1. Layered Diffusion based Coverage Control in Wireless Sensor Networks Wang, Bang; Fu, Cheng; Lim, Hock Beng; Local Computer Networks, 2007. LCN 2007. 32nd IEEE Conference on 15-18 Oct. 2007 Page(s):504 - 511 Digital Object Identifier 10.1109/LCN.2007.90 Local Computer Networks, 2007. LCN 2007. 32nd IEEE Conference on 報告 : 林美佐

2. Outline  一、 Introduction  二、 System Model and Basic Idea  三、 Layered Diffusion based Coverage Control  四、 Performance Evaluation  五、 Conclusions

3. 一、 Introduction (1/2)  Coverage control is used to select as few active nodes as possible from all deployed sensor nodes such that sufficient coverage of the monitored area can be guaranteed, while reducing the energy consumption of each individual sensor node to prolong the network lifetime.  This paper presents a Layered Diffusion based Coverage Control (LDCC)

4. 一、 Introduction (2/2)  The LDCC protocol does not require information on the node location coordinates when selecting active nodes. Instead, it exploits the hop count information, which is easily obtained in a WSN.  The LDCC protocol is very simple and does not require any sophisticated computation.

5. 二、 System Model and Basic Idea(1/6)  It is to apply a triangular tessellation to cover the sensor field.

6. 二、 System Model and Basic Idea(2/6)  In this paper, an active node (or a node in an active state) is a node with its sensing unit on and a sleep node (or a node in a sleep state) is one with its sensing unit off.  The main objective for coverage control is to select as few active sensor nodes as possible to cover as much of the sensor field as possible at any moment of time.

7. 二、 System Model and Basic Idea(3/6)  Given that all sensor nodes use a fixed transmission power, the hop count measures how many transmissions are needed for a sensor node to deliver its packets to the base station.  If the transmission power is set such that the transmission range Rc equals to the length of each side of the equilateral triangle, then the sensor nodes on the circle with radius Rc are those nodes with hop count H = 1.

8. 二、 System Model and Basic Idea(3/6) H=1

9. 二、 System Model and Basic Idea(4/6)  The process of coverage control normally starts by a sensor node with a small hop count sending out an ACTIVE message to its neighbors upon expiration of its timer.

10. 二、 System Model and Basic Idea(5/6)  For example, suppose that sensor node 1 first sends out an ACTIVE message attached with its hop count information.  After receiving an ACTIVE message, each node either needs to set its state to sleep or needs to reset its timer to a different time period according to its hop count information.  If a sensor node receives two or more ACTIVE messages sent from its neighbors with the same hop count as itself, it can then set itself to sleep.

11. 二、 System Model and Basic Idea(6/6)  To enable the rotation between the sleep and the active state for each sensor node, the ACTIVE message can also attach a time period value stating how long is its active state, which will also be used for a sleep node to set its sleep time period.  Therefore, after some interval, all nodes become active again and the above procedure repeats for selecting the active sensor nodes for the next round.

12. 三、 Layered Diffusion Based Coverage Control (1/9)  Layered diffusion is used to describe a message exchange process in which messages of some particular types are likely to be first generated by the base station or the nodes closest to the base station, and diffused from the nodes in a ring closest to the base station to the nodes in a ring farthest from the base station.  That is, a sensor node with a small hop count ignores the messages sent out by another node with a larger hop count.

13. 三、 Layered Diffusion Based Coverage Control (2/9)  Hop count setup  The hop count information can be obtained by a layered diffusion process starting from the base station which sends out a HOP message to initialize the process.  At the time when sensor nodes are deployed, all nodes set their hop count to a very large value HMax, i.e., Hi = HMax for all i.  The process of the layered diffusion starts when the base station first sends out a HOP message with the hop count set to 0, H 0 = 0.  Each HOP message also contains the information of the transmission power (P r ) used for this message.

14. 三、 Layered Diffusion Based Coverage Control (3/9)

15. 三、 Layered Diffusion Based Coverage Control (4/9)  In an errorfree and collision-free environment, the above process of hop count setup can be performed only once if the transmission power does not need to be changed.  For the rest of this paper, the basic version of the proposed LDCC protocol does not consider transmission error and collision.

16. 三、 Layered Diffusion Based Coverage Control (5/9)  Protocol description  The coverage control process is also carried out in a layered diffusion manner and can be combined with the hop count setup process (by merging the HOP and ACTIVE message into one message).  The advantage of this approach is to reduce the message exchange overhead.  The disadvantage is that the hop count message may not be accurate before the hop count setup process is completely finished, and this might introduce some performance loss.

17. 三、 Layered Diffusion Based Coverage Control (6/9)  The execution of the LDCC protocol is divided into rounds and in each round, the base station initiates the coverage control process by sending out an ACTIVE message.  An ACTIVE message indicates that the sender has set itself to an active state and includes the sender’s hop count (H r ), transmission power (P r ) and active time in this round (T r ).  Each sensor node maintains a boolean variable(B) as a flag to indicate that a new round of coverage control has started. B is initialized to true at the time when the sensor network is deployed.

18. 三、 Layered Diffusion Based Coverage Control (7/9)  Each sensor node also maintains several counters. These counters are initialized to 0. N1: to record the number of ticks passed after a node receives the last ACTIVE message. N2: to record the number of received ACTIVE messages sent from the node with the hop count smaller than its own. N3: to record the number of received ACTIVE messages sent from the node with the same hop count as its own.

19. 三、 Layered Diffusion Based Coverage Control (8/9)

20. 三、 Layered Diffusion Based Coverage Control (9/9)

21. 四、 Performance Evaluation (1/4) The more sensor nodes go to sleep, the less area will be covered.

22. 四、 Performance Evaluation (2/4) The proposed LDCC protocol results in very small message overhead even for very large number of deployed nodes.

23. 四、 Performance Evaluation (3/4)

24. 四、 Performance Evaluation (4/4) The message overhead of LDCC is almost negligible compared with that of PEAS.

25. 五、 Conclusions  The LDCC protocol exploits the hop count information, which is very easy to obtain in a WSN, to select active sensor nodes.  Simulation results show that the LDCC protocol achieves high coverage ratio with the least number of active sensor nodes compared with that of the RIS and PEAS algorithms.  The message overhead of the LDCC is much smaller than that of the PEAS. Furthermore, the message overhead of LDCC is not very sensitive to the node density of the deployed network, and hence very large scale networks can greatly benefit from LDCC.