Recent advances in miniaturization and low-cost, low-power design have led to active research in large-scale, highly distributed systems of small, wireless, low-power, unattended sensors and actuators [1,7,4]. The vision of many researchers is to create sensor-rich ``smart environments'' through planned or ad-hoc deployment of thousands of sensors, each with a short-range wireless communications channel, and capable of detecting ambient conditions such as temperature, movement, sound, light, or the presence of certain objects.
Time synchronization is a critical piece of infrastructure for any distributed system. Distributed, wireless sensor networks make particularly extensive use of synchronized time: for example, to integrate a time-series of proximity detections into a velocity estimate [3]; to measure the time-of-flight of sound for localizing its source [5]; to distribute a beamforming array [13]; or to suppress redundant messages by recognizing that they describe duplicate detections of the same event by different sensors [6]. Sensor networks also have many of the same requirements as traditional distributed systems: accurate timestamps are often needed in cryptographic schemes, to coordinate events scheduled in the future, for ordering logged events during system debugging, and so forth.
The broad nature of sensor network applications leads to timing requirements whose scope, lifetime, and precision differ from traditional systems. In addition, many nodes in the emerging sensor systems will be untethered and therefore have small energy reserves. All communication--even passive listening--will have a significant effect on those reserves. Time synchronization methods for sensor networks must therefore also be mindful of the time and energy that they consume.
In this paper, we argue that the heterogeneity of requirements across sensor network applications, the need for energy-efficiency and other constraints not found in conventional distributed systems, and even the variety of hardware on which sensor networks will be deployed, make current synchronization schemes inadequate to the task. In sensor networks, existing schemes will need to be extended and combined in new ways in order to provide service that meets the needs of applications with the minimum possible energy expenditures.
In this framework, we present our idea for post-facto
synchronization, an extremely low-power method of synchronizing
clocks in a local area when accurate timestamps are needed for
specific events. We also present an experiment that suggests this
multi-modal scheme is capable of precision on the order of
sec--an order of magnitude better than either of the two
modes of which it is composed. These results are encouraging,
although still preliminary and performed under idealized laboratory
conditions.
In Section 2, we present a number of metrics that can be used to classify both the types of service provided by synchronization methods and the requirements of applications that use those methods. Section 3 describes our post-facto synchronization idea and presents an experiment that characterizes its performance. Future work is described in Section 4, and our conclusions are drawn in Section 5.