing Cross-Network Interference

0.40.8

0 200 400

0.40.8

-100

-80

0 200 400-100

-80

R1

PR

R

R2

R1

No

ise

(db

m)

Time (sec)

R2

Two receivers' PRR

The concurrent noise increase

Interference leads to correlated packet loss:

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Furthermore, Correlated Shadow Fading

Closely located devices may suffer correlated lose since wireless signals suffer shadow fading when obstacles appear in the propagation path of the radio waves.

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Furthermore, Correlated Shadow Fading

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0.4

0.8

0 200 400

0.8

-90

-80

0 200 400-90-80

R1

PR

R

R2

R1

Sig

na

l(d

bm

)

Time (sec)

R2

Two receivers' PRR

The concurrent RSSI reduction

Closely Located

Appearance of Obstacles

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Synthetic Independent Trace Empirical Trace13

Wireless links are correlated!

0 20 40 60 80 100

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2

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Packet Sequence Number

Rec

eive

r

0 20 40 60 80 100

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Packet Sequence Number

Rec

eive

r

University of Minnesota

1 Source node9 Receivers100 Packets

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How Link Correlation Affects Broadcast？

(a) Negative Correlated:

(b) Positive Correlated:

In order to accurately estimate the broadcast performance, we MUST consider link correlation.

Link quality: 0.8# of packets need to be retransmitted: 4

Link quality: 0.7# of packets need to be retransmitted: 3

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The expected number of transmissions :

Theoretical Analysis

E[ ]

1 21

(K (u))1 1(u)

(e ) (e ) (K (u))

M M ii i

i i i

p

p p p

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Transmissions due to Link Quality

1

1

(e )

M

iip

Reduced transmissions by Link Correlation

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(K (u))1

(e ) (K (u))

M ii

i i

p

p p

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: the probability that all nodes in K(u) successfully receive a packet. .

(K (u))ip

Ki(u) is a subset of N(u) with size i, where N(u) is node u’s one-hop neighbor set.

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1

1(u) M 1

(e )

M

iip

Special Case – when links are independent:

The Property of

(u)

1 21

(K (u))1 1(u)

(e ) (e ) (K (u))

M M ii i

i i i

p

p p p

Property 1:

Property 2:

The higher the link correlation - 1

(K (u))

(K (u))i

i

p

p

The fewer the transmissions - (u)

Link Blacklisting for Better Correlation

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The average number of transmissions before blacklisting is mainly concentrated around 4.5 and it's 2.4 after blacklisting.

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1 2 3 4 5 6 70.0

0.2

0.4

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0.8

1.0

C

umul

ativ

e fr

actio

n of

test

set

s

Number of transmissions

Before blacklist After blacklist

2.4 4.5

Empirical Study:

Blacklisting leads to a significant reduction in transmission!

CorLayer: Goals

• Goals: Design a supporting layer by blacklisting low correlated links to help upper layer protocols save transmissions.

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Neighbor Discovery

CorLayer

Broadcast Protocols

Original Physical Topology

Updated LogicalTopology

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CorLayer: Challenges

• How to guarantee the network connectivity when blacklisting is executed? • A localized light-weight algorithm for connectivity check.

• How to blacklist links thus the updated topology can benefit the upper layer broadcast protocols?• Assess the cost of covering one-hop neighbors, taking

link correlation into consideration.

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CorLayer: Design – Connectivity Check

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Key Idea – link blacklisting requires the existence of an alternative path.

W

U V

Asynchronously Blacklisting – two-phase locking is used to avoid a race condition.

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CorLayer: Design – Link Blacklisting

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Key Idea - Triangular Blacklisting Rule: blacklisting a link if the source node could take fewer transmissions via an alternative path.

U V

W

x

z

y

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CorLayer: Design – Link Blacklisting

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Key Idea - Triangular Blacklisting Rule: blacklisting a link if the source node could take fewer transmissions via an alternative path.

W

U VDirect Broadcast

Cost for 1st Hop Cost for 2nd Hop

N(u)-{v}(u)

| N(u) 1|

(w)

| N(w) |

N(u)-{v}(u) {v}

(u) (w)(u) (u) +

| N(u) 1| | N(w) |N

(u) {v}(u) (u)N

(u)(u) {v}(u)N

U V

W

x

U V

W

x

Evaluation

Testbed Settings

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Platform Location Environment Nodes/APs

MICAz UMN Lab 36/5

TelosB SIAT Office 30/8

GreenOrbs TRIMPS Outdoor 20/0

Physical Size Degree Channel Power

8m*2.5m 7~23 Ch16 -25dBm

18m*13m 6~21 Ch16, Ch26 -25dBm

15m*5m 4~13 Ch16 -25,-19.2dBm

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Supported Protocols (1/2)

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• Integrated Protocols:I. Tree based:

1). S-Tree: A. Juttner et al. Mobile Networks and Application’05

2). C-Tree: K. Alzoubi et al. HICSS’02

II. Cluster based:

3). Cluster: J. Wu et al. Wireless Communication and Mobile Computing’03

4). Intermediate: J. Wu et al. Telecommunication Systems’01

5). Clustering: I. Stojmenovic et al. TPDS’02

6). P-Clustering: T. J. Kwon et al. SIGCOMM’02

III. Multiple point relay:

7). MPR: A. Qayyum et al. HICSS’02

8 – 9). Min-id MPR, MPRCDS: C. Adjih et al. INRIA-Rapport’02

Supported Protocols (2/2)

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• Integrated Protocols:IV. Pruning based:

10-11). SP, DP: H. Lim et al. Computer Communications Journal’01

12-13). PDP, TDP: W. Lou et al. TMC’02

14). RNG: J. Cartigny et al. IJFCS’03

V. Location based:

15). CCH: M. T. Sun et al. CS-NMC’05

VI. Network Coding:

16). COPE: S. Katti et al. SIGCOMM’06

16). CODEB: E. L. Li et al. INFOCOM’07

• Evaluation Metrics: • The total number of transmissions needed to deliver one packet to all

the nodes in the network.

Extensive evaluation with 16 protocols run on 3 testbeds!

Evaluation

• Main Performance Results

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38%48% 52% 49%

36%

39%

On average, CorLayer reduces transmissions by 47%!

Evaluation

• Impact of blacklisting rules• R_: Random Blacklisting;• WL_: Worst Link Blacklisting;• CorLayer_: Our Design;

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R_: CorLayer Saves “R_”50% Transmissions!WL_: CorLayer Saves “WL_” 20% Transmissions!

MPR Cluster Pruning Network Coding

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Conclusion

• We have presented CorLayer, a link correlation-based layer that enhances the energy efficiency of reliable broadcasting.

• We integrated CorLayer transparently with sixteen state-of-the-art broadcast algorithms and evaluated the design on three real-world multi-hop testbeds.

• The results indicate that with CorLayer, reliable broadcast avoids unnecessary transmissions caused by wireless links that are less positively correlated.

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Thank you!

Q&A

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