[ntp:questions] Minimum no. of stratum 1 servers

Richard B. Gilbert rgilbert88 at comcast.net
Fri May 18 11:24:43 UTC 2007

LALL, Suki S wrote:
> We want to implement high integrity NTP for use within our organisation.
> For this reason our stratum 2 servers will poll more than one stratum 1
> server which will be synchronised to GPS via an aerial. What is what is
> the minimum number of stratum 1 servers we should be polling upstream in
> order to ensure that the full benefit of the clock selection and
> clustering algorithms within NTP is realised.  

The minimum number of servers is four.  Four servers are necessary and 
sufficient to protect against one bad server (false ticker).  Five 
servers provide protection against two false tickers.  Seven servers 
protects against three false tickers.  If you need more than seven, 
something is very wrong with the way you select servers!

Note that, in most parts of the world, it can be difficult to find that 
many good servers.  Most stratum one servers have time accurate to 
within a few milliseconds and many are within a microsecond or two.  The 
problem is the network that connects you to those servers!  The longer 
the network path, the poorer the quality of the delivered time.  For 
this reason, try to select servers that are close to you; e.g. with low 
values of delay.

Also note that stratum has nothing to do with accuracy!  Stratum simply 
counts the number of "middlemen" between your server and the atomic 
clock!  A system using an short wave receiver to get time from WWV is 
stratum one but the vagaries of HF radio propagation can introduce 
errors of many milliseconds!

More information about the questions mailing list