R2 signalling

R2 is a 1950s- and 1970s-era channel-associated-signalling used outside of the former Bell System to convey information along a telephone trunk between two telephone switches in order to establish a single telephone call along that trunk.

R2 is the name given to two broad groups: R2 line signalling and R2 register signalling.

A signalling protocol is best visualized by two contexts: what information it conveys and where its participants are in the network. This section presents those two contexts.

Each national variant in the family of R2 signalling protocols conveys at least the following, where forward is the direction from the dialling telephone's switch to the called telephone's switch and where backward is the direction from the called telephone's switch to the calling telephone's switch:

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.

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.

R2 signalling refers to a vast number of variants of R2 that resemble each other to varying degrees. For most R2 variants, each such variant of R2 is promulgated by the PTT of each nation and/or by a telecommunications equipment manufacturer who addressed that national market with a nation-specific adaptation of one of their products.

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