Signalling System R2

Last updated May 05, 2024

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.^[1] 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,^[2] 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:

**R2 at a glance**
line signalling	acquisition (termed seizure) of an idle DS0 channel
forward register signalling	the digits 1 through 10 of the destination telephone number (termed called-party address)
forward register signalling	the digits 1 through 10 of the origin telephone number (termed calling-party address)
forward register signalling	the digits 11 through 14 for special-service requests (e.g., route to operator, add echo-suppression)
forward register signalling	the calling party's category (e.g., normal subscriber, high-priority subscriber, operator, coin-operated telephone)
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)
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)
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)
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

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 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

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.

Multifrequency tones

All R2 register signalling variants use a combination of two frequencies of audio signals. Each pair of 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.)

Nomenclature

R2 is a mnemonic for Regional System No. 2. Regional System No. 1, R1, was the CCITT designation for North American multi-frequency signaling (MF) practiced in the Bell System, although in practice that term is rarely used for MF signalling.

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. A more open set of technical standards developed, particularly under the auspices of CEPT and later ETSI. 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. European derived standards were adopted more readily around the world than the proprietary standards of Bell System and Western Electric. As a result, R2 has even found sparse use in Canada,^{[ where? ]} which is largely under the influence of Bell System standardization.

R2 signalling was also used for register signaling within many European switching systems of that era, notably in Ericsson’s AR- crossbar systems. It was used for inter station communication and for communication between parent and remote switches.

Related Research Articles

<span class="mw-page-title-main">DTMF</span> Telecommunication signaling system

Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed in the Bell System in the United States, and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. DTMF is standardized as ITU-T Recommendation Q.23. It is also known in the UK as MF4.

Modulation is defined as the process by which some characteristics like amplitude, frequency, and phase of a carrier signal are varied in accordance with a modulating wave.

In telecommunication, signaling is the use of signals for controlling communications. This may constitute an information exchange concerning the establishment and control of a telecommunication circuit and the management of the network.

The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls.

The E-carrier is a member of the series of carrier systems developed for digital transmission of many simultaneous telephone calls by time-division multiplexing. The European Conference of Postal and Telecommunications Administrations (CEPT) originally standardised the E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). It was widely used in almost all countries outside the US, Canada, and Japan. E-carrier deployments have steadily been replaced by Ethernet as telecommunication networks transition towards all IP.

Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also performs number translation, local number portability, prepaid billing, Short Message Service (SMS), and other services.

A blue box is an electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voice circuits. This allowed an illicit user, referred to as a "phreaker", to place long-distance calls, without using the network's user facilities, that would be billed to another number or dismissed entirely as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network.

<span class="mw-page-title-main">Answering machine</span> Telephone answering device

An answering machine, answerphone, or message machine, also known as telephone messaging machine in the UK and some Commonwealth countries, ansaphone or ansafone, or telephone answering device (TAD), is used for answering telephone calls and recording callers' messages.

<span class="mw-page-title-main">Telecommunications device for the deaf</span> Electronic text communication device

A telecommunications device for the deaf (TDD) is a teleprinter, an electronic device for text communication over a telephone line, that is designed for use by persons with hearing or speech difficulties. Other names for the device include teletypewriter (TTY), textphone, and minicom.

The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists of telephone lines, fiber-optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables interconnected by switching centers, such as central offices, network tandems, and international gateways, which allow telephone users to communicate with each other.

In telephony, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after World War II. It uses a combination of audible tones for address transport and supervision signaling on trunk lines between central offices. The signaling is sent in-band over the same channel as the bearer channel used for voice traffic.

In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even over a separate network. In-band signals may often be heard by telephony participants, while out-of-band signals are inaccessible to the user. The term is also used more generally, for example of computer data files that include both literal data, and metadata and/or instructions for how to process the literal data.

<span class="mw-page-title-main">Hook flash</span> Telephone signal

On analog telephone lines with special services, a flash or register-recall signal is used to control functions on the public telephone exchange, PBX or VoIP ATA.

Line signaling is a class of telecommunications signaling protocols. Line signaling is responsible for off-hook, ringing signal, answer, ground start, on-hook unidirectional supervision messaging in each direction from calling party to called party and vice versa. After an off-hook, line signaling initiates register signaling to accomplish the exchange of telephone numbers of called party and in more modern line-signaling protocols, the calling party as well. While register signaling occurs, line signaling remains quiescent unless the calling party goes on-hook or an abnormal cessation of the call occurs, such as due to equipment malfunction or shutdown or due to network outage upstream in that call-attempt's series of spanned trunks.

Ringing tone is a signaling tone in telecommunication that is heard by the originator of a telephone call while the destination terminal is alerting the receiving party. The tone is typically a repeated cadence similar to a traditional power ringing signal (ringtone), but is usually not played synchronously. Various telecommunication groups, such as the Bell System and the General Post Office (GPO) developed standards, in part taken over by the European Telecommunications Standards Institute (ETSI) and other standards bodies. With modern cell phone and smartphone technology ringing tone can be customized and even used for advertising.

High-bit-rate digital subscriber line (HDSL) is a telecommunications protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum over copper, twisted pair cables. HDSL was developed to transport DS1 services at 1.544 Mbit/s and 2.048 Mbit/s over telephone local loops without a need for repeaters. Successor technology to HDSL includes HDSL2 and HDSL4, proprietary SDSL, and G.SHDSL.

In telecommunication, supervision is the monitoring of a telecommunication circuit for telephony to convey to an operator, user, or a switching system, information about the operational state of the circuit. The typical operational states of trunks and lines are the idle and busy states, seizure, and disconnect. The states are indicated by various electrical signals and electrical conditions depending on the type of circuit, the type of terminating equipment, and the type of intended service.

The Signaling System No. 5 (SS5) is a multi-frequency (MF) telephone signaling system that was in use from the 1970s for International Direct Distance Dialing (IDDD). Internationally it became known as CCITT5 or CC5. It was also nicknamed Atlantic Code because it was used for the first IDDD connections between Europe and North America.

Compelled signalling is a class of telecommunications signalling protocols that acknowledge the receipt of each discrete signal before the next signal can be sent.

A telephone exchange, also known as a telephone switch or central office, is a crucial component in the public switched telephone network (PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enabling telephone calls between subscribers.

References

↑ ITU-T Recommendation Q.400-Q.490 - Specifications of Signalling System R2
↑ IVth Plenary Assembly (Mar Del Plata, 23 September-25 October 1968), White Book Volume VI, Telephone Signaling and Switching, Recommendations Series Q, ITU 1969.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[q400-1] ITU-T Recommendation Q.400-Q.490 - Specifications of Signalling System R2

[2] IVth Plenary Assembly (Mar Del Plata, 23 September-25 October 1968), White Book Volume VI, Telephone Signaling and Switching, Recommendations Series Q, ITU 1969.

[1]

[2]