[ntp:questions] Isolated Network Drift Problem

David Woolley 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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