US Robotics 56K Series Manual page 61

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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