[ntp:questions] Time server question

William Unruh unruh at invalid.ca
Thu Jun 27 05:20:56 UTC 2019


On 2019-06-25, Chris <xxx.syseng.yyy at gfsys.co.uk> wrote:
> On 06/25/19 19:35, William Unruh wrote:
>> On 2019-06-25, Chris<xxx.syseng.yyy at gfsys.co.uk>  wrote:
>> ...
>>>
>>> Thanks again for the replies. Did a bit of digging this morning and
>>> find that the 1pps sync stuff has been done before. Well, many
>>> years ago in fact and more or less how I had visualised it - ntp
>>> data augmented by the 1 pps signal. Several pointers to the way
>>> forward and it's also supported in the FreeBSD kernel, using either a
>>> pin on a serial or parallel port for the 1 pps. Probably go for
>>> the parallel port, as that avoids the hardware to convert 1 pps
>>> ttl to rs232 levels.
>>
>> Except most serial ports actually used on computers handle ttl levels
>> just fine.
>>
>
> True, but would not rely on that sort of hack for anything serious,
> as it won't have good noise immunity for what is a nS scale timing
> signal.  The 1pps from the gps is at ttl or cmos level on a different
> piece of kit in another box. A diff line driver and receiver might
> be the best way, though that would need a bit more hardware. Devil
> is in the detail as usual, but just need to get it working to start
> with.

Not clear what you mean. Note that a driver would introduce its own
delays, and line termination also has  effect.

>
>
>>
>> Stratum 1 says nothing about accuracy. You could have stratum 1 come off
>> smoke signals and have an accuracy of minutes, and a stratum 7 have an
>> accuracy of usec. All it indicates is how many steps the time server is
>> from a hardware time source. It says nothing about how good that time
>> source is, or how good the connection is between servers.
>
> The docs i've looked at seem to say that stratum 1 is generally
> assumed to be caesium or gps, though the number of links would affect

It is basically any hardware. It could be WWV radio signals. Yes most of
the time it is probably gps, but then you do not know how long the line
from the gps receiver to the computer is,or how rapidly the computer
responds to interrupts, etc. Ie, stratum 1 does not imply accuracy.

> it. Just trying work it out so what appears on the net is as good
> as the standard it refers to...
>
> Chris
>
>



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