Hady Abdel-Salem's PhD Defense Slides Department of Computer Science Old Dominion University November 1, 2010 Note: You may need to download the file to see all of the animations.

2. Agenda
 Introduction. [5 min]
 Motivation & Related work. [5 min]
 Backbone Construction Protocol. [10 min]
 Mitigating Network Challenges:
 Clustering & Sensor Localization. [5 min]
 Energy-Aware Task Management.
 Workforce Selection [10 min]
 Centralized Approach.
 Distributed Approach.
 Sensor Sleep Scheduling [10 min]
 Data Aggregation & Routing. [5 min]
 Energy Hole Problem. [5 min]
 Conclusions & Future Work [5 min]

3. • Collected data are locally aggregated then forwarded for
further processing at sink nodes which in terms of energy,
computation, and communication have more powerful
capabilities compared to sensors.
Introduction
• Sensor Networks are special type of Ad-Hoc networks which
include two types of nodes, “Sensor nodes’’& “ Sink nodes’’.
• Sensor nodes are tiny electronic devices with limited
sensing, computational, and communicational capabilities.
• In a typical scenario, sensors are massively deployed in an area
of interest to collect data that serve the mission of the network.

4. • Sensor networks have many applications …
some of them are military related:
 battlefield surveillance.
 borders monitoring.
• Others are civilian:
 fire and habitat monitoring
 home automation.
 traffic control.
 target tracking.
 body sensor networks
Sensor Network
Applications

5. Design Limitations
• Limited and non-renewable energy budget.
• Short duty cycle.
• Limited computing power.
• Small transmission range.
• High failure rate (unreliable).
• Location unawareness (no GPS).
• Random nature of deployment.
• Working unattended (autonomously).
• Dynamic topology.
Sensor:
Network:

6. Motivation & Related Work
• Several techniques have been proposed to address different
problems in WSNs.
 For Localization, we have APIT, DV-Hop, Centroid, Weighted Centroid, …
 For Routing, we have GPSR, GFS, MFR, …
 For clustering, we have Hierarchical clustering, LEACH, …
• A common drawback when using any of these techniques, they
attempt to solve one problem in isolation from the others,
hence protocol designers have to face the same common
challenges again and again.
• This, in turn, has a direct impact on the complexity of the
proposed protocols and on energy consumption.

7. Motivation
• In this thesis, we present a different approach to address the problems mentioned earlier.
• Specifically,
 In a one-time investment, we construct a lightweight network backbone which
provides some kind of an infrastructure that makes the network easier to manage.
 On top of the constructed backbone, we provide solutions for the following
problems:
 Sensor Localization
 Clustering
 Energy-Aware Task Management
 Workforce Selection
 Sleep Scheduling.
 Data aggregation & routing.
 Energy hole problem.

8. Backbone Construction
Protocol

9. Construction Protocol
• Initially, the deployment area is hypothetically tiled with identical hexagons starting
from the sink outward.
• The first hexagon is placed so its center coincides with the center of the sink node.
• Hexagons are placed next to each other in the following directions:
• The geometry of the gaps between the hexagons in any two consecutive directions allow
perfect tiling.
• The size of the hexagon is determined by the distance between the centers of any two
neighbored hexagons which is chosen to be ≈ tx
Sensor
Sink
Tx

10. • Hence, the deployment area is divided into six sectors.• We use a ternary coordinate system <sector, row, column> to uniquely identify hexagons.• The closest sensor to the center of each hexagon is determined (backbone sensor). These
sensors collectively form the network backbone.
Construction Protocol (cont.)

11. Order of Backbone Sensors Selection
• Selection process goes recursively,
backbone sensors in any row are responsible
for selecting backbone sensors in the next row.
• Each backbone sensor with odd column coordinate
selects 2 sensors in the next row.
i.e Sensor <s, r, 2c-1> selects sensors <s, r+1, 2c-1> and <s, r+1, 2c >
• Backbone sensors in the first column in even rows select 3 sensors
i.e Sensor <s,2r,1> selects sensors <s,2r+1,1> , <s,2r+1,2> and <s-1,r+1,r+1 >
• Backbone sensors can be selected in many ways that may result in different order.
To reduce collisions, we propose the following rules.
• Selection of backbone sensors with the same row and column coordinates in different
odd sectors (i.e. s=1,3,5) occurs in the same time. A similar rule applies for even
sectors (i.e. s=2,4,6).

