David, Since motes A and B that receive the RadioCountToLeds message with counter value i at the same time, ideally, both A and B should return the same global timestamps of the message reception. You can, for every counter value i, take the difference of global timestamps by A and by B. You can use simple statistics (mean, variance) of these differences for a series of counter values to characterize the time synchronization error between motes A and B.
Janos On Mon, Nov 3, 2008 at 11:59 AM, Yi-Tao Wang <[EMAIL PROTECTED]> wrote: > Thanks for your help Janos. > > If I want the clock error between the root and another mote, would > that be the offset? Also, what's is are the timestamps in > milliseconds? > > David > > On Mon, Nov 3, 2008 at 8:22 AM, Janos Sallai <[EMAIL PROTECTED]> > wrote: > > David, > > > > The local timestamp is the time of reception of a RadioCounToLeds message > > according to the motes local clock, whereas the global timestamp is the > time > > of reception of a RadioCounToLeds message according to the global time in > > the network. When a mote is synchronized, they are typically different, > so > > what you're seeing is correct. > > > > Regarding the skew being zero: FTSP maintains two local state variables, > > skew and offset, which are used to translate local time to global time in > > the following way: > > > > global_time = (1 + skew) * local_time + offset; > > > > The test app print out skew*1000000, which, per the above definition, > should > > be pretty close to zero.The 32kHz crystal on the mote has a +-50ppm skew > > within the whole operating temperature range. If all the motes are > operating > > at the same temperature, the skew should be much less than that. I'm > seeing > > mostly zeros, and -1 here and there. As long as the global times for the > > same counter value agree when the network is synchronized, seeing zeros > > should not be a problem. > > > > Please note that, according to the readme file in tests/TestFtsp, a mote > > flag is synced if the synced flag is zero.The skew value is only > meaningful > > when the mote is synced (i.e. the synced flag is zero). > > > > Janos > > > > On Mon, Nov 3, 2008 at 9:55 AM, Yi-Tao Wang <[EMAIL PROTECTED]> wrote: > >> > >> Hi, > >> > >> I'm having some trouble interpreting the results of the testftsp > >> application on tinyos 2.x. The skew value is always 0 even when the > >> synched flag is false and global timestamp doesn't equal local > >> timestamp. Does anyone know why? > >> > >> Thanks, > >> > >> David > >> _______________________________________________ > >> Tinyos-help mailing list > >> [email protected] > >> > https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help > > > > >
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