3Com 56k User's Manual And Reference page 46

V.32 bis
An ITU-T standard that extends the V.32
connection range: 4800, 7200, 9600, 12,000, and
14,400 bps. V.32 bis modems fall back to the next
lower speed when line quality is impaired, fall back
further as necessary, and also fall forward (switch
back up) when line conditions improve.
See online fall back/fall forward.
V.34
An ITU-T standard that currently allows data rates
as high as 28,800 bps and 33,600bps.
V.42
An ITU-T standard for modem communications
that defines a two-stage process of detection and
negotiation for LAPM error control.
42
Glossary
V.42 bis
An extension of ITU-T V.42 that defines a specific
data compression scheme for use during V.42
connections.
V.90
The ITU-T standard for 56 Kbps modem
communications.
Xmodem
The first of a family of error control software
protocols used to transfer files between modems.
These protocols are in the public domain and are
available from many bulletin board services.
Xon/Xoff
Standard ASCII control characters used to tell an
intelligent device to stop/resume transmitting data.