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EnviroTrack and JAM

Presented by Chien-Liang Fok

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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

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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

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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, Le

Critical mass constraint, Ne

Simplify application development Language support for defining context labels, aggregate

state variables, and tracking objects Directory service

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Programming Model

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Context Label Specification

To declare a context label of type e, the following must be provided: sensee() – specifies a pattern in the environment

to watch for statee() – the state data to be maintained within

the context label EnviroTrack provides a library of statee() functions Accessible by all tracking objects

Which tracking objects to associate with the label

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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 sensee()

There is at most one majority leader within the group

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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

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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

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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

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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 Ne

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Approximate Aggregate State

The leader of the group maintains the approximate aggregate data state

Leader sends each member Le

The member periodically sends sensor data to leader with a period of Pe = Le – d d = network delay + processing time

The leader performs aggregation every Pe A valid flag is set if Ne is not met

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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

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Language

EnviroTrack provides a language for declaring context labels and aggregate data state

Example code:

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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)

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Tracked tank trajectory

Nodes located at integer coordinates Actual path: y = 0.5 Nodes have no notion of proximity

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Leadership Handover

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Communication Performance

System handles message loss Message loss not due to link usage Protocol uses very little bandwidth

Scalable to greater tracking difficulty

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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

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Maximum Trackable Speed

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Communication Radius & Sensing Radius Ratio

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Conclusions

EnviroTrack is a middleware for tracking entities

Relieves applications from details of Group management Directory service Data aggregation

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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

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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

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Architecture

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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

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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

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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

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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

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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

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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

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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

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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

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Example

[1] [2]

[3] [4]

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Example (Cont.)

[5] [6]

[7] [8]

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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

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Simulation Results

Varying jammer range

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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

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Simulation Results Cont.

Node failure rate

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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