1. Toward Reliable and Efficient Reporting in Wireless Sensor Networks Authors: Fatma Bouabdallah Nizar Bouabdallah Raouf Boutaba

2. Introduction Energy efficiency, major concerns in WSNs, impacts the network lifetime; Most related mechanisms aim at achieving further energy conservation by reducing the energy wastage resulting from the frequently occurring collisions in WSNs; The key performance metrics in WSNs are both the network lifetime and the average time required to report reliably an event; The goal is to determine the optimal number of reporting nodes that both minimizes the energy required to report reliably an event and respects the latency constraints.

3. Related Work Some schemes aim at decreasing the energy consumption by using sleep schedules.  Some suggest wake-up scheduling schemes at the MAC layer to activate sleeping nodes when it is needed;  Some address the problem at the network layer by proposing new routing solutions that take into account the sleep state of some network nodes. Some works focus on the elimination of the useless redundant information.  At the data originator level by regulating the access;  At the intermediate sensor nodes routing the information to the sink by means of aggregation mechanisms;  Such schemes affect the reliability of the information transmitted to the sink.

4. Related Work Using a small subset of the nodes in the event area, to report the detected event reduces considerably the energy consumption; Vuran and Akyildiz determine the minimum number of representative nodes N min that need at least to be activated in order to comply with the required data reliability at the sink; Advantages compared to the work of Vuran and Akyildiz  The required reliability could be maintained, even if the number of active reporting nodes N is less than the minimum boundary N min ;  Reducing the number of reporting nodes beyond N min decreases the energy wastage due to collisions and idle listening.

5. Problem Statement R c, coverage range; R t, transmission range; N tot, the total number of sensor nodes within the event area; f, the network reporting frequency, generally fixed to achieve the optimal energy consumption; R(N), desired event reliability, the number of data packets required by the sink to consider the event as reliable; N, the number of active reporting nodes; All the sensor nodes are within one hop from the sink.

6. Relationship between Information Reliability and the Number of reporting Nodes Expressions of the observed information distortion at sink D; D max, a predefined tolerable information distortion; One report vs multiple reports;

7. Relationship between Information Reliability and the Number of reporting Nodes The transmitted data to the sink becomes highly redundant when the number of transmitted reports is large; The correlation among the transmitted data increases when N decreases.

8. Relationship between Information Reliability and the Number of reporting Nodes R(N) decreases with N; For each distortion value, there exist a minimum number of reporting nodes to achieve the desired information reliability.

9. Access in WSN Communications in current deployed WSN are usually carried using the basic IEEE 802.11 DCF protocol and its optional RTS/CTS mechanism; The IEEE 802.11 DCF access method is based on the CSMA/CA technique

10. Access in WSN CC-MAC protocol exploits the information about correlation, sent by the sink node, to select only a small subset of sensor nodes among all the potential ones to report the detected event.  Correlation radius R corr, indicates the average distance allowable between selected representative nodes.  A new set of reporting nodes us elected for each event occurrence. Calculate the appropriate R corr

11. Performance Analysis Two modes of functioning according to the network reporting frequency f  Saturated regimes: f is high enough. For each new transmission cycle, all the reporting nodes compete to access the common channel;  Unsaturated regimes: f is relatively low, channel may remain free.

12. Probability of Collision

14. Average Time to Report an Event In the saturated regime  Overall transmission time of a report  The average time to report reliably an event is R(N)*T sat (N) In the unsaturated regime  The average time to report reliably an event

15. Sensor Network Lifetime T network_lifetime (N), the network lifetime; E initial, the total initially provided amount of energy; M, the rate of event occurrence; f, the reporting frequency;

16. Sensor Network Lifetime In the saturated regime In the unsaturated regime

17. Performance Evaluation N total =50, the number of active reporting nodes N varies between 1 and 50;

18. Probability of Collision Collisions become more frequent when the number of competing access nodes increases, which leads to extra energy expenditure and increases the average time to report an event

19. Average Backoff Time as a function N

20. Average time to report an event as a function N

21. Average time to report reliably an event as a function N Assume D max =8 The fastest way to report reliably an event is to let only N opt_latency nodes to report an event

22. Average time to report reliably an event as a function f When f exceeds approximately 900 reports/s, we deal with the saturated regime

23. Average RTC Energy (  j) as a function N Increasing N amplifies the wasted energy due to collision; Increasing N means waking up more sensor nodes within the event radius; Optimal number of active reporting nodes that enables the minimal energy consumption is N=1

24. Average Energy Consumption as a function N The minimal consumption is obtained when only N opt_energy =9 reporting nodes are activated;

25. Average Energy E WSN (  j) vs Observed Event Distortion

26. Average Energy E WSN (  j) as a function f Energy increases slightly when reach the saturated regime, since more energy is wasted due to collisions.

27. Sensor Network Lifetime The maximum network lifetime is obtained when only N opt_energy =9 reporting nodes are activated

28. Conclusion The optimal way of consuming energy is to activate only N opt_energy reporting nodes that operate in the unsaturated regime; The fastest way to report reliably an event is to plan a network with N opt_latency active reporting nodes operating in the saturated regime.

29. Future Work Multiple events occur concurrently in the network; The mobility of the same event over time.

30. Thank You ! Questions?