[ntp:questions] How bad is USB?

Richard B. Gilbert rgilbert88 at comcast.net
Fri May 1 12:57:02 UTC 2009


David J Taylor wrote:
> Garrett Wollman wrote:
>> In article <K9WdnS4y5pIuymfUnZ2dnUVZ_t2dnZ2d at giganews.com>,
>> Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
>>
>>> USB is nearly useless for NTP!  USB has latencies sufficiently large
>>> and variable to render it unsuitable for use with NTP.
> []
> 
> Garrett, Richard,
> 
> You've both commented that USB has drawbacks, but in reality what 
> performance might be obtained?  Not everyone needs microsecond 
> precision, and USB might allow millisecond precision - i.e. possibly 
> better than what might be obtained by Internet access alone, or by using 
> a radio source.
> 
> Has anyone made any actual tests or measurements?  Are there any results 
> available from an actual installation of USB on either Windows or UNIX?
> 
> Thanks,
> David

You can try it and see what happens!  The results may be sufficiently 
good for your purposes.  You will almost certainly not get microsecond 
accuracy.  If you are willing to settle for +/- 10 milliseconds you can 
almost certainly get that.

There are (at least) two groups of people who hang out here:
a. The "chimeheads" in pursuit of the "one true time", and
b. The "synchs" who don't really care what time it is as long as all 
their machines have the SAME time.

Both groups use the same tool, NTPD.  It turns out that having a stable 
and accurate source of time makes it much easier to get a whole herd of 
machines to agree on what time it is.




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