12. Order of Backbone Sensors Selection

13. How backbone sensors are selected ?
• Backbone sensors are selected to be the closest sensors to
the centers of the hexagons they represent.
• To determine these sensors, our approach requires sensors
to be able to measure their angle to the sink node.
• Hence, we find it more appropriate to start by showing
how a sensor can estimate it angle to the sink node.

14. Directional Transmission Pattern
The transmission pattern of directional antennas consists of a
major lobe oriented towards the transmission direction and
several minor and side lobes oriented in other directions.

15. Measuring Angles
Received Power
Initially, the sink uses it omnidirectional transmitter to send a sequence of wakeup
messages to make sure sensors in its neighborhood are awake.
While rotating its unidirectional antenna, the sink sends a sequence of beaconing messages
attaching current transmission angle to each message.
From received messages, a sensor can estimate its angle to the sink as the
average of the received angles weighted by the received power pr.
θ
Sensor
Sink

16. Measuring Angles
Received Power
The process can be repeated for better accuracy.
θ

17. • Sensors that receive the message, verify that the absolute difference between their measured angle
and ϕ is less than a predetermined threshold, otherwise, they ignore the message.
• Sensor S1 estimates the angle ϕ between the sink node and the target hexagon and starts the search
process by broadcasting a message that includes ϕ to its neighbors.
ϕ
S1
3
7
5
6
14
9
2
6
4
5
13
8
1
5
3
4
12
7
New Backbone
Sensor
∞
∞
∞
Selection of backbone sensors
• Sensors within range, estimate the square of their distance to the center of the target hexagon, and
initialize a countdown timer to this value.
• The sensor that has the first timer to expire broadcast a message to its neighbors to announce itself
as the newly selected backbone sensor.
Sink

18. Estimation of Distance e2
Case 1 Case 2

19. Common intermediate terms can be
reused to evaluate the Sine and the
cosine functions using 10 multiplication
operations only, with an accuracy up to
4 decimal digits.

20. Problem of Voids

21. Problem of Voids (Cont.)
Void

22. Even Neighbor Replacement
• Recall that in our basic selection rules, only sensors with
odd column coordinates are allowed to select.
• Note that, every even neighbor can receive selection
messages transmitted from its two immediate odd
neighbors.

23. Even Neighbor Replacement
• If an odd sensor is missing due to voids, then all its
selection tree will be pruned.
• Idea: allow immediate even neighbor sensors to replace
missing sensors in order to continue the selection chain.

24. Recovering From Voids
(Even Neighbor Replacement)

25. Recovering From Voids
Backward Selection
• If selection of a backbone sensor was initiated by a sensor
other than the one determined by the initial rules.
(e.g. Even neighbor replacement or backward selection),
Then the newly selected sensor should try to select the odd
sensor that was supposed to select it.

26. Recovering From Voids
(Backward Selection)

27. Backbone Switching
θ
Rotation Angle θ
• To balance the load on backbone sensors, we use alternative backbones.• Idea: rotate sector orientation angles by θ1, θ2…

28. Actual Hexagons obtained through Simulation

29. Actual vs. theoretical positions of backbone sensors

30. Sensor Localization & Clustering
 During the selection of backbone sensors, sensors can use their knowledge
about their angle to the sink along with the computed distance to the sink
to fully localize themselves.
 Moreover, the hexagonal structure of the backbone provides an implicit
clustering mechanism in which each hexagon represents a cluster and the
backbone sensor around its center represents its cluster head.
 Hence, sensor localization & clustering are direct by-products of the
backbone construction protocol.

31. Average Localization Error for Different Localization Protocols

32. Energy-Aware
Task Management

33. Workforce Selection
• Sensing tasks are issued to sensors through sink nodes.
• A task T identified by the tuple (x, y, w), where
 (x,y): is the position where data need to be collected.
 w : QoS requirements expressed in terms of number of sensors
participating in the task (we refer to them as task workforce).
• Obviously, only sensors whose sensing range cover the point (x,y) can
join the workforce for the task T.

