[Tinyos-devel] Time Sync Requirements

[Michael Newman]

I think very fine grain time is critical. I would opt for microsecond
granularity and can make an argument for nanosecond granularity.

Consider communication protocols where nodes sleep and wake up at specific
times to communicate.

The accuracy of the time sync is equivalent to the cost in power for the
nodes to communicate. For the communication to succeed the listening node
must be on for plus and minus the skew in time between the nodes plus the
message time. The skew is usually much larger than the message time.

While it is possible for time sync to be accurate with coarse time
granularity it is very difficult. More straightforward implementations
require fine granularity in time. In straightforward implementations with
coarse time granularity the skew will be driven by the time granularity.

I have worked on frequency hopping protocols where these issues are
critical. It is my experience that listening for messages dominates the
power consumption and that time skew is a huge contributing factor.


A second reason for fine granularity time:

Nanosecond or sub-nanosecond granularity allows for timing of radio waves
and would be used for marking time in systems like GPS receivers.