Impact of Data Rate on Accuracy and Overhead

Figure 5.5 shows the synchronization error of TPSN and TSS under different data rates.

On one hand, the synchronization error of TPSN exhibits similar behavior to the simu-lation results. The line remains level until the congestion point (at the packet sending interval approximately 2⁻³second), and from that a sudden raise seems to grow

dramati-40 CHAPTER 5. TAROKO MOTE EXPERIMENTS

Figure 5.5: [Taroko Mote Experiment] Synchronization errors for TPSN and TSS un-der different data rates.

cally. On the other hand, the synchronization error of TSS presents almost at. Again, due to same cause noted in Subsection 5.5.1, the pair-wise synchronization error Esync domi-nates the overall error T^tss, and alleviates the impact of data rate on the synchronization error.

Figure 5.6 shows the protocol overhead ratios of TPSN and TSS under different data rates. Similar to the simulation results shown in Figure 4.3, because the protocol overhead is piggybacked on the data packet, the overhead ratio of TSS displays a xed overhead ratio. And the overhead ratio of TPSN drops with increasing data rate since TPSN ex-changes synchronization messages independent of data trafc volume.

Taroko mote experimental results have shown that the synchronization error of TSS is smaller than of TPSN, yet the overhead ratio of TPSN is lower under high data rates.

5.5. TAROKO MOTE EXPERIMENTAL RESULTS 41

˃

˃ˁ˄

˃ˁ˅

˃ˁˆ

˃ˁˇ

˃ˁˈ

˃ˁˉ

˃ˁˊ

˃ˁˋ

˅ ˄ ˃ ˀ˄ ˀ˅ ˀˆ ˀˇ

˗˴̇˴ʳ˥˴̇˸ʳʻ̆˸́˷˼́˺ʳ˼́̇˸̅̉˴˿ʳ˼́ʳ˅˱̋ʳ̆˸˶̂́˷̆ʼ

ˢ̉˸̅˻˸˴˷ʳʻ̅˴̇˼̂ʼ

˧˦˦

˧ˣ˦ˡ

Figure 5.6: [Taroko Mote Experiment] Protocol overhead for TPSN and TSS under different data rates.

42 CHAPTER 5. TAROKO MOTE EXPERIMENTS

Chapter 6

Conclusions and Selection Guideline

In conclusion, we achieve in (1) modeling the average error of two time synchronization mechanisms, TPSN and TSS, (2) validating the analytical models with thorough simu-lations and real sensor experiments, and (3) deriving the following selection guideline for choosing a suitable time synchronization protocol based on the network size, node mobility level, and data rate.

• When network size is large, TSS is dominantly better both in terms of accuracy and overhead.

• When mobility level is high and energy consumption is critical, TSS will be the choice. If energy consumption is not critical, the choice should depend on the actual mobility level. TSS appears to be better in the error to reference clock. However, the error of TSS has a steep growing trend. When the mobility level goes beyond the parameters used for the experiments, the error of TSS could potentially be worse than that of TPSN.

• When data rate is high and energy consumption is critical, TPSN will be the choice.

If energy consumption is not critical, the error of TSS is lower and scales better.

43

44 CHAPTER 6. CONCLUSIONS AND SELECTION GUIDELINE

TSS will be the choice in this case.

Although TSS outperforms TPSN in accuracy under most of the cases, TSS has a fundamental limitation. It only synchronizes the events' generation times to the local clock of the events' sink node. Consider the case that there are more than one sink nodes in the wireless sensor network. Since the clocks of different sink nodes are not synchronized, the generation times of events that go to different sink nodes are not synchronized. If the application requires events to be labeled using a global reference clock, TSS is not applicable.

Appendix A

Publication of Jr-ben Tian

Below is a list of publications that I have achieved in the study of master program:

1. Keng-hao Chang, Shih-yen Liu, Jr-ben Tian, Hao-hua Chu, Cheryl Chen, ”Dietary-Aware Dining Table - Tracking What and How Much You Eat”, in Proceedings of Workshop on Smart Object Systems, in conjunction with the Seventh Interna-tional Conference on Ubiquitous Computing (ACM UbiComp 2005), Tokyo, Japan, September 11, 2005, pages 61-68

1

2 APPENDIX A. PUBLICATION OF JR-BEN TIAN

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