opportunistic flooding in low-duty-cycle wireless sensor networks with unreliable links Shuo Guo, Tian He

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Introduction
Low-Duty-Cycle Wireless Sensor Networks
Flooding in Low-Duty-cycle Networks
Review: Typical Issues in Flooding
Motivation
Fit for Intermittent Receivers
Traditional methods with Low-Duty-Cycle
Preliminaries
Network Model
Assumptions
Performance Metrics
Main Design
Design Overview
Flooding Energy Cost and Delay
The Delay pmf of the Energy-Optimal Tree
Decision Making Process
Decision Conflict Resolution
Shape of Opportunistic Flooding
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Practical Issues
On Node Failures
On Link Quality Change
Evaluation
Simulation Setup
Baseline I : Optimal Performance Bounds
Baseline II : Improved Traditional Flooding
Performance Comparison
Investigation on System Parameters
Evaluation of Practical Issues
Overhead Analysis
Implementation and Evaluation
Experiment Setup
Performance Comparison
Why Opportunistic Flooding is Better
Conclusion

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Duty Cycle of
Humans
Lions
Sensors
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6:00 7:00 8:00 17:00 00:00
: 75%
: 5%

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Why low-duty-cycle?
A cubic centimeter package
Limited amount of energy
Need for sustainable deployment of sensor system
To reduce operational cost and ensure service continuity
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Different wake-up time
If its receivers do not wake up at the same time
A sender has to transmit the same packet multiple times
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Sender
On
Off
Unreliable wireless link
due to wireless loss
A transmission is repeated if the previous transmissions are not successful
Combination of the two features
Make the problem more difficult
… …

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Efficiency or Reliability
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Source
Relay
Destination
Tradeoff Relationship
If # of the relay nodes is increased, Broadcast Storm occurs
If # of the relay nodes is reduced, the next node could fail to receive a broadcast packet
Blind flooding Routing tree
in always-wake networks
In low-duty-cycle networks
If # of the relay nodes is increased, they cost of high energy consumption
If # of the relay nodes is reduced, the cost of long delays

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Major Energy Drain
1.3 ms to transmit a TinyOS packet using a CC2420 radio
3 ~ 4 orders of magnitude longer duration waiting for reception
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17.4
19.7
16
17
18
19
20
Energy Consumption of CC2420 Radio
Transmission Idle Listening / Receiving
mA
Energy Consumption of Zigbee
Need for Low-Duty-Cycle Operation
To reduce the energy penalty in idle listening

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If applied directly
① A node broadcasts a packet as soon as it receives it
② Becomes even worse when unreliable links
③ Collisions are taken into account
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①
②
③
Possible Arguments
Multiple transmissions based on the neighbor schedules
ARQ-based mechanism to deal with unreliable links
 Not suitable for low-duty-cycle networks,
If used directly
Sender
On
Off
Collision Redundant
Need for a New Flooding Design
To address these limitations

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Two Possible Sensor States
Active
Dormant
A node can only receive a packet when it is active, but can transmit a packet at any time
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1
0 : Turning off all its modules except a timer to wake itself up
: Able to sense an event, or receive a packet
Working Schedules: 𝑤𝑖, 𝜏
T : working period of the whole network
𝑤𝑖 : string of ‘1’ and ‘0’s denoting the schedule
𝜏 : time units of length, T can be divided into
Each node picks one or more time units as its active state

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Only one flooding will be in process at any time
Working schedules are shared with all its neighbors when joining the network
Unreliable links and collision are exist
Link quality is measured using probe-based method and updated infrequently
Do not consider “capture effect”
Local synchronization can be achieved in an accuracy of 2.24 us
as described in Flooding Time Synchronization Protocol (FTSP), SenSys ‘04
Hop count is to denote the minimum number from a node to the source
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Flooding delay
Due to the imperfection of the links, the flooding delay exhibits inherent randomness
The average flooding delay is used
Energy consumption
As the receiver-side energy is determined by their predefined working schedules
Only the sender-side energy
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Directed Acyclic Graph (DAG)
Edge weight : link quality
Energy-Optimal Tree : Default path in SOAR
Smaller hop count  larger ones,
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Opportunistic Flooding (this study defines)
To utilize links outside an energy-optimal tree
If these links have a high chance of receiving the packet “statistically earlier” than its parent
not specified only 1-hop anywhere in this paper

