The VVRB.*exe and VVSB.*exe files provide Ymodem protocol
receive and send file capability under VMS. One issues a
command like

vvsb -switches file

to send a single file. By using a command like

vvsb -k file

you send files in 1K packets. Ymodem is one of the enhancements
to the older xmodem protocol for asynchronous transfer that uses
larger packets and less acks sent back to get better thruput.
It needs an 8 bit transparent channel for this, but for MNP4
or X.PC or better modems can give significantly faster transfers
than Xmodem.

Zmodem is a still further enhancement.