R2 signalling

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R2[1] is a 1950s- and 1970s-era channel-associated-signalling signalling protocol used outside of the former Bell System, mostly Europe and Latin America, 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 protocol groups: R2 line signalling and R2 register signalling.

Information conveyed along an R2-signalled trunk[edit]

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 at a glance
R2 line signalling acquisition (termed seizure) of an idle DS0 channel
R2 forward register signalling the digits 1 through 10 of the destination telephone number (termed called-party address)
R2 forward register signalling the digits 1 through 10 of the origin telephone number (termed calling-party address)
R2 forward register signalling the digits 11 through 14 for special-service requests (e.g., route to operator, add echo-suppression)
R2 forward register signalling the calling party's category (e.g., normal subscriber, high-priority subscriber, operator, coin-operated telephone)
R2 forward register signalling the disposition of the routing of the telephone call attempt (e.g., called-party's telephone is currently busy, called-party's telephone is now ringing, called-party's telephone is out of service)
R2 line signalling the called party has now lifted the handset from its hook to answer this call attempt in order to transition from R2 signalling toward the establishment of speech-capable audio to fully establish the call (termed going off hook)
R2 line signalling the release of the call (e.g., the handset of the called-party's telephone has now been returned to its on-hook position, ending this call; the telephone company or trouble in the network is explicitly or implicitly forcing the ending of this call)
R2 backward register signalling the set of explicit requests corresponding to each of the forward register-signalling data. The client–server requesting of each of these data differentiates R2 from the Bell System R1 MF-tone signalling, where in R1 the called-party's switch unilaterally sends some of these data as a timed sequence without explicit intervening requests from the calling-party's switch.

Line signalling[edit]

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[edit]

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.

The international version of R2 is an inter-nation protocol for R2 signalling that crosses national boundaries, and is neither a superset nor a subset of any national variant of R2. Conversely, each national variant of R2 is intra-nation. The international standard R2 was first promulgated by CEPT in the 1970s and then later by ITU-T with only modest evolution.

Signalling variants[edit]

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 channel #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[edit]

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 multi-frequency 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 apt modern-era proleptic terms, 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, which request information from the outgoing register, are client–server, where the client is the incoming register and the server is the outgoing register.)


R2 "signalling" is consistently "signalling" (with a double ell) in standardization documents, which are consistently written using British spelling, as the standard was not adopted for wide use in the United States.

R2 is mnemonic for Region Two signalling to differentiate it from R1 signalling, the North American MF signalling. 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 signalling, except to contrast with its European contemporary, R2. 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. 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,[where?] which is largely under the influence of Bell System standardization in North America.

See also compelled signalling.


  1. ^ ITU-T Recommendation Q.400-Q.490 - Specifications of Signalling System R2