[ntp:questions] Befuddled and confused, again...

David Lord snews at lordynet.org
Mon Feb 15 22:50:09 UTC 2010


unruh wrote:
> On 2010-02-15, Dave Baxter <spam at goes.nowhere.com> wrote:
>> On 2010-02-14, Dave Baxter <spam at goes.nowhere.com> wrote:
>>> [quoted text muted]
>> GPSD as in Global Positioning System Diciplined.
> 
> No idea what gpsd stands for. It is program used to read the signals
> from a gps receiver and deliver them to ntpd via the shm refclock. 
> 
>> Yes, it uses the DCD line (I think) to input the PPS signal from the GPS 
>> receiver, a Garmin GPS16LVS in this case.  It's all configured and 
>> working on a Win2k box, with the Meinberg port, but I want that physical 
>> machine for something else, so I'm trying to get the FreeBSD version 
>> going.


gpsd = interface daemon for GPS receivers.
<http://www.freebsdsoftware.org/astro/gpsd.html>

NetBSD pkgsrc version is working ok here on NetBSD-5 but
I've not yet tested if PPS support is working.

That should give you 10ms at worst and < 10us if PPS is
working, you can also use NMEA with pps support or NMEA + ATOM
to get offsets <10us.

> 
> Now, I am not sure whether gpsd will run on bsd, but I suspect it should
> not be hard to convert it to do so. The ioctl calls to the serial port
> or parallel port may well be different, but there are very few of them
> in the code. 
> 
> 
>> NTP timekeeping via the internet is not reliable at the mS level, I 
>> suffer time of day related WAN congestion, sometimes the end to end 
>> latnecy can be over 1.5 seconds, but no packet loss, so the ISP says 
>> that's good!   Hence my need for a local NTP source, as 'net based is 
>> not a viable option for me at this time.
> 
> WOW. What kind of a wan is that? 
> 

This is the UK, we're nearly back to tin cans and lengths of string.

Systems seem heavily overloaded.

My 3G mobile broadband has seconds of latency whilst initiating
connections, that is how the system works, maybe 10 seconds, but
whilst traffic is flowing the latency can drop to around 150ms.
That's why I'm using "minpoll 11 burst" against my server at
home and that gets ntpd to within 10ms or so.

My adsl broadband has around 17ms latency and checked vs GPS,
ntpd using internet servers gets within 2ms. Sustained heavy
downloads have recently been causing massive latency spikes.
Up to end of last year they were rare, now they are a feature
and can start ntpd into oscillation.

David




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