[ntp:questions] Re: NTP4 has 3 different time formats! Namly (32, 64, 128) bits wide

Brian Inglis Brian.Inglis at SystematicSW.Invalid
Mon Jul 17 18:05:38 UTC 2006


On Mon, 17 Jul 2006 00:42:44 -0400 in comp.protocols.time.ntp,
"Richard B. Gilbert" <rgilbert88 at comcast.net> 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

Brian.Inglis at CSi.com 	(Brian[dot]Inglis{at}SystematicSW[dot]ab[dot]ca)
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