Re: [Linuxptp-users] Interpretation of ptp4l results

[Chandra M. <sma...@al...>]

Hi Miroslav et al,

Perhaps I am not clear about my question - just would like to understand the mathematical interpretation of the following for the offset and frequency corrections. For example, in the following output,

Frequency correction: Average frequency deviation observed is 156403 ppb with the maximum deviation of 7 pbb.

Offset correction: the 'rms' (root mean squre) is the mean offset of all the offsets in the sample observed in the interval and 'max' is the maximum deviation observed from the rms value.


However, you mention that 'max' for the offset is simply the maximum offset observed in the sample. In that case, do frequency correction and offset correction values differ in their definitions (with 7ns in the offset defining the max offset in the sample whereas 7 pbb in the frequency showing the maximum deviation from the mean)?


ptp4l[833.369]: rms    2 max    7 freq -156403 +/-   7 delay    28 +/-   0

Thanking you in anticipation,
Regards,
Chandra

(c) : +60.175508142
(O): +60.4.636.6412

"Knowledge speaks, Wisdom listens"


-----Original Message-----
From: Chandra Mallela
Sent: Saturday, June 20, 2015 12:41 AM
To: 'Miroslav Lichvar'
Cc: lin...@li...
Subject: RE: [Linuxptp-users] Interpretation of ptp4l results



Hi Miroslav,

My question is about mathematical interpretation of the output from the ptp4l stack. If I see the output like the one below, is it fair to assume that the 'rms' (root mean squre) is the mean offset of all the sample offsets observed in the interval and 'max' is the maximum deviation observed from the rms value? I am likening it to kinda gaussian/normal distribution but without sign interpretation. Otherwise, I am fully in line with what you have mentioned below.

ptp4l[833.369]: rms    2 max    7 freq -156403 +/-   7 delay    28 +/-   0

Thanking you in anticipation,
Regards,
Chandra

(c) : +60.175508142
(O): +60.4.636.6412

"Knowledge speaks, Wisdom listens"



-----Original Message-----
From: Miroslav Lichvar [mailto:mli...@re...]
Sent: Friday, June 19, 2015 4:23 PM
To: Chandra Mallela
Cc: lin...@li...
Subject: Re: [Linuxptp-users] Interpretation of ptp4l results

On Fri, Jun 19, 2015 at 07:53:52AM +0000, Chandra Mallela wrote:
>
>
> Hi Friends,
>
> I would like to double-check whether my following interpretation is correct. Please elucidate me.
>
> For the following ptp4l output, the ptp4l reports rms value and the max deviation from the mean offset. Thus, if I have to report the worstcase accuracy, I take the (rms+max). For example, , looking at the first line, I see that the maximum offset uncertainty/inaccuracy  observed is 2 +7 = 9ns. The maximum frequency uncertainty is (156403 + 7) pbb.

The max value is just the maximum measured offset in that interval. It doesn't mean it's also the worst accuracy of the clock. There could be an asymmetry in the network delay and ptp4l wouldn't know the clock is actually behind or ahead of the time source. To evaluate the accuracy you would to measure the error independendly from PTP, for example by comparing PPS signal generated by the synchronized clock with the reference on a scope.

--
Miroslav Lichvar

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