[ntp:questions] The libntp resumee...

Uwe Klein uwe_klein_habertwedt at t-online.de
Tue Sep 9 19:08:57 UTC 2008


Unruh wrote:
> Uwe Klein <uwe_klein_habertwedt at t-online.de> writes:
> 
> 
>>Unruh wrote:
>>
>>>Yes, that is with reference to the road. Car three should thus completely
>>>ignore the other two cars and use his speedometer. 
>>>
>>>Ie, put up a GPS receiver with a PPS and use that as your time source, and
>>>ignore all the other ntp time sources, except perhaps as sanity checks (eg
>>>if you r speedometer breaks you should get to know about it by occasionally
>>>looking at the other cars)
> 
> 
>>A)One GPS to each box or
>>B) a single GPS with PPS line to all boxes?
> 
> 
> Whichever you want. Up to you.
> 
> 
>>A:
>>Doesn't that impact reliability?
> 
> 
>>You add the failure probability of a GPS-unit to each Box
>>where one failure will make the whole system fail.
> 
> 
> So, that is why ntp has backup servers. You have a single failure point
> anyway-- the network. It goes down, and nothing can get the time.


That actually is _three_ different scenarios.

time over the network:

network fails
	1: time
	2: the system as a whole

	failure of network infrastructure
	thus does not add to the probability of the complete system failing.

time over PPS/GPS 1 unit with signaling to each box:

network fails
	2: the system as a whole

GPS fails
	1: time
		-> the system as a whole

	This adds up to a higher failure rate/probability.

time over PPS/GPS unit per box:

network fails
	2: the system as a whole

GPS fails
	1: time
		-> the system as a whole

	This adds up to a higher failure rate/probability.

	With the added disadvantage that GPS failure overall
	is single failure times number of boxes.


uwe




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