Before starting our discussion in earnest, we will first define a set of metrics that we have found useful for characterizing time synchronization in sensor networks. In studying both methods and applications, we have found five metrics to be especially important:
The services provided by different time synchronization methods fall into many disparate points in this parameter space. All of them make tradeoffs--no single method is optimal along all axes.
To illustrate the use of these metrics, consider the time service provided by the U.S. Global Positioning System [Kap96] (described in more detail in Section 4). Consumer GPS receivers can synchronize nodes to a persistent-lifetime time standard that is Earth-wide in scope within a maximum of 200ns [MKMT99]. However, GPS units often can not be used (e.g., inside structures, underwater, during Mars exploration), can require several minutes of settling time. In some cases, GPS units might also be large, high-power and expensive compared to small sensors.
In contrast, consider a small group of nodes with short-range, low-power radios. If one node transmits a signal, the others can use that signal as a time reference--for example, to compare the times at which they recorded a sound. The synchronization provided by this simple ``pulse'' is local in scope and its error comes primarily from variable delays on the radio receivers and propagation delay of the radio waves. For a given error bound, the lifetime of the synchronization is also finite as the nodes' clocks will wander after the initial pulse. However, the pulse is fast and energy-efficient because it only requires the transmission of a single signal.