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In telecommunications, 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.
Signaling systems may be classified based on several principal characteristics.
In the public switched telephone network (PSTN), in-band signaling is the exchange of call control information within the same physical channel, or within the same frequency band, that the message (the callers' voice) is using. An example is dual-tone multi-frequency signaling (DTMF), which is used on most telephone lines to customer premises.
Out-of-band signaling is telecommunication signaling on a dedicated channel separate from that used for the message. Out-of-band signaling has been used since Signaling System No. 6 (SS6) was introduced in the 1970s, and also in Signalling System No. 7 (SS7) in 1980 which became the standard for signaling among exchanges internationally. [1] [2]
In the mid-20th century, supervision signals on long-distance trunks in North America were primarily in-band, for example at 2600 Hz, necessitating a notch filter to prevent interference. Late in the century, all supervisory signals had been moved out of band. With the advent of digital trunks, supervision signals are carried by robbed bits or other bits in the E1-carrier dedicated to signaling.
Line signaling is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (answer supervision and disconnect supervision, together referred to as supervision ), ringing, and hook flash. Register signaling is concerned with conveying addressing information, such as the calling and/or called telephone number. In the early days of telephony, with operator handling calls, the addressing formation is by voice as "Operator, connect me to Mr. Smith please". In the first half of the 20th century, addressing formation is done by using a rotary dial, which rapidly breaks the line current into pulses, with the number of pulses conveying the address. Finally, starting in the second half of the century, address signaling is by DTMF.
Channel-associated signaling (CAS) employs a signaling channel that is dedicated to a specific bearer channel.
Common-channel signaling (CCS) employs a signaling channel which conveys signaling information relating to multiple bearer channels. These bearer channels, therefore, have their signaling channel in common.
Compelled signaling refers to signaling where the receipt of each signal from an originating register needs to be explicitly acknowledged before the next signal can be sent. [3]
Most forms of R2 register signaling are compelled, while R1 multi-frequency signaling is not.
The term is only relevant in the case of signaling systems that use discrete signals (e.g. a combination of tones to denote one digit), as opposed to signaling systems which are message-oriented (such as SS7 and ISDN Q.931) where each message is able to convey multiple items of formation (e.g. multiple digits of the called telephone number).
Subscriber signaling refers to the signaling between the telephone and the telephone exchange. Trunk signaling is the signaling between exchanges.
Every signaling system can be characterized along each of the above axes of classification. A few examples:
Whereas common-channel signaling systems are out-of-band by definition, and in-band signaling systems are also necessarily channel-associated, the above metering pulse example demonstrates that there exist channel-associated signaling systems which are out-of-band.
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.
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. During that period, charges associated with long-distance calling were commonplace and could be significant, depending on the time, duration and destination of the call. A blue box device allowed for circumventing these charges by enabling 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 by the telecom company's billing system as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network.
In telecommunications, common-channel signaling (CCS), or common-channel interoffice signaling (CCIS), is the transmission of control information (signaling) via a separate channel than that used for the messages, The signaling channel usually controls multiple message channels.
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.
Professional mobile radio are person-to-person two-way radio voice communications systems which use portable, mobile, base station, and dispatch console radios. PMR systems are based on such standards as MPT-1327, TETRA, APCO 25, and DMR which are designed for dedicated use by specific organizations, or standards such as NXDN intended for general commercial use. These systems are used by police, fire, ambulance, and emergency services, and by commercial firms such as taxis and delivery services. Most systems are half-duplex, in which multiple radios share a common radio channel, and only one can transmit at a time. Transceivers are normally in receive mode, the user presses a push-to-talk button on his microphone when he wants to talk, which turns on his transmitter and turns off his receiver. They use channels in the VHF and UHF bands, giving them a limited range, usually 3 to 20 miles depending on terrain. Output power is typically limited to 4 watts. Repeaters installed on tall buildings, hills or mountain peaks are used to increase the range of systems.
2600 hertz (2600 Hz) is a frequency in hertz that was used in telecommunication signaling in mid-20th century long-distance telephone networks using carrier systems.
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. It is suitable for signaling on analog as well as digital circuits.
In telephony, a special information tone (SIT) is an in-band international standard call progress tone consisting of three rising tones indicating a call has failed. It usually precedes a recorded announcement describing the problem.
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.
Loop start is a telecommunications supervisory protocol between a central office or private branch exchange (PBX) and a subscriber telephone or other terminal for the purpose of starting and terminating a telephone call. It is the simplest of the telephone signaling systems, and uses the presence or absence of loop current to indicate the off-hook and on-hook loop states, respectively. It is used primarily for subscriber line signaling. An extension of the protocol that adds disconnect supervision is often called kewlstart.
The Elektronisches Wählsystem Digital (EWSD), translated to Electronic Digital Switching System in English, is a widely installed German telephone exchange system, originally introduced in 1975 by Siemens AG, but discontinued in 2017.
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 push-button telephone is a telephone that has buttons or keys for dialing a telephone number, in contrast to a rotary dial used in earlier telephones.
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.
Dialling is the action of initiating a telephone call by operating the rotary dial or the telephone keypad of a telephone.
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).
