hi, Yes, we also made this observation, although we could not pin point the exact reasons for it. I agree that it cannot be because of aging. It can be because of the variation in other ambient conditions such as humidity, pressure etc. Since we didn't log these things during the experiment, I can only be speculating out here. I would also like to point out that the granularity of temperature measurements is 1 deg. Thereby, this might also be because of quantization errors, i.e. the temperature might be changing during that interval although that change is within a degree. Note that clearly the variations in drift are not as drastic as compared to the scenarios, when the temperature actually changes by more than a degree.
The units are ppm (sorry for the typo). Also, note that what is mentioned in the datasheets (-20 to +20ppm) corresponds to the drift variation of a mote with the UTC, whereas this figure indicates the relative clock evaluation between two motes (for calculating the actual relative drift, rate, you should be taking a differential of this graph over time). bye, saurabh On Fri, 10 Mar 2006, Sharmistha Maitra wrote: > > Hi Saurabh, > > I have another question about the paper > http://nesl.ee.ucla.edu/document/show/153 , figure 11a drift and > temperature variation. > > We know - "The rate of change of drift is highly correlated with the > rate of change of temperature." > > But from 4 to 8 hours, the temperature is constant at 18 deg C, but I see > lot of drift variations during that period. Why this is so ? I know that > drift depends on certain other factors like normal aging of the crystal > and mechanical shock. But it is unlikely for a crystal to have much aging > effect within 4 hour duration...so what else is causing this variation ? > > My next query....drift in figure 11a has been specified as unitless > fugures. Are you considering drift as some kind of ratio ? What is that > ? I ask this becasue we generally specify drift as parts per million. > Datasheets specify drift due to temperature as 20 ppm, I cannot relate > your values of -4.428...-4.438 to common notion of drift. Please help me > with my doubts. > > Thanks a lot for your time, > > Smaitra. > > > > . > > ----- Original Message ----- > > From: Saurabh Ganeriwal <[EMAIL PROTECTED]> > > Date: Thursday, March 9, 2006 5:35 pm > > Subject: Re: [Tinyos-help] Mote Drift > > > > > hi Sharmistha, > > > > I think you are getting confused between the actual error between the > > mote's clocks and the prediction error as used in this paper. Let > > me take > > a stab at clearing out your doubts. Let us represent the clocks of two > > motes by c1(t) and c2(t). Here t represents the real time t, say UTC. > > > > The error between these two clocks at a given time t1 will be > > given by > > c1(t1) - c2(t1). This error can be attributed to many factors such as > > offset in starting the nodes, drift and skew over time. And the worst > > case / average case numbers (assuming no offset) is something that can > > be obtained by the datasheets. > > > > Now, what this paper does is maintain a relative clock model > > between these > > two clocks, some function G(.), i.e. applying G(.) to c1 gives you the > > expected time in c2. The prediction error, as defined in this > > paper and in > > Figure 6 corresponds to > > > > c2(t1) - G(c1(t1)). > > > > No, FTSP does not have that much amount of error. I think you are > > referring to the line in Introduction. By sampling at a lower sampling > > period, FTSP guarantees that the error can never exceed 90 > > microseconds.The actual error attained by FTSP will be definitely > > of much smaller > > magnitude. > > > > Please let me know if this is clear. I shall be glad to answer your > > further queries on the paper. > > > > bye, > > saurabh > > > > On Thu, 9 Mar 2006, Sharmistha Maitra wrote: > > > > > > > > Dear All, > > > > > > This is a question about the paper 'Estimating Clock Uncertainty for > > > Efficient Duty-Cyclicng in Sensor Networks', found at > > > http://nesl.ee.ucla.edu/document/show/153 > > > > > > The paper entions - 'time prediction error of a sensor node' > > increases> monotonically with 'Sampling Period of time synch > > messages' (figure 6 in > > > paper). I agree that it will increase monotonically, but how did the > > > authors arrive at this graph ? I mean how did they arrive at the > > > particular error values as shown in the graph ? Elsewhere in the > > paper> they have mentioned that FTSP (a quite popular protocol) > > with 1 minute > > > sampling period has prediction error of 90 us. But here in this > > graph I > > > see ~5us error for 1 minute sampling period. By what method has > > this been > > > calculated ? > > > > > > I dont know if this is the right forum to talk about the paper, I > > > apologise if it is not. Anybody who has read this paper, or the > > authors> themselves (if they are in the mailing list), if they can > > clarify , I > > > will appreciate. > > > > > > Thanks, > > > > > > Smaitra. > > > > > > ----- Original Message ----- > > > > > > From: Vinayak Naik <[EMAIL PROTECTED]> > > > > > > Date: Thursday, March 9, 2006 12:19 pm > > > > > > Subject: Re: [Tinyos-help] Mote Drift > > > > > > > A relevant paper. http://nesl.ee.ucla.edu/document/show/153 > > > > > > > > - Vinayak > > > > > > > > On 3/9/06, Sharmistha Maitra <[EMAIL PROTECTED]> wrote: > > > > > > > > > > Dear All, > > > > > > > > > > I am working on modelling the clock drift of a mote (mica2 ) > > > > caused by > > > > > environmental conditions like temperature. Can anybody guide me > > > > where can I > > > > > find any information on this, something like a relation of drift > > > > amount and > > > > > operating frequency ? > > > > > > > > > > My other question- In general we know that mote frequency can > > > > drift by 40 > > > > > ppm (natural drift, not due to temperature)....can anybody give > > > > me an idea > > > > > how frequently does this change happen....I mean in the absence > > > > of all > > > > > external phenomenon, approx how long will a mote hold on to its > > > > previous> frequency before 'naturally' jumping off to the next. > > > > All this because > > > > > current literatures suggest that short term stability of the > > > > motes are good. > > > > > What is the extent of this short term ? > > > > > > > > > > Thanks... > > > > > > > > > > Smaitra. > > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > > > Tinyos-help mailing list > > > > > [email protected] > > > > > https://mail.millennium.berkeley.edu/cgi- > > > > bin/mailman/listinfo/tinyos-help > > > > > > > > > > > > > > > > > > > > > > > > > _______________________________________________ Tinyos-help mailing list [email protected] https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
