[time-nuts] SLIP vs Ethernet for NTP

Magnus Danielson magnus at rubidium.dyndns.org
Sun Oct 23 14:09:29 EDT 2011


On 10/23/2011 07:15 PM, Chris Albertson wrote:
> On Sun, Oct 23, 2011 at 2:28 AM, Iain Young<iain at g7iii.net>  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).
>
>
> I think this is only true if the Ethernet is using a shared media.
> Like the old coax 10BaseT cables.   Modern system use switched
> 100BaseT that works much like a point to point link.   In fact if you
> can do point to point RS232 why not point to point 100BaseT.  Just use
> a crossover cable between two computers.

If you do a crossover cable and do the home-work in both ends on keeping 
timing and delays where they should be, then it should be a good 
solution. However, it is much easier to break it by myriads of reasons 
than RS-232. Point-to-point and care in packet timing and you have a 
good stable solution.

> The "best" why to distribute time over an area that is small enough to
> run cable is to distribute PPS.   Most cat-5 cable has unused pairs,
> makes an easy way to send PPS.

You can lock up the 25 MHz oscillator and provide PPS and clock. Sync-E 
locks up the baud-rate like that, but doesn't provide any solution for 
time (it is not intended to).

There is also the AES packet solution which distributes the word-clock 
on the remaining pairs such that audio over wordclock + Fast Ethernet 
comes in the same connector. Cool solution which nobody seems to be using.

Cheers,
Magnus




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