Data-Centric Storage in Sensornets with GHT, A Geographic Hash Table

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Data-Centric Storage in Sensornets with GHT, A
Geographic Hash Table

• Sylvia Ratnasamy, Scott Shenker,
• Brad Karp, Ramesh Govindan, Deborah Estrin,
• Li Yin and Fang Yu
• ICSI/UCB/USC/UCLA

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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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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 gtgt maxDq, Q

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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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Review GPSR

• Greedy forwarding algorithm
• Perimeter forwarding algorithm

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GHT

• Home node
• to be the node geographically nearest the
  destination coordinates of the packet
• Home perimeter
• the entire perimeter
• that encloses the
• destionation.

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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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Perimeter Refresh Protocol

• PRP stores a copy of a key-value pair at each
  node on the home perimeter.
• PRP generates refresh packets periodically.

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Structured Replication

• Use a hierarchical decomposition of the key
  space.
• For a given root r and a given hierarchy depth d,
  one can compute 4d-1 mirror images of r

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Simulation

• Success rate
• the mean over all queries of the fraction of
  events returned in each response, divided by the
  total number of events known to have been stored
  in the network for that key.
• f
• the fraction of nodes that remain up for the
  entire simulation.

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Simulation

• Stable and Static Nodes

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Simulation

• Static but Failing Nodes

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Simulation

• System parameters
• N, the number of nodes in the system
• T, the number of event types, T 100
• Q, the number of event types queried for
• Di, the number of detected events of event type
  i. Di 100

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Simulation

• Three version of DCS
• Normal DCS (N-DCS) a query returns a separate
  message for each detected event
• Summarized DCS (S-DCS) A query returns a single
  message regardless of the number of detected
  events
• Structured Replication DCS (SR-DCS)?

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Simulation

• Test 1 Varying Q

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Simulation

• Test 1 Varying Q

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Simulation

• Test 2 Varying n

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Simulation

• Test 2 Varying n

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Conclusion

• Advantages
• In DCS, relevant data are stored by name at nodes
  within the sensornets.
• 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.

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Conclusion

• Disadvantages
• GHT requires approximate knowledge of a
  sensornet's boundaries
• Only supports binary events, not range queries.

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Questions?Thanks