[time-nuts] SLIP vs Ethernet for NTP

Dale J. Robertson dale at nap-us.com
Sun Oct 23 06:41:42 EDT 2011

Just off-hand I would think that at best serial might perform as well as 
ethernet, But could be (much) worse.
First off, modern ethernet implementations are not really CDMA. they run 
full duplex into buffered switches.
I would think that serialization delay and packet processing variations 
at serial line speeds could be worse than equivalent delays at 100 or 
1000 Mbit. I think also that the implementation of multiple slip or ppp 
terminations on the single tcp/ip stack of your S2 machine might 
introduce lots of packet process ordering jitter etc. It has been many 
years since this was a mainstream application on a host. So who knows 
what shape the OS's are in in regards to real-time performance on serial 
line pools.
Having said all that, Assuming you have the serial line hardware 
available. It would certainly be an interesting experiment!
Dale J. Robertson

On 10/23/2011 5:28 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).
> (Especially if you know how many bits you need to squirt over the
> RS-232 link at what baud rate)
> Since I am shortly going to have quite a few Srata 1 time sources,
> my first thought of just plugging them all into a switch, and then
> having an S2 box sat on the LAN, and distribute from there.
> And remembering the rules about RS-232 etc, the (possibly mad) thought
> of running SLIP or PPP over dedicated serial lines to link the Strata 1
> boxes to the Strata 2 machine.
> (I can't connect all the Strata 0 devices to the same Strata 1 box,
> as there will be other processing being done on the GPS data using
> RTKLIB on a couple of them)
> To be honest, this is for nothing more than fun, and nothing critical
> (this is time *nuts* after all), although it may well mean I have less
> copper Ethernet hanging around working like tuned antennas, *and*
> wouldn't need to go fibre on quite so many machines!
> What are people's opinion and experience of doing such a thing ?
> Best Regards
> Iain
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