Minji Wu, Jianliang Xu, Xueyan Tang, Wang-Chien Lee

Professor : 陳朝鈞 教授　　　 Speaker : 邱志銘

Minji Wu, Jianliang Xu, Xueyan Tang, Wang-Chien Lee, “Top-k Monitoring in Wireless Sensor Minji Wu, Jianliang Xu, Xueyan Tang, Wang-Chien Lee, “Top-k Monitoring in Wireless Sensor Networks”, Knowledge and Data Engineering, IEEE Transactions on Volume 19,  Issue 7,  July Networks”, Knowledge and Data Engineering, IEEE Transactions on Volume 19,  Issue 7,  July 20072007

Introduction Top-k Monitoring

◦ FILA Overview◦ Filter Setting (Uniform & Skewed)◦ Filter Update (Eager & Lazy)

Performance Evaluation◦ Simulation◦ Eager versus Lazy Filter Update◦ Performance Comparison with TAG and Cache

Conclusions

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Top-K Query◦ Environmental Monitoring

A top-k query is issued find out the nodes and their corresponding areas with the highest pollution indexes for the purpose of pollution control or research study.

◦ Network Management A top-k query may be issued to continuously monitor

the sensor nodes with the least residual energy.

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This paper focuses on continuously monitoring top-k queries in sensor networks.◦ Utilize previous top-k result to obtain a new top-k

result.

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Monitoring a Top-1 query◦ TAG Base Station

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總共需要 9 次傳送

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Monitoring a Top-1 query◦ FILA Base Station

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probe node C

[39, 47] [47, 80]

[20, 39]

總共需要 6 次傳送

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Base station has a continuous power supply.

Sensor nodes powered by battery.

Each sensor node measures the local physical phenomenon at a fixed sampling rate.

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1. Filter Setting◦ the base station computes a filter [li, ui] for each

sensor node i and sends it to the node for installation.

2. Query Reevaluation3. Filter update

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Tinternal： the set of internal updates Tjoin： the set of join updates Tleave： the set of leave updates T ： the old top-k set

◦ If |T’|=|T|-|Tleave|+|Tjoin|≧k The new top-k set must be a subset of T’

◦ Otherwise, if |T’|<k The nodes that are not in T have to be probed.

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Uniform filter setting

◦ It is simple and favorable when the readings of sensor nodes follow a similar changing pattern.

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Skewed filter setting◦ Taking into account the changing patterns of

sensor readings.◦ Suppose the average time for the reading of node

I to change beyond is fi(δ) 1/fi(δ): the rate of sensor-initiated updates by node i

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Eager filter update◦ If a new filtering windows [li’, ui’] is different from

the old one [li, ui] then the new filter [li’,ui’] is immediately sent to node i

Lazy filter update◦ If a new filtering windows [li’, ui’] fully contains the

old one [li, ui], then the base station delays the filter update until node i’s reading violates the old filter [li, ui] .

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Simulation Setup◦ Energy cost in transmitting a message

s: message size α: distance-independent term β: coefficient q: distance-dependent term d: distance

◦ Energy cost in receiving a message γ is set at 50 nJ/b

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A Sensor initiated update message:◦ Sensor ID: 4 bytes◦ Sensor Reading: 4 Bytes

A filtering windows is characterized by 8 bytes.

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10 Sensor 120 Sensor

Simulated using the real traces provided by the Live from Earth and Mars (LEM) project at the University of Washington.

Two kinds of sensor readings are used◦ Temperature (TEMP)◦ Dew point (DEW)◦ Logged by the station at the University of Washington

from August 2004 to August 2005 Total 500000 sensor readings

◦ Extract many subtraces stating at different dates◦ Each subtrace contains 20000 readings◦ The subtraces were used to simulate the physical

phenomena in the immediate surroundings of different sensor nodes.

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Network Lifetime◦ The network lifetimes is defined as the time

duration before the first sensor node runs out of power.

Average Energy Consumption◦ It is defined as the average amount of energy

consumed by a sensor node per time unit. Monitoring Accuracy

◦ This is defined as the mean accuracy of monitored results against the real results.

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This paper exploited the semantics of top-k query and proposed a novel energy-efficient monitoring approach called FILA.

Two filter setting algorithms (that is, uniform and skewed) and two filter update strategies (that is, eager and lazy) have been proposed.

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