Xmodem - the beginning.  128 byte packets as documented previously.
Ymodem - add CRC and filename negotiation.  Permits downloading of
         multiple files with a single command.  I don't think that
         ymodem added 1k packets, although that variant might exist.
Zmodem - added large packets and a sliding window for full bandwidth
         throughput.

These should all be out there and pretty well documented.  I can dig
up IBM BasicA code to do Xmodem if anyone is interested...

BillW