[ntp:questions] Getting precision when server side only offersseconds... Ideas?

Jason Rabel jason at extremeoverclocking.com
Mon Feb 25 15:26:35 UTC 2008


Soleth,

> Explaining it better, it is a kind of network switch for which I don't
> have total access,
> and can only talk through a protocol, that offers me a date in format
> YYYY-MM-DD hh:mm:ss. Period. 

Does the server auto-output the info "on-time" every second, or only
dynamically reply when requested by the client? Some hardware that I have
seen has an option to start an auto-output where each line of time is
printed within so many ms of the zero mark.

> Am I right and is it impossible to get the desired precision or does
> anyone have
> any idea of how could I accomplish this? Is it possible to perhaps
> "guess" the
> millisecond "behind" the seconds I receive? The satellite delay is
> something on
> the order of 700-1200 ms.

With the high latency of a satellite link it might be hard to pound it into
submission (querying multiple times per second until the second changes).

You can account for the bulk of the delay just in the round-trip time. If
you wanted to dig deeper you could figure out the distance to the satellite
and the delay caused by that, then delay caused by the hardware, etc... 

At the same time you could be querying approx once a second (either more or
less often depending on latency) and as the second changes you can slowly
converge on the correct time by adjusting your query time to hover around
the time-change.

However, from what people tell me with satellite links, they are expensive
and usually people end up with less available bandwidth than what they need
/ want (because of the cost), so as a negative side-effect the link is
constantly saturated. That might make getting any sort of time a major pain
in the rear.


Jason




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