34. • Assume that each of the tasks T1 and T2 requires 3 sensors.
• Case 2: assign < a, b, c> to T1 We can run T2 using < e, g, i >.
• Case 1: assign < b, e, f > to T1 No way to run T2.
• Considering sensor remaining energy during workforce selection can
enhance network reliability and durability.
• Assigning tasks to sensors improperly can consume sensor energy
unevenly which in turn may result in many problems
e.g. reducing network density, creating energy holes, …
Example

35. Workforce Selection
Centralized Approach

36. • A task starts when the sink sends a sequence of CTW to get the attention of enough
number of sensors (including backbone sensors across routing path)
Workforce Selection
• CTW message should contain:
• W: required workforce w,
• (x,y): task position,
• Emax: sink estimate of maximum
energy in target hexagon.
• S: number of bidding slots in first
bidding round.

37. 10
14
14 12
12
13
14
13
• Candidate sensors whose sensing range cover the point (x, y) participate in the task
with probability
• Obviously, the formula shown above gives sensors with higher energy higher chance
to participate in the task than other sensors.
• Immediately, after the last CTW message, time
line is divided into s time epochs (slots). Sensors
willing to participate should choose a random
slot and transmit a short message that contains its
current energy level.
( For the example assume S = 8)
Last
CTW 13, 14 14 10,12 13 14 12
Workforce Selection

38. Last
CTW 13, 14 14 10,12 13 14 12
• Immediately, after the last bidding slot, the task coordinator (the backbone
sensor within the hexagon) transmits a message that contains the results of the
last bidding round.
Last
Bidding Slot 0 0 1 0 2 0 3 4 Info
.
• While evaluating the bidding process, only single-bidder slots are considered,
other slots are ignored. A temporary id is assigned to sensors that bid in any of
these slots, this id determines the order by which sensors send their sensory data
to the task coordinator during data aggregation.
• The info field in the bidding result message should include information
needed for next bidding round (if any):
• S: number of slots ,
• W: number or remaining sensors needed,
• Emax: maximum energy among sensors.

39. • The bidding process continues till the whole workforce is recruited or a
maximum number of bidding rounds is reached.
• Task execution starts immediately after collecting required workforce.
When task execution is completed, sensors start to send their sensory data to the
backbone sensor in the order assigned during workforce selection.
• Finally, the backbone sensor within the hexagon sends the locally aggregated
sensory data to the sink node along with Emax, the maximum energy among
sensors within this hexagon, hence the sink can attach this value with CTW
messages transmitted for any upcoming tasks within this hexagon.

40. • We provided mathematical derivation of all parameters needed for our
workforce selection protocol, including
Number of slots in any bidding round
Expected number of candidate sensors
collected by CTW messages
Awake probability of sensors.
Number of CTW messages needed

41. Workforce Selection
Distributed Approach
• Estimation of Maximum Energy
• Workforce Selection

42. • Assume that sensor energy can be quantized into 2n
levels, (i.e it can be encoded in a string of n bits).
Estimation of Maximum Energy
For illustration, we assume, n = 4
E1
Last
CTW
E2 E3 E4
clock drift
margin
Time line is divided into 4 time epochs
• Immediately after the last CTW message, time line is
divided into n time epochs.

43. • Sensors in the same sensing area transmit short messages that
represent their energy levels bit by bit starting from the most
significant bit.
• A value of 0 is not transmitted, while a value of 1 is transmitted.
• Sensors stop transmitting if they have a 0 and received a packet or
detected a collision in the corresponding epoch.
Estimation of Maximum Energy (cont.)
For illustration, assume we have 3 sensors with the following energy
levels, S1 [1110], S2 [1100], S3 [1011]
clock drift
margin
E1
Last
CTW
S2
S1 S1
S2
S1
S3
E2 E3 E4

44. • Sensors that pick up the values transmitted should use the following
disambiguation scheme:
 No packets are received: 0 is recorded.
 A packet is received or collision is detected: 1 is recorded.
Estimation of Maximum Energy (cont.)
S1 [1110], S2 [1100], S3 [1011]
clock drift
margin
E1
Last
CTW
S2
S1 S1
S2
S1
S3
E2 E3 E4
Maximum: 1 1 1 0
Using appropriately long epochs, synchronization should not be a problem.

