US Robotics 56K Series Manual page 59

V.22bis
An ITU-T standard for modem
communications at 2400 bps. The
standard includes an automatic link
negotiation fallback to 1200 bps and
compatibility with Bell 212A/V.22
modems.
V.23
An ITU-T standard for modem
communication at 1200 bps with a 75
bps back channel.
V.27ter
An ITU-T standard for facsimile
operations that specifies modulation at
4800 bps, with fallback to 2400 bps.
V.29
An ITU-T standard for facsimile
operations that specifies modulation at
9600 bps, with fallback to 7200 bps.
V.32
An ITU-T standard for modem
communications at 9600 bps and 4800
bps. V.32 modems fall back to 4800 bps
when line quality is impaired.
V.32bis
An ITU-T standard that extends the
V.32 connection range: 4800, 7200,
9600, 12,000, and 14,400 bps. V.32bis
modems fall back to the next lower
speed when line quality is impaired, fall
back further as necessary, and also fall
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