Time-Synchronization Challenges and Techniques

Abstract

The advancement of Micro Electro-Mechanical Systems technology, wireless communications, and digital electronics has enabled the development of lowcost, low-power, multifunctional sensor nodes that are small in size and communicate untethered in short distances. The sensor nodes may be used in military, environmental, and commercial applications [1]. Each of these applications may require a different level of clock precision among the sensor nodes. For example, the clock precision required for security time-stamping and target tracking may be different. In addition, sensor nodes with a common view of time may fuse and display voice and video data in a meaningful way. Also, they may be able to use Time Division Multiple Access (TDMA) to access the channel and turn off the radio whenever possible to save energy. Thus, time synchronization is needed for different types of services that require coordination and precision. If services do not require time, then time synchronization may be an overhead for the sensor networks. Since some applications and protocols need time synchronization, this chapter explores the design challenges, factors influencing time precision, and the state-of-the-art in time synchronization for wireless sensor networks. The purpose of any time synchronization technique is to maintain a similar time within a certain tolerance throughout the lifetime of the network or among a specific set of nodes in the network. To achieve the purpose, the time synchronization protocol has to take into account the energy as well as the low-end nature of the sensor nodes in a multi-hop environment. Hence, this requirement makes a challenging problem to solve. In addition, the sensor nodes may be left unattended for a long period of time, e.g., in a cave or on an ocean floor. When messages are exchanged using