45. 1 10 0
1 01 0
1 110
0 11 1
# Channel Status Bit
4 <c> 1
3 Ambient 0
2 Collision <b,c> 1
1 Collision <a,b,c> 1
Maximum: 1101Estimation of Maximum Energy

46. Workforce Selection
• The sensing area is divided into k regions with equal size
• Workforce selection goes in decision rounds, each round is
associated with an energy level (Emax, Emax-1, Emax-2,…etc).
• Each decision round has k slots corresponding to the regions
determined above.
k
R2
.
Max
Estimation
Decision Round
Emax
Decision Round
Emax-2
Decision Round
Emax-1
……
clock drift
margin
S1
S2 Sk
Round
E+1
Round
E-1

47. • Based on task and sensor position, each sensor estimates its region
and transmits a short message in the decision round/slot that
matches its energy and region.
• Protocol terminates when required workforce is recruited.
• Sensors keep track of number of recruited sensors as follows:
 Collision is detected: increment by 2.
 No packets are received: 0 is recorded.
 One packet is received: increment by one.
S1
Round
E+1 S3
S2
S1
S2 S3 S4
Round
E-1
+2 +0 +1
S6
S5S4
+2
Workforce Selection

48. # Sensors Wnew Wtotal
4 Ambient 0 3
3 c<S4 , S5> 2 3
2 Ambient 0 1
1 S2 1 1
Round-1 (E = 10)
Assume required workforce is 4

49. # Sensors wnew Wtotal
1 S1 1 4
Round-2 (E = 9)
Workforce collected,
Protocol terminates

53. Sensor Sleep Schedule
 Due to their limited energy budget, sensors spend most of their lifetime in
a sleep mode.
 Various deterministic and probabilistic schemes can be used to
determine the schedule based on which sensors sleep and wake up.
 Each scheduling scheme has its impact on :
 The effective sensor density (ESD),
defined as the density of awake sensors.
 The coverage capability of the network.

54. Scheduling Schemes
 Static Schemes
 Dynamic Schemes
 Energy-Aware Schemes
Sensor sleep for a random amount of time uniformly selected in
the interval [Ts, TS], and stay awake for another random amount
of time uniformly selected in the interval [Ta, TA].
Dynamic scheduling schemes which adaptively change the upper bounds
TA and TS based on sensor remaining energy.
In general, we can classify scheduling schemes into,
Sensors sleep for Ts time units and stay awake for Ta time units.

55. PASTA & Sleep Scheduling in WSN
 The PASTA property stands for Poisson Arrivals See Time Average.
 We show that the PASTA property can be applied to effective
sensor density in WSNs.
 Specifically, we proved that fraction of events that occur in a
certain area while k sensors are awake equals the fraction of time
this area is under the surveillance of k sensors.
 This result allowed us to use the time-invariant probability
distribution of k-coverage to analyze different scheduling
schemes.
 PASTA has been used for too long in queuing systems and in general it
implies the equivalence between the time average view seen by an
internal observer who has been watching the system for long time and
the view seen by external observers that arrive at the system according to
a Poisson process.

56. Energy-Aware Scheduling
 We proposed a backbone-guided energy-aware scheduling scheme
designed to extend network reliable lifetime by balancing energy
consumption among sensor nodes.
 The main idea of our scheme is to continuously and probabilistically adjust
sleep and awake times of sensors based on the differences in their
remaining energy.
 In other words, the proposed scheme prolongs the sleeping periods of
sensors with relatively low energy and compensate for their absence by
shortening sleeping periods of sensors with relatively high energy.
 This goal has to be done without probabilistically affecting the effective
sensor density.

57. Energy-Aware Scheduling
 We proved that if sensors independently updated their upper bounds for
TA & TS after the execution of each task according to the following
equations, their energy consumption will almost be even.
 The figure below show the Energy-Aware scheme in action
S9
S8
S7
S6
S5
S4
S3
S2
S1
Sensors
Sleep / Awake Cycle

60. Sink
Local Aggregation
Backbone Sensor
Regular Sensor
Inter-backbone sensor aggregation through
reversed selection rules
Selection rules
Data Aggregation & Routing

