The CCITT protocol for transmission of 1200bps one way, 75bps the other way.
600/1200 bps, half duplex dial up lines.
CCITT standard for modem communications at 1200bps with a 75bps back channel. It is used in the United Kingdom.
600/1200bit/s modem for use over PSTN.
An ITU-T standard for modem communication at 1200 bps with a 75 bps back channel.