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I'm currently thinking about the various sources that influence time when it is synchronized with the NTP protocol. One of them is the physical layer (OSI-model).

When sending a bit over a cable, what influences the variance in transmission time?

Please note that I am NOT interested in the latency, but the variance. So what makes the time the transmission needs change?

I guess for metal cables it is mostly differences in temperature. Is there anything for a given glass fiber cable that changes the signals need to be transfered over it? Are there any aging processes that change the time?

What about WLAN signals? On which order of magnitude does the time from sending and receiving differ, depending on conditions of the environment (air temperature, maybe increased error rates when many devices are sending?)

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Here I am discussing some of the other sources of variance you have mentioned. Since the propagating medium is essentially linear for all practical intensities any variance of travel time is due to the finite bandwidth of the transmitting and receiving equipment and the definition of what that the travel time actually means. (A nonlinear amplifier will contribute to delay variance, but let us ignore this.) In other words, you have to define when, say, a pulse starts and end; this is, of course, unambiguous for an ideal rectangular pulse but it is not so for a pulse that has gone through a finite bandwidth filter or channel. You may define any way you wish so for a single pulse but then a random signal made out of such pulses stacked one after another will behave randomly so the rise and fall times will vary form sequence to sequence, buzzword: intersymbol interference. Temperature, humidity, aging will affect the zeros and poles of all real filters made of capacitors, inductors, cavities, etc., hence the rise and fall times and their distributions, and directly its group delay as well.

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