Image Processing Reference
In-Depth Information
5
Overview of Time Synchronization
Issues in Sensor Networks
.
Introduction .........................................
-
. DesignChallenges ...................................
-
.
FactorsInluencingTimeSynchronization ...........
-
. BasicsofTimeSynchronization ......................
-
.
Time Synchronization Protocols for Sensor
Networks.............................................
-
. Conclusions..........................................
-
Weilian Su
Naval Postgraduate School
References .................................................
-
5.1 Introduction
In the near future, small intelligent devices will be deployed in homes, plantations, oceans, rivers,
streets, and highways to monitor the environment []. Events such as target tracking, speed esti-
mating, and ocean current monitoring require the knowledge of time between sensor nodes that
detect the events. In addition, sensor nodes may have to time-stamp data packets for security rea-
sons. With a common view of time, voice and video data from different sensor nodes can be fused
and displayed in a meaningful way at the sink. Also, medium access scheme such as time division
multiple access (TDMA) requires the nodes to be synchronized, so the nodes can be turned off to save
energy.
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.
Combining with the criteria that sensor nodes have to be energy-efficient, low-cost, and small in
a multihop environment, 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 deep space or on an ocean floor.
When messages are exchanged using short-distance multihop broadcast, the software and medium
access time and the variation of the access time may contribute the most in time fluctuations and
differences in the path delays. Also, the time difference between sensor nodes may be significant over
time due to the drifting effect of the local clocks.
In this chapter, the backgrounds of time synchronization are provided to enable new develop-
ments or enhancements of timing techniques for the sensor networks. The design challenges and
factors influencing time synchronization are described in Sections . and ., respectively. In addi-
tion, the basics of time synchronization for sensor networks are explained in Section .. Afterwards,
different types of timing techniques are discussed in Section .. Lastly, the chapter is concluded in
Section ..
