On 23 Apr 98 at 9:49, Roy Schriftman, MS, MBA wrote:

> CRC works as follows:
>
> I send data 852963741, the sum of the digits is 45.  The modem software
> will transmit 8529637415, which is an EXTRA digit.  The software at the
> receiving end will sum the first nine digits in the block and compare the
> ones digit to the last digit received.  If things don't match,  you have a
> CRC error and the block is retransmitted.

  In principle, this is a reasonable illustration.  In practice, the
CRC calculation is somewhat more complex.  Consider, for instance,
that if the host sends 825963415, the procedure given above still
checks out.  But a real CRC will come up with a different value,
because I switched the order of the second and third digits.
  In the particular case which sparked this thread, CRC failures are
probably occurring as a side-effect of the missing data reported as
Overrun errors.  i.e., fix the Overruns, and the CRC errors should go
away.

  [CRC is interesting in a couple of other respects, too.  It's
abstract definition involves a large polynomial that looks like a lot
of work, but if you have RAM/ROM for a couple of 256-byte tables the
CPU effort becomes negligible, making it feasible to build this into
a variety of cheap equipment.]

> sorry to be so wordy its the teacher in me ;)

  It's the networking/communications software engineer in me....

David G