tailieunhanh - Smart Wireless Sensor Networks Part 10

Tham khảo tài liệu 'smart wireless sensor networks part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Time Synchronization in Wireless Sensor Networks 259 asymmetric delay is bounded by one-half the round trip time between the two nodes Levine 1999 . Clock glitches Clock glitches are sudden jumps in time. This may be caused by hardware or software anomalies such as frequency and time steps. Besides dealing with these factors a time synchronization protocol for sensor networks should be automatically self-configured and be sensitive to energy requirement. Synchronization Problems in WSNs Network time protocol NTP Mills 1991 has been widely used in the Internet for decades. The NTP clients synchronize their clocks to the NTP time servers with accuracy in the order of milliseconds by statistical analysis of the round-trip time. The time servers are synchronized by external time sources typically using GPS. The NTP has been widely deployed and proved to be effective secure and robust in the internet. However traditional synchronization schemes and GPS-equipped systems are not suitable for use in WSNs due to the specific requirements of those networks Yoon et al. 2007 Precision WSNs may require much higher precision than traditional networks depending on the deployed applications. For example a precision of a few milliseconds is satisfactory for NTP in the Internet while microsecond precision may be required in order to significantly improve the performance of the WSN beam-forming application. Cost Nodes in WSNs typically have limited batteries computational resources and storage capacity. However most of the protocols designed for wired environments need to exchange many messages and also store them for statistical processing. The problem in a modern sensor network scenario is that nodes can only communicate locally to their neighbors. The localized communication makes the problem much harder in that 1 a valid consensus has to be computed locally and 2 the local consensus must be conveyed to other parts of the network this is even harder because the relay nodes may

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