On Mon, 17 Jul 2006 00:42:44 -0400 in comp.protocols.time.ntp,
"Richard B. Gilbert" <[EMAIL PROTECTED]> wrote:
>David L. Mills wrote:
>
>> Richard,
>>
>> I can't claim preconition, as the current NTP timestamp format was
>> invented in 1978 when nominal accuracies were in the 16-ms range.
>> However, the resolution limit of 232 picoseconds is likely to be
>> exceeded when the CPU clock rate approaches 4 GHz, which might not be
>> long off.
>>
>
>I suppose that even a 2 GHz machine could slice time into 500 picosecond
>increments. But I was thinking in terms of the ability to set a clock
>that accurately. There's no way that I can think of that it could be
>done over a network using today's technology. I'm seeing a ~4us delays
>on my 100 Mb full duplex LAN. I think that means I can't pass time from
> machine A to machine B over my LAN without an uncertainty of ~2us.
>The error is probably less than that but probably is the best we can say.
Close to state of the art on 10GbE is 450ns switching delay and 6ns/m
propagation delay; 232ps allows for ~40mm cable, with a theoretical
limit of ~70mm at c, so is likely to be adequate for any connection.
Internal resolution *may* require another byte, but current machine
architecture seems to be a limiting factor on instruction thruput near
4GHz, vendors are going multithreaded and multicore, and further
advances may require a new architecture or different technology.
>So you could get delta time measurments with 232 picosecond resolution
>but getting absolute time accurately with that precsion is not going to
>be easy.
The caesium standard time transitions are only about 110ps, so either
that's good enough for almost all purposes, or they need a newer
standard before NTP is likely to be affected.
--
Thanks. Take care, Brian Inglis Calgary, Alberta, Canada
[EMAIL PROTECTED] (Brian[dot]Inglis{at}SystematicSW[dot]ab[dot]ca)
fake address use address above to reply
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