This Recommendation defines signals, messages and the procedures for exchanging these over the PSTN and on leased point-to-point telephone-type circuits, for the identification and selection of common modes of operation between Data Circuit-terminating Equipments (DCEs) and Data Terminal Equipments (DTEs). These are used when the modes of operation of DCEs and DTEs communicating over the connection need to be automatically established and selected, but before signals are exchanged which are specific to a particular Recommendation. Considerations are included for the standard analogue telephony mode of operation.