3Com 56k User's Manual And Reference page 47

Glossary
Ymodem
An error-checking protocol that can send several
files of data at a time in 1024-byte (1K) blocks.
This protocol can use either checksums or CRC for
error checking.
Ymodem G
This is similar to the Ymodem, except it relies on
the modem for error checking, which makes it
faster.
Zmodem
This is similar to Xmodem and Ymodem, except it
includes batch transfer, the ability to recover from
a partially complete transfer, an autostart feature,
and improved efficiency.
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