Signalling System R2 is a signalling protocol for telecommunications that was in use from the 1960s mostly in Europe, and later also in Latin America, Asia, and Australia, to convey exchange information between two telephone switching systems for establishing a telephone call via a telephone trunk. It is suitable for signaling on analog as well as digital circuits. R2 signaling specifications were first published by the International Telegraph and Telephone Consultative Committee (CCITT) in ITU White Book Volume VI of 1969, and are maintained by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) in Recommendations Q.400 through Q.490. The name R2 is a derived from a designation as Regional System No. 2. R2 signaling methods may be logically divided into two protocol groups. The line signaling group comprises supervisory signals for call setup and termination, while interregister signaling uses in-band multifrequency signals to transfer calling-party and called-party addressing information. Signaling information A signalling protocol may be visualized by two contexts: the information it conveys, and the location of participants in the network. Each R2 national variant conveys at least the following. Forward is the direction from the dialling telephone's switch to the called telephone's switch, and backward is the inverse direction: Line signalling R2 line signalling is a family of protocols that govern the resource acquisition and resource release related to a two-party telephone call attempt and, if successful, the establishment of a two-party telephone call. Although in the 1960s R2 line signalling was represented as electrical pulses on a two-wire or four-wire circuit, by the latter 1970s these analog electrical pulses also could be represented in digital form by a signalling DS0 channel in the trunk, which is normally channel 16 in an E1 trunk. Register signalling R2 register signalling is a family of protocols that govern the conveyance of addressing information during the addressing phase and how the call attempt turned out during the disposition phase. Although in the 1960s R2 register signalling was represented by electromechanical devices that could generate multi-frequency audio tones and by electromechanical devices that could detect those audio tones, by the latter 1970s these electromechanical registers also could be represented by digitized PCM audio in DS0 channels of an E1 other than the R2 line signalling DS0 channel in that E1. Standards R2 signalling refers to a vast number of variants of R2 that resemble each other to varying degrees. For most R2