Event-Driven Reporting Scenarios

In the following, we assume that sensors’ reporting activities are triggered by events occurred at random locations in the network with a rate λ. The sensing range of each sensors is 3

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0 5 10 15 20 25 30 35 40

7 8 9 10 11 12

L(G) x slot-size (in second)

BO Theoretical

Actual(λ=1/5s) Actual(λ=1/15s) Actual(λ=1/30s)

0 10 20 30 40 50 60 70 80 90 100

7 8 9 10 11 12

Goodput (%)

BO λ=1/5s

λ=1/15s λ=1/30s

(a) (b)

Figure 12: Simulation results of event-driven scenarios: (a) theoretical v.s. actual report latencies and (b) goodput.

meters and each event is a disk of a radius of 5 meters. A sensor can detect an event if its sensing range overlaps with the disk of that event. Each router has an 1 KB buffer. When a sensor detects an event, it only tries to report that event once. All other settings are the same as those in Section 5.2.

Fig. 12 shows the simulation results whenλ = 1/5s, 1/15s, and 1/30s. From Fig. 12(a), we can observe that whenBO is small, the report latency can not achieve to the theoretical value. This is because that an active portion is too small to accommodate all reports from sensors, thus lengthening the report latency. WhenBO becomes larger, the theoretical and actual curves would meet. However, the good put will degrade, as shown in Fig. 12(b). This is because reports are likely to be dropped due to buffer overflow. How to determine a proper BO, which can contain most of the reports and guarantee low latency, is an important design issue for such scenarios.

6 Conclusions

In this paper, we have defined a new minimum delay beacon scheduling (MDBS) problem for convergecast with the restrictions that the beacon scheduling must be compliant to the ZigBee standard. We prove the MDBS problem is NP-complete and propose optimal

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lutions for special cases and two heuristic algorithms for general cases. Simulation results indicate the performance of our heuristic algorithms decrease only when the number of in-terference neighbors is increased. Compared to the random slot assignment and greedy slot assignment scheme, our heuristic algorithms can effectively schedule the ZigBee routers’

beacon times to achieve quick convergecast. In the future, it deserves to consider extending this work to an asynchronous sleep scheduling to support energy-efficient convergecast in ZigBee mesh networks.

7 Acknowledgements

Y.-C. Tseng’s research is co-sponsored by Taiwan MoE ATU Program, by NSC grants 93-2752-E-007-001-PAE, 96-2623-7-009-002-ET, 058-MY3, 95-2221-E-009-060-MY3, 95-2219-E-009-007, 95-2218-E-009-209, and 94-2219-E-007-009, by Realtek Semiconductor Corp., by MOEA under grant number 94-EC-17-A-04-S1-044, by ITRI, Tai-wan, by Microsoft Corp., and by Intel Corp.

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