[ntp:questions] frequency adjusting only

Unruh unruh-spam at physics.ubc.ca
Wed Apr 23 00:50:05 UTC 2008

hal-usenet at ip-64-139-1-69.sjc.megapath.net (Hal Murray) writes:

>>                          If you compensate for cable
>>delays you can probably get to within 50-100 usec.

>Do the arithmetic.  Cable delay is interesting for ns, not microseconds.

>The speed of light is 1 ft/ns.  That's in vacuum.  It's slower in cable.
>Inexpensive cable is about 1/2 c.  Good cable is faster than that.
>The conversion from miles to km is about right.  5 microseconds
>per mile (in vacuum) is 5 microseconds per km in cable.  So 100 meters
>would be 0.5 microsecond.

>So if we are discussing a handful of machines within ethernet/CAT5
>distances, you don't have to correct for cable lengths unless your
>target is better than a few microseconds.

Since the cable delay is symmetric, ntp will compensate anyway, no matter
how long the cable. Only if the cable is asymmetric( cable one way,
passenger pigeon the other) is there a problem.

>These are my opinions, not necessarily my employer's.  I hate spam.

More information about the questions mailing list