3Com 56K Command Reference Manual page 107

Getting Maximum Throughput 12-11
Overhead is minimal with ZMODEM, resulting in throughput that is
almost equal to that obtained with no file-transfer protocol. ZMODEM
should also be used for non-ARQ connections. Leave the Business
Modem at its &M4 and &K1 settings for both error control and data
compression.
YMODEM-G is another good choice, but never use it unless both the
local and remote devices are using error control: if YMODEM-G
detects an error, it aborts the transfer. Do not use either protocol with
software flow control (XON/XOFF signaling).