The V.22bis modem is a duplex modem for general data use on the PSTN, at rates of 1200 and 2400 bits/second. It is a compatible extension of the V.22 modem, which is a 1200 bits/second only design. It is a band-split design, using carriers of 1200Hz and 2400Hz. It is the fastest PSTN modem in general use which does not use echo-cancellation.
V.22bis uses 4PSK modulation at 1200 bits/second or 16QAM modulation at 2400 bits/second. At 1200bps the symbols are so long that a fixed compromise equaliser is sufficient to recover the 4PSK signal reliably. At 2400bps an adaptive equaliser is necessary.
The V.22bis training sequence includes sections which allow the modems to determine if the far modem can support (or is willing to support) 2400bps operation. The modems will automatically use 2400bps if both ends are willing to use that speed, or 1200bps if one or both ends to not acknowledge that 2400bps is OK.