[Tinyos-devel] Time Sync Requirements

[Philip Levis]

On Dec 19, 2007, at 2:08 PM, Michael Newman wrote:

> 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.

The question isn't what is desired, but rather, what's a reasonable  
minimum functionality that most hardware can easily achieve. Nano- 
second granularity is kind of difficult with an 8MHz oscillator.

Phil