DataCentric Storage in Sensornets

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Data-Centric Storage in Sensornets

• Sylvia Ratnasamy, Scott Shenker,
• Brad Karp, Ramesh Govindan, Deborah Estrin
• ICSI/UCB/USC/UCLA
• Presenter Vijay Sundaram

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Outline

• Background
• Existing Schemes
• Data-Centric Storage
• Conclusion

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Background

• Sensornet
• ? A distributed sensing network comprised of a
  large number of small sensing devices equipped
  with
• processor memory radio
• ? Great volume of data
• Data Dissemination Algorithm
• ? Scalable
• ? Self-organizing
• ? Energy efficient

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Observations/Events/Queries

• Observation
• ? Low-level output from sensors
• ? E.g. detailed temperature and pressure
  readings
• Event
• ? Constellations of low-level observations
• ? E.g. elephant-sighting, fire, intruder
• Query
• ? Used to elicit the event information from
  sensornets
• ? E.g. locations of fires in the network
• Images of intruders detected

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Existing Schemes

• External Storage (ES)
• Local Storage (LS)
• Data-Centric Storage (DCS)

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External Storage (ES)
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Local Storage (LS)
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Local Storage (LS)
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Data-Centric Storage (DCS)

• Events are named with keys
• DCS provides (key, value) pair
• DCS supports two operations
• ? Put (k, v) stores v ( the observed data )
  according to the key k, the name of the data
• ? Get (k) retrieves whatever value is stored
  associated with key k
• Hash function
• ? Hash a key k into geographic coordinates
• ? Put() and Get() operations on the same key k
  hash k to the same location

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DCS Example
(11, 28)
(11,28)Hash(elephant)
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DCS Example
Get(elephant)
(11, 28)
(11,28)Hash(elephant)
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DCS Example contd..
elephant
fire
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Geographic Hash Table (GHT)

• Builds on
• ? Peer-to-peer Lookup Systems
• ? Greedy Perimeter Stateless Routing

GHT
GPSR
Peer-to-peer lookup system
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Problems

• Not robust enough
• ? Nodes could move (new home node?)
• ? Home nodes could fail
• Not scalable
• ? Home nodes could become communication
  bottleneck
• ? Storage capacity of home nodes

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Solutions

• Perimeter Refresh Protocol
• ? Extension for robustness
• ? Handles nodes failure and topology change
• Structured Replication
• ? Extension for scalability
• ? Load balance

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Comparison Study

• Metrics
• ? Total Messages
• total packets sent in the sensor network
• ? Hotspot Messages
• maximal number of packets sent by any
  particular node

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Comparison Study - contd..

• Assume ? n is the number of nodes
• ? Asymptotic costs of O(n) for floods
• O(n 1/2) for point-to-point
  routing

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Comparison Study -contd..

• Dtotal, the total number of events detected
• Q , the number of event types queries for
• Dq, the number of detected events of event
  types
• No more than one query for each event type, so
  there are Q queries in total.
• Assume hotspot occurs on packets sending to the
  access point.

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Comparison Study contd..

• DCS is preferable if
• Sensor network is large
• Dtotal maxDq, Q

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Conclusion

• In DCS, relevant data are stored by name at nodes
  within the sensornets.
• GHT hashes a key k into geographic coordinates,
  the key-value pair is stored at a node in the
  vicinity of the location to which its key hashes.
• To ensure robustness and scalability, DCS uses
  Perimeter Refresh Protocol (PRP) and Structured
  Replication (SR).
• Compared with ES and LS, DCS is preferable in
  large sensornet .