61. Energy Hole Problem
Energy Hole

62. Rendezvous-based Routing

63. Source 2
Δ c
Source 1
Destination 2
Δ r
Δ c
Δ r
Destination 1
Δ𝑐 = 𝑐 𝑑𝑠𝑡 − 𝑐 𝑠𝑟𝑐
+ Δc anti-clockwise
- Δc clockwise
𝑐 𝑠𝑟𝑐 = 𝑠𝑠𝑟𝑐 − 1 𝑟 𝑚𝑖𝑛 + 𝑐 𝑠𝑟𝑐
𝑐 𝑑𝑠𝑡 = 𝑠 𝑑𝑠𝑡 − 1 𝑟 𝑚𝑖𝑛 + 𝑐 𝑑𝑠𝑡
𝑟 𝑚𝑖𝑛 = Min(𝑟𝑑𝑠𝑡, 𝑟𝑠𝑟𝑐)
If (Δ𝑐 > 3𝑟 𝑚𝑖𝑛)
Δ𝑐 = 6𝑟 𝑚𝑖𝑛 − Δ𝑐else If (Δ𝑐 < −3𝑟 𝑚𝑖𝑛)
Δ𝑐 = Δ𝑐 − 6𝑟 𝑚𝑖𝑛
Hexagon to Hexagon Routing
Δ𝑟 = 𝑟𝑑𝑠𝑡 − 𝑟𝑠𝑟𝑐 + Δr outward direction
- Δr inward direction
(𝒔 𝒔𝒓𝒄, 𝒓 𝒔𝒓𝒄, 𝒄 𝒔𝒓𝒄) >> (𝒔 𝒅𝒔𝒕, 𝒓 𝒅𝒔𝒕, 𝒄 𝒅𝒔𝒕)

64. Source 2
Δ c
Source 1
Destination 2
Δ r
Δ c
Δ r
Destination 1• Once Δr & Δc are known, each sensor can
adaptively select its next hop as follows:
 If (|Δr| > 0) & (|Δc| > 0) , whenever
possible, select the hop which decrements
both |Δc| and |Δr|. Otherwise,
 If (routing outward i.e Δr > 0)
select the next hop which decrements |Δc|
 If (routing inward i.e Δr < 0)
select the next hop which decrements |Δr|
Hexagon to Hexagon Routing (𝒔 𝒔𝒓𝒄, 𝒓 𝒔𝒓𝒄, 𝒄 𝒔𝒓𝒄) >> (𝒔 𝒅𝒔𝒕, 𝒓 𝒅𝒔𝒕, 𝒄 𝒅𝒔𝒕)

65. Conclusions
 In this work, we explored the construction and the advantages of having an
infrastructure for WSNs.
 The proposed backbone proved to be useful in mitigating many of the typical
challenges inherent to WSN including:
 Sensor Localization.
 Data Aggregation & Routing
 Energy-Aware Workforce Selection
 The construction protocol initially tiles the deployment area around sink nodes
using identical hexagons.
 After that backbone sensors are selected to be the closest sensors to the centers of
the hexagons they represent.
 Clustering
 Energy Hole Problem
 Energy Aware Sleep Schedule

66. Future Research Directions
Despite the encouraging results, many important challenges and research questions
remained unanswered.
 In the construction protocol, it would be useful to reduce the amount of
inaccuracy due to RSS. One way around this difficulty would be to:
 Estimate the initial distance through several readings.
 Continuously update distance estimates form different messages received
throughout the network lifetime.
 Given the limited on-board energy budget available to sensors, it would be of
interest to see how far can one streamline the computational requirements of the
construction protocol.
 If backbone sensors in different columns can be selected in parallel,
can we also select backbone sensors in different rows in parallel ?

67.  Let the sink node estimate the positions of all the hexagon centers and
broadcast them to sensors which can work in parallel to estimate the
closest sensor to the center of each hexagon.
Future Research Directions (cont.)
Any Problems?
 Sensor Synchronization is one of the most challenging problems in sensor
networks. It would be of an interest to show, if possible, how our proposed backbone
can be useful in simplifying the synchronization problem.
 Network Security is something that we completely overlooked in our presented
work. Revisiting the proposed protocols from a security point of view would
definitely open new dimension of research challenges.
 Finding more applications of the PASTA property in in sensor networks analysis.

68. Publications
Journals:
Conferences/Workshops:

69. Conferences/Workshops (cont.)

70. Conferences/Workshops (cont.)

71. Questions ?
Thank You !