Hi,
I agree with Dale that ppb is dimensionless and it is a ratio.
So 1 ns is 1 ppb of a second.
Meanwhile, as far as I know, ppb and ppm are mainly used to indicate the
stability of an oscillator.
When we talk about ppb / ppm, it might be more related to the frequency.
Best regards,
Hao
On 30 August 2015 at 07:55, 林志剛 <[email protected]> wrote:
> so, the unit of master offset is nanosecond?how to convert to ppb?
> 2015/8/30 上午2:10於 "Richard Cochran" <[email protected]>寫道：
>
>> On Sat, Aug 29, 2015 at 06:09:56PM +0100, Guo Hao wrote:
>> > ptp4l[7726.711]: master offset -8 s2 freq +3835 path delay
>> > 825
>> >
>> > Does this mean the clock offset between the 82574 NIC and the Master
>> Clock
>> > is in the range less than 100 ns?
>>
>> Yes.
>>
>> > Does that mean the system time is synchronized to the ptp hardware
>> clock on
>> > the 82574 NIC?
>>
>> Yes, but remember that there is probably some offset in the low
>> microseconds range. The "delay" tells you that reading the PHC time
>> over the PCIe bus takes about 13 microseconds.
>>
>> > However I cannot run ptp4l and phc2sys as a daemon process under Ubuntu:
>> >
>> > root@Hao-Ubuntu:~# service ptp4l start
>> > ptp4l: unrecognized service
>> > root@Hao-Ubuntu:~#
>> >
>> > Any idea on this?
>>
>> Just start the programs by hand. To start them automatically, one
>> easy way is to put the commands into /etc/rc.local.
>>
>> > Therefore, I doubt whether there is a inherent delay between the point
>> when
>> > the Ethernet capture card request for the ToD and point the Ethernet
>> > capture card really gets the information or the system time is not
>> > synchronized by the hardware clock of the 82574 NIC.
>>
>> But you said,
>>
>> the Ethernet capture card gets the 1-PPS from the same GPS clock.
>>
>> so the ToD from the host is irrelevant. The capture has the GPS's
>> 1-PPS, and so the phase of its time stamps should be correct to a few
>> hundred nanoseconds.
>>
>> The ~1 usec difference is probably just the sum of the GPS device's
>> egress latency and the capture card's ingress latency. The number is
>> perfectly reasonable, if one or both of the devices has MAC time
>> stamping.
>>
>> HTH,
>> Richard
>>
>>
>>
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