[ntp:questions] Isolated Network Drift Problem
david at ex.djwhome.demon.co.uk.invalid
Thu Nov 27 07:39:46 UTC 2008
Richard B. Gilbert wrote:
> "c" is not correct. "c" is the speed of light in vacuum! Electricity
> in copper is somewhat slower. 1 nanosecond per foot is a useful rule of
We are talking about a guided electromagnetic wave that is actually
almost entirely excluded from the copper.
> thumb! It's an approximation but a useful one. If you need something
> more exact, you might consult the manufacturer. Cat 5 cable consists of
> four twisted pairs with two turns to the inch. The physical and
A key point of the design of cat 5 is actually that the twist rates are
different for each pair.
> electrical lengths of the cable are slightly different!
The velocity factor is actually quite variable for both coax and twisted
pair in the baseband range of interest for timing. The limits for
twisted pair baseband transmisssion tend to be set by signal distortion
resulting from this, unless you apply equalisation.
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