R2 signaling specifications were first published by the International Telegraph and Telephone Consultative Committee (CCITT) in ITU White Book Volume VI of 1969,[2] and are maintained by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) in Recommendations Q.400 through Q.490.
A signalling protocol may be visualized by two contexts: the information it conveys, and the location of participants in the network.
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 signalling refers to a vast number of variants of R2 that resemble each other to varying degrees.
Later in the 20th century, use of R2 signalling spread beyond Europe to all regions of the globe, including Mexico on the North American continent.
The European market consisted of a large number of national telecommunications administrations, but unlike the Bell System, these were not usually integrated with an equipment manufacturer.
Instead a large number of independent vendors of equipment emerged in Europe though the 20th century in a very competitive market, tendering for business from European PTTs.
This allowed interconnection, interoperability and communication between a more diverse range of equipment types and telecommunication administrations.
European equipment vendors such as Ericsson, Siemens and the European ITT affiliates, amongst others became major suppliers to telecommunication administrations around the world bringing technologies like Ericsson’s ARF and ITT’s Pentaconta crossbars to many markets.