1 st oct 2013 1 corlayer: a transparent link correlation layer for energy efficient broadcast shuai...
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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.
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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
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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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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[ ]
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(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
0.6
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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0
5
10
15
20
Nu
mb
er
of T
ran
smis
sio
ns
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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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