[ntp:questions] 500ppm - is it too small?

Joseph Gwinn joegwinn at comcast.net
Thu Nov 12 13:12:20 UTC 2009

In article <87r5s3syxz.fsf at pc9454.klinik.uni-regensburg.de>,
 Ulrich Windl <Ulrich.Windl at RZ.Uni-Regensburg.DE> wrote:

> "nemo_outis" <abc at xyz.com> writes:
> > "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in
> > news:TLSdnQ2E26bBLBnXnZ2dnUVZ_sydnZ2d at giganews.com: 
> >
> >> nemo_outis wrote:
> > ...
> >>> I fail to see the value or relevance of "500ppm satisfies 98% of
> >>> computer clocks" if some other number, perhaps 5000 ppm, could
> >>> satisfy yet even more than 98% of computer clocks with no downside -
> >>> as indeed seems to be the case!  Chrony, whatever its other merits
> >>> and demerits, is an "existence proof" for this proposition.
> >
> >
> >> I can't follow Dave's math but I'm reasonably sure that there is a
> >> good reason for the 500 PPM limit.  Since almost all computer clocks
> >> can meet this criterion I'm not going to worry about it.
> >
> > Hmm, "faith-based" ntp?  Not for me.  If there is a "good reason" I'd 
> > like to hear it - 500 ppm has the smell of arbitrariness about it.
> As arbitrary as there are 8 bits in a byte.

No, 8 bits isn't arbitrary.  

Computer hardware is simplified if the various word lengths are all 
powers of two.

Eight bits was the smallest power-of-two size that allowed the full  
Roman alphabet including punctuation and control characters to be coded.

There are 5, 6, and 7 bit codes, all now obsolete:

Five-bit: Baudot, used in teletypes.

Six-bit:  Fieldata (Univac and Control Data, and others I assume.)

Seven-bit:  ASCII without parity bit.

Eight bit:  ASCII with parity bit, and EBCDIC 

ASCII came from AT&T, while EBCDIC came from IBM.

And now sixteen bit: Unicode.  

Joe Gwinn

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