[ntp:questions] Help appreciated

David Woolley david at ex.djwhome.demon.co.uk.invalid
Wed May 13 19:20:55 UTC 2009

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.

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