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About Energy Optimality
Flooding in low-duty-cycle is realized by multiple unicasts
 The probability that a node has two neighbors with identical schedules?
 Combination with repetition
Energy-optimal tree’s Energy optimality
 Proof by contradiction
If multiple nodes wake up simultaneously
 MCDS problem, NP hard
 But is rare
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About Delay Optimality
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F
D
E
D and E receives the packet at time t
F wake up at time instances t +4, t +8, …
4 × 0.8
0.8
0.7
+8 × 1 − 0.8 × 0.8 +12 × 1 − 0.8 2
× 0.8 = 𝑡 + 4.999 ⋯
4 × 0.7 +8 × 1 − 0.7 × 0.7 +12 × 1 − 0.7 2
× 0.7 = 𝑡 + 5.71 ⋯
Delay in the case DF
Delay in the case EF
Delay in the case DF | EF
𝑡 +
𝑡 +
𝑡 + 4 × 1 − 1 − 0.8 1 − 0.7 +8 × 1 − 0.8 1 − 0.7 × 1 − 1 − 0.8 1 − 0.7
+12 × 1 − 0.8 1 − 0.7
2
× 1 − 1 − 0.8 1 − 0.7
= 𝑡 + 4.26 ⋯

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0.9
0.8
D
A
Computation of pmf
Source S generates a packet at time slot 0
Intermediate A wakes up at every 10t time slot
Intermediate D wakes up at every 10t +5 time slot
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S
E
C
D
G
B A
F
S
0
1.00
0
0.90
10
0.09
20
0.009
30
…
t
t
35
0.05 …
t5
0.72
15
0.22
25
𝑖:𝑡 𝑙 𝑖 <𝑡 𝑙+1(𝑗)
𝑝𝑙 𝑖 𝑞 1 − 𝑞 𝑛𝑖𝑗
The probability that it receives the flooding packet at its j-th active time slot
𝑝𝑙+1 𝑗 = (1)

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Complexity Analysis
Theoretically, the delay pmf may have infinitely many entries
The Eq. (1) takes quadratic time 𝑂 𝑛2
But linear time is achievable
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𝑝𝑙+1 𝑗 =
𝑖:𝑡 𝑙 𝑖 <𝑡 𝑙+1(𝑗)
𝑝𝑙 𝑖 𝑞 1 − 𝑞 𝑛 𝑖𝑗
=
𝑖:𝑡 𝑙 𝑖 <𝑡 𝑙+1(𝑗−1)
𝑝𝑙 𝑖 1 − 𝑞 𝑛 𝑖,𝑗−1(1 − 𝑞) +
𝑖:𝑡 𝑙+1 𝑗−1 ≤𝑡 𝑙 𝑖 <𝑡 𝑙+1(𝑗)
𝑝𝑙 𝑖 𝑞
= 𝑝𝑙 𝑗 − 1 (1 − 𝑞) +
𝑖:𝑡 𝑙+1 𝑗−1 ≤𝑡 𝑙 𝑖 <𝑡 𝑙+1(𝑗)
𝑝𝑙 𝑖 𝑞
2) = +0.009 × 0.8
= 0.512 ≈ 0.535
0.09
D
0.009
A
0
0.90
10 20 30
…
t
0.05
0
0.72
0.22 …
t5 15 25
D
A
0.8
(2)
1) = +0.09 × 0.8
= 0.216 ≈ 0.22
1) 2)
0.72 × 1 − 0.8
0.22 × 1 − 0.8

