1. 1 EnviroTrack and JAM Presented by Chien-Liang Fok

2. 2 EnviroTrack A entity-tracking middleware for sensor networks Provides an single context label abstraction that is dynamically created and logically associated with an entity Abstracts details of inter-node communication Performs of group maintenance Programmer interacts with context label rather than changing set of nodes

3. 3 Context Label A form of attribute-based naming Serves as an address to the entity Is distinguished by type (e.g., fire or car) Contains aggregate state about the entity Hosts application-defined tracking objects Active elements that perform context-specific computation or action Abstracts a group of nodes that can sense the entity

4. 4 Problem Definition Maintain context labels such that Each entity has one context label Context label follows the entity Group management, leader maintenance Approximate aggregate state is maintained Freshness constraint, L e Critical mass constraint, N e Simplify application development Language support for defining context labels, aggregate state variables, and tracking objects Directory service

5. 5 Programming Model

6. 6 Context Label Specification To declare a context label of type e, the following must be provided: sense e () – specifies a pattern in the environment to watch for state e () – the state data to be maintained within the context label EnviroTrack provides a library of state e () functions Accessible by all tracking objects Which tracking objects to associate with the label

7. 7 Group Maintenance A group of sensors that detect the entity should produce a single context label for it The algorithm must be lightweight and dynamic All members of the group must satisfy sense e () There is at most one majority leader within the group

8. 8 Group Maintenance Algorithm If a leader exists, the leader sends heartbeat containing e Propagates h hops beyond group boundary Tells member nodes leader is still present Notifies non-members of the existence of context label e

9. 9 G. M. Algorithm Cont. When a member receives a heartbeat, it sets a timer. If timer expires before receiving another heartbeat, a leadership takeover protocol is triggered This is a backup to the standard leadership handoff

10. 10 G. M. Algorithm Cont. When a non-member node receives a heartbeat, it sets a timer If the entity is sensed before expiration, the node joins the group If the entity is sensed after timer expires, the node creates a new group (and context label) and declares itself leader

11. 11 Spurious Leaders The group member algorithm may result in more than one leader Solution: Each context label’s leader contains a weight, initially zero Increment weight each time data is received from the members Weight is inherited during leadership handoff Context label with greater weight wins Unlikely for spurious leaders attain N e

12. 12 Approximate Aggregate State The leader of the group maintains the approximate aggregate data state Leader sends each member L e The member periodically sends sensor data to leader with a period of P e = L e – d d = network delay + processing time The leader performs aggregation every P e A valid flag is set if N e is not met

13. 13 Directory Service Provides access to a particular type of context label Use a hash function context label type  physical location (x,y) Nodes within one hop of (x,y) are called directory object Context labels periodically send their state and location to directory object

14. 14 Language EnviroTrack provides a language for declaring context labels and aggregate data state Example code:

15. 15 Evaluation Theoretical example scenario: A T-72 tank moves through a sensor field Sensors can detect tank 100m away Sensors arranged in grid 140m apart At max speed, tank moves one hop every 11.2 seconds (45km/h) Actual test setup: 1000:1 model of the above scenario 10s/hop (50km/h) and 15s/hop (33km/h)

16. 16 Tracked tank trajectory Nodes located at integer coordinates Actual path: y = 0.5 Nodes have no notion of proximity

17. 17 Leadership Handover

18. 18 Communication Performance System handles message loss Message loss not due to link usage Protocol uses very little bandwidth Scalable to greater tracking difficulty

19. 19 Maximum Trackable Speed Affected most by heartbeat period Best results when Receive timer = 2.1 HB Time to take over leadership Wait timer = 4.1*HB Time to create new group

20. 20 Maximum Trackable Speed

21. 21 Communication Radius & Sensing Radius Ratio

22. 22 Conclusions EnviroTrack is a middleware for tracking entities Relieves applications from details of Group management Directory service Data aggregation

23. 23 JAM A protocol that allows applications to view a jammed region as an entity rather than a collection of broken links and congested nodes Allows data to be rerouted around a jammed region and other evasive actions

24. 24 General Algorithm Nodes within the jammed area notify nodes outside of the jammed area that they are jammed Border nodes just outside jammed area coordinate to form groups of jammed nodes jammed area

25. 25 Architecture

26. 26 Jam Detection A node considers itself jammed when the utility of its communication link falls below a certain threshold Based on local data (failed channel access, protocol violations, low SNR) When a node considers itself jammed, it periodically sends a JAMMED message Modify MAC to bypass carrier sense phase Hopefully an un-jammed neighbor receives it

27. 27 Mapping protocol Border nodes receive a JAMMED messages from its jammed neighbors and calculates normal direction vector to the jammed nodes These nodes are called mapping nodes If it is not aware of a compatible group, it creates a new one with a random ID, waits a while, and broadcasts a BUILD message Two groups are compatible if the angular difference between their direction vectors is below some threshold

28. 28 Group Coalescing If two groups are compatible, they are coalesced after a certain time The group with the larger group ID dominates Jammed nodes in the subordinate group is merged with the dominant group Dominant group’s BUILD message is augmented with subordinate group ID

29. 29 BUILD Message Contains: ID of original sender Group ID Sequence number List of known jammed nodes List of subordinate group IDs When received If a regular node, store data locally If a mapping node, update the list of known jammed nodes and rebroadcast it

30. 30 Eager Eavesdropping JAM attempts to quickly diffuse knowledge of jammed region: Forward information diffusion: As a BUILD message is relayed around mapping nodes, each node updates list of jammed nodes Back Flooding: When a a BUILD message is rebroadcasts, the single hop upstream mapping node processes it Updates its list of jammed nodes Does not relay it

31. 31 Edge Nodes A mapping node that does not have a neighbor left or right of the direction vector If a neighbor is not discovered within some time, it periodically sends a PROBE message to detect neighboring mapping node(s) or form bridge nodes jammed area edge node

32. 32 Bridge Nodes Does not actually receive any JAMMED messages Connects two edge nodes Masks areas of low connectivity A node becomes a bridge node when it detects a PROBE message from a group it knows about A bridge node’s coalescing timer is longer to prevent spurious bridging nodes

33. 33 Recovery When a jammed node becomes un-jammed, it periodically sends an UNJAMMED message When a mapping node receives it, it updates the list of jammed nodes and propagates an TEARDOWN message The opposite of a BUILD message When all of a mapping node’s jammed neighbors are un-jammed, the mapping node becomes a regular node

34. 34 Example [1][2] [3][4]

35. 35 Example (Cont.) [5][6] [7][8]

36. 36 Simulation Methodology GloMoSim simulator 4000 x 4000 meter field 400 nodes placed at 200m interval grid Radio settings inbetween MICA2 and WaveLAN MACA & IEEE802.11 MAC layer Jammed area ranged from 5-144 nodes Connectivity Low: 4 neighbors Moderate: 8 and 12 neighbors

37. 37 Simulation Results Varying jammer range

38. 38 Simulation Results Cont. Time to map region, 12 neighbor case 90% of mapping nodes know 1/3 of jammed region in 0.5-3 seconds

39. 39 Simulation Results Cont. Node failure rate

40. 40 Conclusions JAM provides a protocol for mapping a jammed region Loose group semantics and eager eavesdropping Quick convergence and tolerance to failures when network is moderately connected