[ntp:questions] Stratum 1 GPS Network Time Servers

Richard B. Gilbert rgilbert88 at comcast.net
Tue May 17 00:08:35 UTC 2011

On 5/12/2011 3:14 AM, Alan Decelis wrote:
> In the context of a requirement to set up a redundant configuration of
> internal (peered) GPS Network Time Servers that operate at Stratum 1, is
> there a best practice of the number of GPS Network Time Servers to set?
> Would it be risky to rely on two reliable (meaning rather renowned and
> expensive) GPS Network Time Servers each connected to a separate GPS antenna
> with a TCXO? We are talking of a configuration whereby the number of ntp
> requests to these GPS Network Time Servers will be low (10 to 20 nodes at
> Statum 2 with 10 to 20 requests every 5 to 15 minutes) but the context
> requires a high level of security (thus eliminating connections to the
> internet) and a moderate level of accuracy (millisecond).
> http://support.ntp.org/bin/view/Support/SelectingOffsiteNTPServers#Section_5.3.3,
> in line with how NTP functions, would suggest a minimum of four upstream
> servers. My understanding of this is that this applies to a scenario whereby
> external time sources are being used, for instance official time sources
> over the internet.  Does this also apply to the scenario specified
> above? Would I thus require to look at the option of having 4 GPS Network
> Time Servers at Stratum layer 1
> Thanks in advance

Four, five, and seven are the magic numbers.  These values allow for the 
failure of one, two, and three servers respectively.  Failure can mean 
no response or a server returning incorrect time.  It happens!  The last 
NTP Survey discovered a server that was returning the wrong year!  Most 
errors are not quite so gross.

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