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2) If ,
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To judge Need or Redundant
opportunistically early packets are forwarded
A node finds its 𝑝-quantile delay based on its pmf, 𝐷 𝑝, shared with its parents
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Expected Packet Delay (EPD)
A level-𝑙 node A receives a packet at its 𝑖 th active time unit with delay 𝑡𝑙(𝑖)
For a link quality 𝑞,
1
𝑞
transmission are expected 
1
𝑞
-th time slot after 𝑡𝑙(𝑖)
𝐸𝑃𝐷 =
𝑗:𝑡 𝑙+1 𝑗 >𝑡 𝑙(𝑖)
𝑞 1 − 𝑞 𝑛 𝑖𝑗𝑡𝑙+1 𝑗 (3)
18 22 26 Time
0.5
0.1 0.04
0.3
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B’s
pmf
1) If 𝑝 = 0.8, then 𝐷 𝑝 = ?
Received by A
1st try to B 2nd try to B
D 𝑝 = 18
𝐸𝑃𝐷 = 22
BA
0.5
then 𝐸𝑃𝐷 = ?
3) If node A received at 9,
Did node A judge the packet
Need or Redundant?
…1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1…
3
)
BA

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Need for Selection of Flooding Senders
To avoid Collision caused by HTP
More likely to occur in a low-duty-cycle-networks
TDMA or RTS/CTS based solutions are not suitable
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Reduced Sender Set
Link quality threshold 𝑙 𝑡ℎ
The best link quality is always included, the rest are selected inductively
All the candidates are tested one-by-one in descending order
Computation Complexity
Denote 𝐻 as the total number of sender candidates
A node consumes 𝑂 𝐻 time
To construct a sender set, totally, 𝑂 𝐻2
time is consumed
Sender
On
Off
Best Candidate
Candidate

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Link-Quality-Based Backoff
To resolve collisions
To reduces redundant transmissions
A node with better link quality  a higher priority to grab the channel : Ideal
Backoff Duration
Backoff Time bound : 𝑇𝑏𝑎𝑐𝑘𝑜𝑓𝑓
Maximum Size of Sender Set : 𝑊
Random period of time : 𝑋
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𝑡 𝑏𝑎𝑐𝑘𝑜𝑓𝑓 = 𝑊 1 − 𝑞
𝑇𝑏𝑎𝑐𝑘𝑜𝑓𝑓
𝑊
+ 𝑋
0 ~ 𝑊 − 1
Transmission Priority
Randomness
To reduce collision
−
𝑇𝑏𝑎𝑐𝑘𝑜𝑓𝑓
𝑊
,
𝑇𝑏𝑎𝑐𝑘𝑜𝑓𝑓
𝑊
After Backoff
The one with the best link quality starts first
Best-link node can keep occupying the channel until the current time slot is passed

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Source
Candidates
S
A
B
C
D
E
F
H
G
(a) Original Network
S
A
B
C
D
E
F
H
G
(b) Sender Selection
S
A
B
C
D
E
F
H
G
(c) B receives the packet early
S
A
B
C
D
E
F
H
G
(d) B receives the packet late

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Source
Candidates
S
A
B
C
D
E
F
H
G
(c) B receives the packet early
S
A
B
C
D
E
F
H
G
(d) B receives the packet late
S
A
B
C
D
E
F
H
G
(g) B receives the packet late
S
A
B
C
D
E
F
H
G
(e) B sends the packet earlier than the others
S
A
B
C
D
E
F
H
G
(f) B sends the packet later than A

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Just makes the use of Elementary mathematics
First and last
Nothing to waste
The 2nd half of this study
Treats the practical issues in the protocol
Evaluates it in diversified ways
Future work
The second half of Opportunistic Flooding in Low-Duty-Cycle Wireless Sensor Networks with Unreliable Links
Flooding Time Synchronization Protocol, SenSys ‘04
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