[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Mike Cook michael.cook at sfr.fr
Wed Feb 15 03:42:40 EST 2017

>> Best practice today is to have two independent NTP servers and two GPS
>> receivers.   It is best if these are independent as you can make them,
>> different buildings if you can.   I would even use different brands of
>> hardware to protect against a bug.

This is an often missed essential, though it also applies to software. 

>>   Then throughout your company all your
>> PCs are configured to look at both NTP servers
>> Each server is configured to use the GPS reference clock, the other "twin"
>> NTP server as well as about five Internet "pool" servers.
>> If your location does not have an Internet connection. ( YES this can
>> happen.  I've worked on computers that process classified information and
>> these computers never have Internet access.)  You can configure them so
>> they run in "orphan mode" that is they all use each other as reference
>> clocks.  Then when GPS is lost thenoormal NTP clock selection algorithm
>> will select the subset of PCs that all agree on what the time is.   The
>> outliers tent to get ignored.    When GPS comes back up the system makes a
>> gradual and graceful recovery.
>> Chris Albertson
>> Redondo Beach, California
> That dual set model is new to me. Interesting to see its fall-back on failures. And the offline model.
> Thanks to all
> Michael

Since when has two clocks been of any use to anyone. It is false economy to think that you can get a way with just two upstream references at any stratum level just because they themselves have many sources. 
For each NTP node except the top stratum (1) you need at least 4 servers to enable the removal of any one for maintenance or allowing for failure and still allow NTPs selection algorithms to work correctly. 

"The power of accurate observation is commonly called cynicism by those who have not got it. ยป
George Bernard Shaw

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