1. 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 Networks”, Knowledge and Data Engineering, IEEE Transactions on Volume 19, Issue 7, July 2007

2.  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 2 2008/12/3

3.  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. 3 2008/12/3

4.  This paper focuses on continuously monitoring top-k queries in sensor networks. ◦ Utilize previous top-k result to obtain a new top-k result. 4 2008/12/3

5.  Monitoring a Top-1 query ◦ TAG Base Station A BC t1t1 t2t2 t3t3 51 56 52 43 45 48 51 56 52 43 45 48 t1t1 t2t2 t3t3 35 38 37 t1t1 t2t2 t3t3 51 56 52 總共需要 9 次傳送 5 2008/12/3

6.  Monitoring a Top-1 query ◦ FILA Base Station A BC t1t1 t2t2 t3t3 48 52 48 52 43 45 48 t1t1 t2t2 t3t3 35 38 37 t1t1 t2t2 t3t3 51 56 52 probe node C [39, 47][47, 80] [20, 39] 總共需要 6 次傳送 6 2008/12/3

7.  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. 7 2008/12/3

8. 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 Reevaluation 3. Filter update 8 2008/12/3

9.  T internal ： the set of internal updates  T join ： the set of join updates  T leave ： the set of leave updates  T ： the old top-k set ◦ If |T’|=|T|-|T leave |+|T join |≧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. 9 2008/12/3

10.  Uniform filter setting ◦ It is simple and favorable when the readings of sensor nodes follow a similar changing pattern. 10 2008/12/3

11.  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 f i (δ)  1 /fi (δ): the rate of sensor-initiated updates by node i 11 2008/12/3

12.  Eager filter update ◦ If a new filtering windows [ l i ’, u i ’ ] is different from the old one [ l i, u i ] then the new filter [ l i ’,u i ’ ] is immediately sent to node i  Lazy filter update ◦ If a new filtering windows [ l i ’, u i ’ ] fully contains the old one [ l i, u i ], then the base station delays the filter update until node i’s reading violates the old filter [ l i, u i ]. 12 2008/12/3

13.  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 13 2008/12/3

14.  A Sensor initiated update message: ◦ Sensor ID: 4 bytes ◦ Sensor Reading: 4 Bytes  A filtering windows is characterized by 8 bytes. 14 2008/12/3

15. 15 10 Sensor120 Sensor

16.  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. 16 2008/12/3

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18.  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. 18 2008/12/3

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22.  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. 22 2008/12/3