1. Aggregation

2. Learning Objectives Understand why we need aggregation in WSNs
Understand aggregation protocols in WSNs
Understand secure aggregation protocols in WSNs

3. Prerequisites Basic concepts in network protocols
Basic concepts of network security

4. Why do we need Aggregation?
Sensor networks – Event-based Systems
Example Query:
What is the maximum temperature in area A between 10am and 11am?
Redundancy in the event data
Individual sensor readings are of limit use
Forwarding raw information too expensive
Scarce energy
Scarce bandwidth
Solution
Combine the data coming from different sources
Eliminate redundancy
Minimize the number of transmissions
Aggregation: Summary
[Aggre_1] Section 1

5. What is Aggregation?

6. One Example of Aggregation - Count
Example: consider a query that counts the number of motes in a network of indeterminate size

7. adopted from slides from S. Madden
Sensor # 2 1 3 4 5 1 2 3 4 5 -
Time
Goal: Count the number of nodes in the network.
Number of children is unknown.
So many redundant message
Scenario: Count
adopted from slides from S. Madden

8. Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time- 2 3
Time
Goal: Count the number of nodes in the network.
Number of children is unknown.
Scenario: Count

9. Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4-
1 + 2 2 3
Time 4
Goal: Count the number of nodes in the network.
Number of children is unknown.
Scenario: Count

10. Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5-
1 + 2
1 + ½ 2 3
Time 4 5
Goal: Count the number of nodes in the network.
Number of children is unknown.
Scenario: Count

11. Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5-
1 + 2
1 + ½
1+3
1+ ½
1+1 2 3
Time 4 5
Goal: Count the number of nodes in the network.
Number of children is unknown.
Scenario: Count

12. Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5-
1 + 2
1 + ½
1+3
1+ ½
1+1
1+2/2
Time
Goal: Count the number of nodes in the network.
Number of children is unknown.
Scenario: Count

13. Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5-
1 + 2
1 + ½
1+3
1+ ½
1+1
1+2/2
1+4
Time
Goal: Count the number of nodes in the network.
Number of children is unknown.
Scenario: Count

14. Count Example – A Better Scheme
Each leaf node in the tree reports a count of 1 to their parents
Interior nodes sum the count of their children, add 1 to it, and report that value to their parent

15. Data Aggregation Process
Sensor nodes are organized into a tree hierarchy rooted at the Base Station
Non-leaf nodes act as the aggregators

16. Example Aggregation
Max, Min
Count, Sum
Average
Median

17. Tiny Aggregation Distribution phase Collection phase
Aggregate queries are pushed down into the network
Collection phase
Aggregate values are continuously routed up from children to parents

18. Energy Consumption

19. Declarative Queries for Sensor Networks
Sensors
Examples:
SELECT nodeid, light
FROM sensors
WHERE light > 400
EPOCH DURATION 1s
Epoch
Nodeid
Light
Temp
Accel
Sound 1
455 x 2
389
422
405 1
Time is partitioned into epochs of duration i A single aggregate value is produced to combine the readings of all devices during the epoch

20. Aggregation Queries Epoch AVG(sound) 440 1 445 2 Epoch roomNo
440 1
445 2
SELECT AVG(sound)
FROM sensors
EPOCH DURATION 10s
Epoch
roomNo
AVG(sound) 1
360 2
520
370
SELECT roomNo, AVG(sound)
FROM sensors
GROUP BY roomNo
HAVING AVG(sound) > 200
EPOCH DURATION 10s
Rooms w/ sound > 200 3

21. Illustration: Aggregation
SELECT COUNT(*) FROM sensors
Slot 1 1 2 3 4 5
Sensor # 1 2 3 4 5
Slot # 1
Section 4.1 of TAG

22. Illustration: Aggregation
SELECT COUNT(*) FROM sensors
Slot 2 1 2 3 4 5
Sensor # 1 2 3 4 5 2
Slot #

23. Illustration: Aggregation
SELECT COUNT(*) FROM sensors
Slot 3 1 2 3 4 5
Sensor # 1 2 3 4 5 1 3
Slot #

24. Illustration: Aggregation
SELECT COUNT(*) FROM sensors 5
Slot 4 1 2 3 4 5
Sensor # 1 2 3 4 5
Slot #

25. Illustration: Aggregation
SELECT COUNT(*) FROM sensors
Slot 1 1 2 3 4 5
Sensor # 1 2 3 4 5
Slot # 1

26. Flow Up the tree during an epoch
How parents choose the duration of the interval in which they will receive values?

27. Topology Maintenance and Recovery
How to address the unreliable nature of WSNs in TAG?
Each node maintains a fixed size of its neighbors – Select a better parent node
If a node does not hear from its parent for some time, it assumes that its parent has failed
Section 7.1 of [Aggre_1]

28. Secure Aggregation

29. Secure Aggregation
It is challenging to design suitable security mechanisms for Wireless Sensor Networks (WSNs)
Stringent resource constraints on energy, processing power, memory, bandwidth, etc.
WSNs need lightweight secure mechanisms
We introduce an LCG-based secure aggregation scheme
Efficiency and simplicity

30. Security Goals Security Goals Assumptions Confidentiality Integrity
Sensor data/readings cannot be disclosed to attackers
Integrity
If an adversary modifies a data message, the receiver should be able to detect this tampering
Authenticity
Ensures that data messages come from the intended sender
Assumptions
The existence of a key management scheme
WSN nodes can negotiate the key and trust setup

31. LCG-based Security Protocols
Basic Hop by Hop Message Transmission
Notations
A, B, C…: Sensor Nodes
E(P, K): Encryption of plaintext message P using key K
P1|P2: Concatenation of message P1 and P2
MAC(K, P): Message Authentication Code (MAC) of message P using key K
X0: seed of the LCG
a, b, m: Parameters of the LCG

32. Integrity and Authenticity
CBC: Cipher Block Chaining