ITU-T Recommendations

This Recommendation defines signals to be exchanged between DCEs over the PSTN when a session of data transmission needs to be established, but before signals are exchanged which are specific to a particular modem Recommendation. It provides means to automatically determine the best available operational mode between two DCEs connected via the PSTN, provides a timely indication to Circuit Multiplication Equipments on the ITU-T V-Series modulation to be employed in a new session of data transmission, provides a means to enable a PSTN call to be passed on automatically to an appropriate DCE, and provides signals for interacting with PSTN echo-control equipment.
This revised Recommendation defines additional call function types, defines procedures for the exchange of non-standard facilities information, and adds support for ITU-T V.90 modulation modes.