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

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Sylvia Ratnasamy, Scott Shenker,

Brad Karp, Ramesh Govindan, Deborah Estrin,

Li Yin and Fang Yu

ICSI/UCB/USC/UCLA

Background

Existing Schemes

Data-Centric Storage

Conclusion

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

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

External Storage (ES)

Local Storage (LS)

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

Put(“elephant”, data)

(11, 28)

(11,28)=Hash(“elephant”)

Get(“elephant”)

(11, 28)

(11,28)=Hash(“elephant”)

elephant

fire

Metrics

♦ Total Messages

• total packets sent in the sensor network

♦ Hotspot Messages

• maximal number of packets sent by any particular node

Assume ♦ n is the number of nodes

♦ Asymptotic costs of O(n) for floods

O(n 1/2) for point-to-point routing

ES

LS

DS

Cost for Storage

O(n 1/2)

0

O(n1/2)

Cost for Query

0

O(n)

O(n1/2)

Cost for Response

0

O(n1/2)

O(n1/2)

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.

ES

LS

DCS

Total

Hotspot

DCS is preferable if

- Sensor network is large
- Dtotal >> max[Dq, Q]

Builds on

♦ Peer-to-peer Lookup Systems

♦ Greedy Perimeter Stateless Routing

GHT

GPSR

Peer-to-peer

lookup system

Greedy forwarding algorithm

Perimeter forwarding algorithm

- Home node
- to be the node geographically nearest the destination coordinates of the packet

- Home perimeter
- the entire perimeter
that encloses the

destionation.

- the entire perimeter

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

Perimeter Refresh Protocol

♦ Extension for robustness

♦ Handles nodes failure and topology change

Structured Replication

♦ Extension for scalability

♦ Load balance

PRP stores a copy of a key-value pair at each node on the home perimeter.

PRP generates refresh packets periodically.

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

- 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.

- Stable and Static Nodes

- Static but Failing Nodes

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

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)

Test 1: Varying Q

Test 1: Varying Q

Test 2: Varying n

Test 2: Varying n

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.

Disadvantages:

GHT requires approximate knowledge of a sensornet's boundaries

Only supports binary events, not range queries.

Questions?Thanks