Re: [Linuxptp-users] Want to reduce offset (which is distance dependant) between boundary clocks.

[Richard C. <ric...@gm...>]

On Fri, Jan 15, 2016 at 03:51:57PM +0530, aniket wrote:
> In my first experiment,  master and slave are 2 Km away from each other. 
> In this case I got offset at slave PHY clock around 50 nano second.
> 
> In second experiment I relocate my slave around 20 Km from master where 
> I got offset of 150 nano seconds.

So you are checking against GPS, right?

> Ideally the offset between PHY clocks should remain constant though we 
> are varying distance between them (correct me if I am wrong).

If the system is properly calibrated, then the offset should be
minimal, and the difference in distance should be reflected in the
path delay.

> Do we need to specify any parameter (which is distance dependant) while 
> issuing above command? Because there are many PROGRAM AND CLOCK OPTIONS 
> in ptp4l. Do we need to set any specific option?

We do have options to calibrate the system:

 delayAsymmetry - corrects for any asymmetry between the Rx and Tx paths
 egressLatency  - corrects the transmit latency at the MAC/PHY
 ingressLatency - corrects the receive latency at the MAC/PHY

What you observe sounds like an asymmetry.  If it is, then it is
surprising to me that the asymmetry grows with distance.  Then again,
I don't know anything about your equipment, and so maybe this is
simply a property of your fibre optic links.

HTH,
Richard