[ntp:questions] Re: a couple of questions

Richard B. Gilbert rgilbert88 at comcast.net
Thu Nov 4 16:32:11 UTC 2004

Folkert van Heusden wrote:

>I've got a couple of questions.
>1. what is the mtbf for a typical atomic clock? how often do these
>devices fail? 

>2. when one uses an atomic clock, should one still use 3
>of them because of fals tickers? or are atomic clocks reliable enough?
If you require the utmost reliability and accuracy, you should have at 
least three.  Most people, however, would consider it an extravagance!!  
Cesium standards are EXPENSIVE!!!!!!  If you buy three of the same make 
and model and place them in service at the same time, it is likely that 
they will all fail within a few months of each other!

>3. when using a pc as a kind of proxy, e.g.: it runs ntpd and connects
>to 4 upstream ntp-servers, and all clients connect to this one system.
>should one still preferably have 3 proxy-systems instead of 1? or only
>when the upstream link is likely to be broken in some cases? 
If all proxy systems use the same upstream servers, the only benefit to 
having more than one is the case where proxies fail.

>4. what is
>the mtbf of a typical gps receiver?

>5. if one has 3 time servers in an intranet, does it make sense to hook
>up a gps receiver to each of them, even if the gps receivers are
>installed at the same location on the roof of the building?
Two or more GPS receivers provide some protection against the failure of 
one of them.

>6. what is the mtbf of a typical dcf77 receiver?
>7. if one has 3 time servers in an intranet, does it make sense to hook
>up a dcf77 receiver to each of them, even if the dcf77 receivers are
>installed at the same location in the building? 
Two or more receivers protect against the failure of one.

>8. are there any documents describing best practices for setting up a time server for an
>intranet? i'm currently looking into an intranet with aprox 5000
>computers (windows/novell/linux/solaris) that need to sync themselves.

>8. about peering servers: should they peer if they use the same
>upstream-servers? or in any case?
I don't think there is any benefit.

You neglected to say what problem you are trying to solve (except in 
VERY general terms).   What degree of accuracy is required?  What degree 
of reliability is required?

I'd suggest  at least three servers, each with a hardware reference 
clock; perhaps one with a cesium or rubidium standard, one with a GPS 
receiver, and one with a DCF77 receiver.  If possible each server should 
have serveral internet sources either as peers or as clients; at least 
one source should be unique to each server.  These servers should peer 
with each other.  These servers will all operate at stratum 1 and should 
be dedicated to time service.

With five thousand clients, you may want to have some stratum 2 servers 
dedicated to serving clients.

The design will depend strongly on your requirements and the available 

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