V.22bis
V.22bis is a transmission standard for modems. The V.22 standard transmits the bits in combination of two as dibits with 600 bit/s. Each character represents two bits, which are transmitted with quadrature phase modulation, but rotated by 45°: Dibit "00" has a phase of 45°, "01" of 135°, "11" of 225°, and "10" of 315°.
With V.22bis, transmission is in duplex over the frequencies 1,200 Hz for the calling modem and 2,400 Hz for the answering modem.
In the V.22bis method, the transmission speed is 2,400 bit/s. The V.22bis duplex modem operates in the public telephone network and on two- wire telephone leased lines using the frequency separation method.
ITU specifications with the suffix "bis" are the second generation of a protocol or interface. It is an older German term for "second".