[time-nuts] SLIP vs Ethernet for NTP

Kasper Pedersen time-nuts at kasperkp.dk
Sun Oct 23 11:27:37 UTC 2011

On 10/23/2011 11:28 AM, Iain Young wrote:

> Hi Guys,
> I have often heard it said that since RS-232 is more "deterministic",
> and suffers from less jitter, and uncertainties, than ethernet, that
> it makes a better medium for time distribution (no CDMA for a start).

Old HDX ethernet uses cSma-cd. On what you are likely to use (junkbox
100Mbit switch and up) this is not a problem.

I once built a 'frame generator' to send precisely&accurately timed
ethernet frames, and did a bit of testing on this:

In short, when I disabled interrupt coalescing on a Pro/1000, the jitter
was better than the jitter from the same signal through the DCD line of
a serial port (1us vs 4us). Also the packet timestamp on the nic was
~5us faster than the DCD one, likely because the RS232 level translator
is pretty slow.

Beware of ARP on ethernet. Either poll quickly enough that the entry
stays cached, or set them static on both ends.

And beware of windows boxes. I did the same experiment across work's

The right answer is obviously to connect them using both, and compare.

/Kasper Pedersen

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