R2 signalling
Encyclopedia
R2 is a 1950s- and 1970s-era channel-associated-signalling
signalling protocol 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 protocol
groups: R2 line signalling and R2 register signalling.
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:
, 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.
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.
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.
s 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.
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 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
, which is largely under the influence of Bell System standardization in North America.
See also compelled signalling
.
Channel Associated Signaling
Channel Associated Signaling , also known as per-trunk signaling , is a form of digital communication signaling. As with most telecommunication signaling methods, it uses routing information to direct the payload of voice or data to its destination. With CAS signaling, this routing information is...
signalling protocol used outside of the former Bell System
Bell System
The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...
to convey information along a telephone trunk
Trunking
In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...
between two telephone switches in order to establish a single telephone call along that trunk.
R2 is the name given to two broad protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
groups: R2 line signalling and R2 register signalling.
Information conveyed along an R2-signalled trunk
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 | 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
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 circuitTelecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
, by the latter 1970s these analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
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-frequencyMulti-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
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 PTTPostal Telegraph and Telephone
A postal, telegraph, and telephone service is a government agency responsible for postal mail, telegraph, and telephone services. Such monopolies existed in many countries, though not in North America or Japan. Many PTTs have been partially or completely privatized in recent years...
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
CEPT
CEPT may refer to:* The European Conference of Postal and Telecommunications Administrations* The CEPT, Ahmedabad* The Committee of European Pharmacy Technicians * Common Effective Preferential Tariff...
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 millisecondMillisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s 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
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
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
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-frequencyMulti-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
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
Peer-to-peer
Peer-to-peer computing or networking is a distributed application architecture that partitions tasks or workloads among peers. Peers are equally privileged, equipotent participants in the application...
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 "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 StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
.
R2 is mnemonic for Region Two signalling to differentiate it from R1 signalling, the North American MF
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
signalling. Here region number two was envisioned in the 1960s to be Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and region number one was envisioned in the 1960s to be the Bell System
Bell System
The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...
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
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, which is largely under the influence of Bell System standardization in North America.
See also compelled signalling
Compelled signalling
The term compelled signalling refers to a class of telecommunications signalling protocols where receipt of each discrete signal needs to be explicitly acknowledged before the next signal is able to be sent....
.