[vcf-midatlantic] A "100%" Efficient BBS Transfer Protocol

Kenneth Gober kgober at gmail.com
Wed Oct 16 14:07:34 EDT 2019

On Tue, Oct 15, 2019 at 7:42 PM John Heritage <john.heritage at gmail.com>

> Ahh interesting - is that because a one byte slip/drop might not allow the
> CRC algorithm to work properly?

Yes.  There are 2 kinds of failures that you need to handle:
1. a byte is corrupted during transmission, and is received incorrectly.
2. a byte is dropped during transmission, and is never received.

The CRC will handle the first case with no trouble, but in the second case
it won't be obvious
to the receiver that the 1024 bytes they got was actually 1023 bytes from
the first block (with
1 byte missing), plus the first byte of the second block.  So the receiver
will calculate the CRC
and it will be wrong, which is fine because the first block needs to be
retransmitted anyway.

But then, the receiver will calculate the CRC for the second block,
starting with the second
byte (because the receiver doesn't know that the last byte of the first
block was actually
supposed to be the first byte of the second), which will also end up wrong,
and this will
continue to happen for every successive block.

Sending the entire file back the other way would work as a confirmation of
success, but if
there are any errors (in either direction) you still need a way to figure
out which parts of
the file need to be retransmitted and communicate that information to the
receiver, which
is kind of the same problem all over again.


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