RE-Pair

Self-Repairing Mobile Node Network

Disabled Sensors

• In James Bond movies, you often see him disabling sensors and cameras and sneaking into the secret hideout.

• So what’s wrong with this picture?

• If the bad guys can spend millions buying sharks with fricken lasers, then why can’t they buy better security?

Mobile Nodes

• What does mobility give us?• Self-deployment• Self-repair

• We consider a sparse deployment of nodes• Depth vs Breadth• Each node has to maintain a connection to

the base station

Applications

• Providing reliable wireless access• Quick, automated deployment• Fast repair, adaptive

• Intrusion detection• Self-repairing perimeter allows recovery

from damage

• Monitor environment

Challenges

• Death of a node bisects the graph• 2 Ways to repair link (distributed vs

centralized)• Nodes detect break and repair on their own• Base station/Master node redeploys the nodes to

optimally cover the area

• 2 Goals of repair• Reconnect all nodes• Maximize coverage

Challenges cont.

• Repair with nodes• Possible to reconnect network• Tough for nodes to cooperatively find the

total slack in the network

• Repair with master node• Able to calculate best dispersal with nodes

it has• Can not reach separated nodes to issue

deployment orders

Re-Pair

• Separate link repair into two stages• Stage 1

• Individual nodes detect break and repair link• Requires knowledge of node locations and

node deaths

• Stage 2• Master node calculates a new deployment once

it determines how many nodes it has left

Keep-Alives

• Nodes broadcast a keep-alive advertising their status and location• Optimization: filter out keep-alives that contain

only old information• Reduces packet traffic

• Assign pairs of neighboring nodes• Responsible for announcing death of paired node

• Store locations and status in a table per node• Not necessary for correctness but for performance

Paired Nodes

Pair 2 & 3

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Pair 3 & 4

Stage 1

• Node’s death detected by its neighboring pairs (network is now split in two)

• Death announced to connected and disconnected nodes

• Disconnected neighbor heads toward the location of the node preceding the dead node• Repeats until its reconnected or has reached the

base station• Allows the network to retain coverage during

repair

Death Detection

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2 announces death of 3

4 announces death of 3

Pair 2 & 4

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Stage 2

• Master node listens for changes in the network

• Requests Update from nodes to get accurate count and locations

• Sends a redeployment packet once nodes have reconnected to use existing slack in the network • Improves coverage

FSM for Master Node

WAIT WAIT UPDATE

LISTEN

Timer (deploy)

Timer (update)

Death/New/Arrival

Death/New/Arrival

Death

Figure 1: Finite state machine for master node

Deployment

• Graph divided into linear sets• Nodes expand to cover the area assigned to

set in case of loss• Extra nodes in a set are reassigned to sets without

enough nodes to maintain coverage

• At the very start, all the nodes are in the same set and are located at the base station

Node Movement: Line

Node Movement: Line

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Distance Moved

CDF

Node Movement: Star

Node Movement: Star

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Distance Moved

CDF

Node Movement: BoxNode Movement for Box Topology

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Distance Moved

CDF

Coverage: BoxCoverage: Box

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97.00%

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Lost Nodes

Coverage

Future Work

• Tweaks• Use distances to calculate timing parameters for

state transitions

• New applications• Coordinated movement to perform sweep-search• Allow nodes to search areas, use rendezvous to

share search results and report progress

Summary

• Mobile nodes enable self-repair and self-deployment

• Re-Pair• Use 2 stages to reconnect network and

maximize coverage