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requirement that cripples GPS's use for sensor networks dispersed within a building or in a
heavily forested area. On the other hand, NTP is one of the first synchronization protocols
used for computer systems, first developed in 1985 (NTP, 2009). This protocol uses a
relatively large amount of memory to store data for synchronization sources, authentication
codes, monitoring options, and access options. As mentioned earlier, typical wireless sensor
nodes have limited onboard memory. A large sensor network will require large files for
synchronization sources and codes. If these configuration files can be programmed into each
node, it would leave very little memory to hold the data monitored by the sensor, limiting
NTP's use for WSNs. Furthermore, NTP's synchronization accuracy is within 10 ms over the
Internet, and up to 200 ps in a LAN (NTP, 2009); these specifications are inadequate for most
sensor network applications. Therefore, new synchronization methods have been developed
specifically for sensor networks, such as the reference broadcast synchronization method
(RBS) (Elson et al., 2002) and the timing-sync protocol for sensor networks (TPSN)
(Ganeriwal, November 2003), (Ganeriwal, 2003).
RBS and TPSN achieve accurate clock synchronization within a few microseconds of
uncertainty nonetheless both are designed for networks with a small number of sensors and
are not specifically geared towards energy conservation. Although these algorithms tend to
work for larger networks, their energy consumption becomes inefficient and network
connectivity is broken once nodes begin lacking power. Simulations show that
synchronizing a large sensor network requires a large number of transmissions, which will
quickly deplete sensors and reduce the network's coverage area.
A time synchronization scheme for wireless sensor networks that aims to save sensor
battery power while maintaining network connectivity for as long as possible is presented
based on a hybrid algortihm that combines both TPSN and RBS.
This algorithm is an extension of our previous work presented in (Akl & Saravanos, 2007). It
focuses on the following aspects of WSNs:
1. Design a hybrid method between RBS and TPSN to reduce the number of
transmissions required to synchronize an entire network.
2. Extend single-hop synchronization methods to operate in large multi-hop
networks.
3. Verify that the hybrid method operates as desired by simulating against RBS and
TPSN.
4. Maintain network connectivity and coverage.
2. Time Synchronization Algorithms in WSNs
Traditional synchronization methods, that are effective for computer networks, are
ineffective in sensor networks. New synchronization algorithms specifically designed for
wireless sensor networks have been developed and can be used for several applications
(Sivirkaya & Yener, 2004). The authors in (Palchaudhuri et al., 2004) present a probabilistic
method for clock synchronization based on RBS. In (Sun et al., 2006), the authors present a
level-based and a diffusion-based clock synchronization that is resilient to some source
nodes. The authors in (He & Kuo, 2006) propose creating spanning trees with multiple
subtrees in which two subtree synchronization algorithms can be performed. Four methods
are described in (Qun & Rus, 2006) to achieve global synchronization: a node-based, a
hierarchal cluster-based, a diffusion-based, and a fault-tolerant based approach. An Efficient
