Christian Frank, Kay Rmer

1
Algorithms for Generic Role Assignment in
Wireless Sensor Networks

• Christian Frank, Kay Römer
• ETH Zurich

2
The Gap
read_sensor() send_msg() get_pos()
read_sensor() send_msg() get_pos()
3
Generic Role Assignment

• Enables automatic assignment of
• Special functions/roles to nodes in the network
• Using programmer-specified rules for assignment
• Rules are based on local and neighborhood
  properties

4
Use Case / Architecture
Gateway
Role Specifications
Sensor Node
RA Algorithm
Property Directory
Simulation Evaluation
Network
App.
battery 80 pos (12.3, 3.4) role ON
5
Clustering Appl.
CLUSTERHEAD battery gt 60 count(1 hop)
role CLUSTERHEAD 0 GATEWAY
chs retrieve(1 hop, 2) role
CLUSTERHEAD count(2 hops) role
GATEWAY chs super.chs 0 SLAVE
else

• p retrieve(scope, num) pred
• At least num nodes in scope must fulfil pred
• Bind p to ids of matching nodes

• count(scope) pred
• Counts nodes matching pred within scope

6
Use Case / Architecture
Gateway
Role Specifications
Sensor Node
RA Algorithm
Property Directory
Simulation Evaluation
Network
App.
7
Distributed Algorithm

• Local cache table on each node
• Contains local and remote properties
• Algorithm consists of three procedures
• 1) Initialize cache table
• 2) Propagate properties to neighbors
• 3) Choose role according to local table
• On change of local table
• Reschedule 2) and 3)
• Iteration through a set of roles
• Notify applications on stable role

Clustering
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Distributed Algorithm

• 1) Initialization
• 2) Property Propagation
• broadcast all rows x with
• dist lt max and
• dirty true
• set x.dirty to false
• 3) Local Rule Evaluation

Dist
Max
Dirty
Value
Key
Src
0
1
true
undef.
role
A
A
B
C
undef.
ON
OFF
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Probabilistic Initialization

• Improve convergence
• Chose initial role smartly
• Approach
• Estimate probability pr for each role
• Draw role r with probability pr
• Estimation can be done offline using static
  information
• Specification
• Node degree estimate
• Extension
• Combine estimate pr and known/certain information
• Later on repair inconsistent role assignments
• Using standard cache table approach

10
Probabilistic Initialization
ON count(1) role ON lt lim

• Given
• Specification
• Estimated n nodes within scope
• Initial role probabilities
• Compute role probabilities from spec.
• Consider above example, probability that
• k out of n nodes are ON
• k nodes less/eq. lim are ON
• Assumption
• Symmetric probabilities
• System of equations
• solved offline using fixpoint iteration

11
Wave Initialization
ON count(1) role ON lt lim

• Additionally given
• Est. role probabilities (last slide)
• Roles of some nodes in scope
• Compute role probability given known roles
• Make use of initial specification flood

Sink
Y nodes are known and ON
12
Use Case / Architecture
Gateway
Role Specifications
Sensor Node
RA Algorithm
Property Directory
Simulation Evaluation
Network
App.
13
Implementation

• Simulation tool
• Discrete event simulator based on JIST/SWANS
• Visualization / specification frontend
• Specification compiler
• Network model
• Based on CC1000 parameters
• Simple CSMA approach, only broadcast is used
• Intentionately, no measures to improve
  reliability
• Initial prototype on real nodes
• Supports subset of specification (count operators)

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Evaluation

• Simulated three specifications
• Coverage / clustering / aggregation
• Studied algorithms
• Basic caching algorithm
• Basic probabilistic initialization
• Basic wave-based initialization
• Examined
• Overhead, while varying nodes in same area
• Convergence, while varying nodes in same area
• no. of role changes until a stable role is
  reached
• Robustness, while varying an additional ratio of
  lost messages
• Proportionality, while varying the maximum scope
  of the specification

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Convergence

• Metric
• Num. of role changes (except 1stprob. choice)
• Coverage results
• No further reconfiguration after wave
• Clustering results
• Probabilistic does not improve

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Limitations / Discussion

• Some specifications may not terminate
• Support user to detect non-terminating
  specifications
• Simulation tool used for testing
• Protect deployed network by limiting role changes
• Cannot describe every algorithm
• Focus on ease-of-use for application domain
  experts
• Extensible by using app.-specific procedures
• Efficiency
• Effort proportional to difficulty of
  specification
• Comparable to specific implementations

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Conclusion/Outlook

• First generic role assignment tool
• System service for WSN configuration problems
• Used to formulate a variety of network
  configuration heuristics
• Rapid prototyping
• System properties
• Proportional effort
• Efficient probabilistic initialization
• Future work
• TinyOS implementation
• More flexible scope definitions
• Adaptation for more heterogeneous networks

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Thank you!