R2 is a 1960s- and 1970s-era Channel Associated Signalling signalling protocol used outside of North America to convey at least the following information along a telephone trunk: seizure of an idle DS0 channel, the called party's digits 1 through 10, the calling party's digits 1 through 10, the calling party's category, the disposition of the routing of the telephone call, the called party's off-hook answer, and the release of the call. R2 is the name given to two broad protocol groups: R2 line signalling and R2 register signalling.

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 signaling DS0 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 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 signaling DS0 in that E1.

Misconceptions & Standards

R2 is not a single family of protocols. R2 line signalling is not a single protocol. R2 register signalling is not a single protocol. These points cannot be emphasized enough, as this forms the most common misconceptions about R2. R2 signalling refers to a vast number of variants of R2 that all 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.

Even the international standard R2 is merely yet another variant of R2 that is neither a superset nor a subset of any national variant of R2. Many people, including companies who attempt to enter the R2 market with telecommunications products, do not realize that international standard R2 is not necessarily a normative reference for any particular national variant of R2. The international standard R2 was first promulgated by CEPT in the 1970s and then later by ITU-T with only modest evolution. The international version of R2 is literally an inter-nation protocol for R2 signalling that crosses national boundaries. Conversely, each national variant of R2 is intra-nation.

Signalling Variants

R2 line signalling variants that support R2 line signalling in its electrical-pulse form by dedicated two-wire or four-wire circuits usually represent each signal as two different pulse lengths (e.g., 150 milliseconds versus 600 milliseconds), where the two different meanings of those two different pulse lengths varies by context.

R2 line signalling variants that support R2 line signalling in its digital form via the A, B, C, and D bits in DS0 #16 of an E1 fall into two general categories: those that use only the A bit to represent each signal and those that use multiple A, B, C, and D bits to represent each signal, with using only two, the A bit and the B bit being by far the most common. Those that use only the A bit, strongly resemble the analog electrical R2 line signalling, where each signal is a timed pulse that differs by length of time that the pulse is in the high voltage (i.e., one-valued bit) state before returning to the untimed resting voltage state (i.e., zero-valued bit). Those that use multiple bits to represent each signal usually are stateful in that time of exhibition of that bit pattern typically plays no role in conveying meaning.

MF tones

One unifying aspect of R2 register signalling variants is that all audio tones are combinations of two frequencies. Each pair of two frequencies is called a multifrequency tone, or MF tone. Each call attempt has an origination end and a termination end, both to the multi-span call as well as for each span within that multi-span call routing. The origination end is called the outgoing register in R2 parlance. The termination end is called the incoming register in R2 parlance. The outgoing register transmits forward MF tones to the incoming register. The incoming register transmits backward MF tones to the outgoing register. The outgoing register detects backward MF tones arriving from the incoming register. The incoming register detects forward MF tones arriving from the outgoing register. The outgoing register transmits the first signal, but from that point forward the outgoing register merely passively responds to the prompting driven by each signal transmitted by the incoming register. (In modern era proleptic vocabulary that retrofits nicely, the outgoing register's first push signal to the incoming register is similar to a peer-to-peer message, whereas the incoming register's pull signals that request information from the outgoing register are client-server, where the client is the incoming register and the server is the outgoing register.)

Nomenclature

The spelling of signalling in R2 signalling is consistently signalling with a double ell instead of signaling with a single ell because R2 is an international protocol whose standardization documents are consistently written using British spelling. There are no R2 signaling (with a single ell) standards in the United States, so the use of United States' spelling conventions is inappropriate for a strictly international standard.

R2 is mnemonic for Region Two signalling to differentiate it from R1 signaling, the North American MF signaling. Here region number two was envisioned in the 1960s to be Europe and region number one was envisioned in the 1960s to be the Bell System in North America. In practice the term R1 is rarely used for North American MF signaling, except to contrast with its European contemporary, R2. As time as marched onward, use of R2 signalling spread beyond Europe to all regions of the globe, including Mexico on the North American continent. This spread is due largely to European telecommunications manufacturers selling their older equipment designs at a discount to developing countries while selling their higher-speed, higher-density post-R2 SS7 equipment at higher prices in more-industrialized countries. There is even sparse use of R2 in Canada, which is largely under the influence of Bell System standardization in North America.

Telephony