[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Bob Camp kb8tq at n1k.org
Thu Feb 16 07:18:20 EST 2017


> On Feb 16, 2017, at 1:30 AM, Ruslan Nabioullin <rnabioullin at gmail.com> wrote:
> On 02/15/2017 01:17 PM, Chris Albertson wrote:
>> Why set up a dedicated NTP server if you only have two computers
>> that will use it?    Your server will be accurate to a few
>> microseconds but your two computers will only by good to a few
>> milliseconds because ethernet is not nearly as good as PPS.
> Well Ethernet can be *extremely* accurate if PTP is used (a whitepaper specifies <= 100 ns accuracy if the LAN is optimized for it).

PTP single shot over ethernet is not at the < 100 ns level, even with proper cards. In real world settings, the traffic level for sub
100 ns PTP can be pretty high. Some situations appear to require > 100K transactions per second. I’ve never seen anything quite that
extreme myself. 


> Well, the assumption here is that one would render this service available to the public, registering the server(s) with the NTP website and/or the NTP Pool Project; n.b. this is still possible for connections lacking a static IP address, by means of an IPv6 tunnel, available at no cost from at least one vendor.  Otherwise yes, by some perspectives it can be considered quite pointless and wasteful to operate dedicated servers, standards, receivers, etc. with no means of time transfer to customers.
> > NTP is almost zero load on the CPU and the best thing is the NTP
> > accuracy is not effected by CPU load  SO you can run other service
> > without degrading the NTP server.
> Well n.b. TVB's hardware PPS timestamping post.  Also WWV and CHU decoding by NTP's modules can be problematic, as well as the obvious case of the server being overloaded.  Finally note that based on others' experimentation, the motherboard's XO temperature is nontrivially-highly correlated with CPU load, so for better motherboard XO-based holdover performance, once must create an ersatz oven utilizing the CPU(s), by running them at full utilization (obviously with proper scheduling priority), so typically volunteer distributed computing project(s) such as BOINC (SETI at home, etc.), Folding at Home, etc.  Of course then power consumption becomes problematic.
> -Ruslan
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