[ntp:questions] Help appreciated
towli at oldboyz.dk
Wed May 13 19:30:33 UTC 2009
"David Woolley" <david at ex.djwhome.demon.co.uk.invalid> skrev i meddelelsen
news:4a0b1dab$0$518$5a6aecb4 at news.aaisp.net.uk...
> Towli wrote:
>> I am really trying to understand this, and i feel a little
>> stupid, -perhaps i dont get it because english is not my native language:
>> Why is my 3750 switch reporting a precison of 2**-6 when using my
>> appliance DCF77 box, but reporting precison of 2**-18 when talking about
>> the external Gps-servers.
> Because the DCF77 box can only read its software clock to the nearest 10
> to 20ms, whereas the external GPS server is presumably running on a recent
> Unix or Linux system and can interpolate its software clock reading to
> better than the nearest 5 micro-seconds.
> I don't remember your providing any output that would indicate the
> precision reported by the router, but it may well be at the poor end.
>> If i erase the ntp clock-period from my switches when they are in synch
>> with my appliance box - would the precision improve ?
> The only way of improving it is to use a better radio clock box. It will
> be fundamental to the software on that box, which might also be
> constrained by the hardware, if it has no means of reading the residual
> timer count, or otherwise interpolating.
> I'm not sure that precision is a problem here. The basic problem is that
> you have a fast connection to GPS clocks and GPS has an intrinsic error of
> about 50 nano-seconds, and a good GPS based time server can achieve about
> 1 microsecond, with no time of flight considerations, whereas as a slow
> code DCF77 receiver may have difficulty getting the time to better than a
> few milliseconds, and will require correcting for the radio propagation
> delay if you want high accuracy.
There you go...Or there i go...Now i get it! Thank you so much ! ! !
Im getting shivers down my spine, because of people like you guys in here
who are prepared to help out newbies like myself, without anything but
appreciation in return.
Thank you !
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