Signalling (telecommunications)
Encyclopedia
In telecommunication
, signaling (signalling in British spelling) has the following meanings:
Signaling systems can be classified according to their principal properties, some of which are described below:
(PSTN), in-band signaling is the exchange of call control information within the same channel that the telephone call itself is using. An example is dual-tone multi-frequency signaling (DTMF), which is used on most telephone line
s to customer premises.
Out-of-band signaling is telecommunication signaling on a channel that is dedicated for the purpose and separate from the channels
used for the telephone call. Out-of-band signaling is used in Signaling System 7 (SS7), the standard for signaling among exchanges that has controlled most of the world's phone calls for some twenty years.
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 current (alerting), and recall. In the middle 20th Century, supervision signals on long distance trunks in North America were usually inband, for example at 2600 Hz, necessitating a notch filter to prevent interference. Late in the century, all supervisory signals were 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.
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 information is by voice as "Operator, connect me to Mr. Smith please". In the first half of the 20th century, addressing information is 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.
(CAS) employs a signaling channel which 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.
is the case where receipt of each signal needs to be explicitly acknowledged before the next signal is able to be sent.
Most forms of R2 register signaling are compelled (see R2 signaling
), 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 information (e.g. multiple digits of the called telephone number).
. Trunk signaling is signaling between exchanges.
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.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, signaling (signalling in British spelling) has the following meanings:
- the use of signals for controlling communications
- the informationInformationInformation in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
exchange concerning the establishment and control of a telecommunication circuitTelecommunication circuitA 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...
and the management of the network, in contrast to userUser (telecommunications)In telecommunications, a user is a person, organization, or other entity that employs the services provided by a telecommunication system, or by an information processing system, for transfer of information....
information transferInformation transferIn telecommunications, information transfer is the process of moving messages containing user information from a source to a sink.Note: The information transfer rate may or may not be equal to the transmission modulation rate.-See also:... - the sending of a signal from the transmitting end of a telecommunication circuitTelecommunication circuitA 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...
to inform a user at the receiving end that a messageMessageA message in its most general meaning is an object of communication. It is a vessel which provides information. Yet, it can also be this information. Therefore, its meaning is dependent upon the context in which it is used; the term may apply to both the information and its form...
is to be sent.
Signaling systems can be classified according to their principal properties, some of which are described below:
In-band versus out-of-band signaling
In the public switched telephone networkPublic switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
(PSTN), in-band signaling is the exchange of call control information within the same channel that the telephone call itself is using. An example is dual-tone multi-frequency signaling (DTMF), which is used on most telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
s to customer premises.
Out-of-band signaling is telecommunication signaling on a channel that is dedicated for the purpose and separate from the channels
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
used for the telephone call. Out-of-band signaling is used in Signaling System 7 (SS7), the standard for signaling among exchanges that has controlled most of the world's phone calls for some twenty years.
Line versus register
Line signalingLine signaling
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...
is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (Answer supervision
Answer supervision
Answer supervision is a term in telephony, describing a situation whereby the called party indicates to the central office that the call is being answered. -CAS E&M Signaling Basics:...
and Disconnect supervision
Disconnect supervision
Disconnect supervision is a term in telephony describing signaling between the telephone exchange and a connected party used to indicate that the connected call is being disconnected....
, together referred to as supervision), ringing current (alerting), and recall. In the middle 20th Century, supervision signals on long distance trunks in North America were usually inband, for example at 2600 Hz, necessitating a notch filter to prevent interference. Late in the century, all supervisory signals were out of band. With the advent of digital trunks
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
, supervision signals are carried by robbed bits or other bits in the E1-carrier dedicated to signaling.
Register signaling
Register signaling
In telecommunications, register signaling provides addressing information, such as the calling and/or called telephone number. R2 register signaling is an example.This is contrasted with line signaling....
is concerned with conveying addressing information, such as the calling and/or called telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
. In the early days of telephony
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
, with operator
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...
handling calls, the addressing information is by voice as "Operator, connect me to Mr. Smith please". In the first half of the 20th century, addressing information is by using a rotary dial
Rotary dial
The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent...
, which rapidly breaks the line current into pulses
Pulse dialing
Pulse dialing, dial pulse, or loop disconnect dialing, also called rotary or decadic dialling in the United Kingdom , is pulsing in which a direct-current pulse train is produced by interrupting a steady signal according to a fixed or formatted code for each digit and at a standard pulse repetition...
, with the number of pulses conveying the address. Finally, starting in the second half of the century, address signaling is by DTMF.
Channel-associated versus common-channel signaling
Channel Associated SignalingChannel 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...
(CAS) employs a signaling channel which is dedicated to a specific bearer channel
B channel
B channel is a telecommunications term which refers to the ISDN channel in which the primary data or voice communication is carried. It has a bit rate of 64 kbit/s in full duplex....
.
Common Channel Signaling
Common Channel Signaling
In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...
(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
Compelled signalingCompelled 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....
is the case where receipt of each signal needs to be explicitly acknowledged before the next signal is able to be sent.
Most forms of R2 register signaling are compelled (see R2 signaling
R2 signalling
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....
), 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 information (e.g. multiple digits of the called telephone number).
Subscriber versus trunk signaling
Subscriber signaling is between the telephone and the telephone exchangeTelephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
. Trunk signaling is signaling between exchanges.
Classification examples
Note that every signaling system can be characterized along each of the above axes of classification. A few examples:- DTMF is an in-band, channel-associated register signaling system. It is not compelled.
- SS7SS7SS-7 can stand for:* Signaling System #7, a set of telephone signaling protocols.* The R-16 missile, with NATO reporting name SS-7 Saddler.* China Railways SS7, an electric locomotive model in China.* Super Socket 7, a chip socket introduced by AMD...
(e.g. TUP or ISUP) is an out-of-band, common-channel signaling system that incorporates both line and register signaling. - Metering pulseMetering pulseIn telecommunications signalling, metering pulses are signals sent by telephone exchanges to metering boxes and payphones aimed at informing the latter of the cost of ongoing telephone calls....
s (depending on the country, these are 50 Hz, 12 kHz or 16 kHz pulses sent by the exchange to payphones or metering boxes) are out-of-band (because they do not fall within the frequency range used by the telephony signal, which is 300 through 3400 Hz) and channel-associated. They are generally regarded as line signaling, although this is open to debate. - E and M signalingE and M signalingE&M is a type of supervisory line signaling that uses DC signals on separate leads, called the "E" lead and "M" lead, traditionally used in the telecommunications industry between telephone switches....
(E&M) is an out-of-band channel-associated signaling system. The base system is intended for line signaling, but if decadic pulses are used it can also convey register information. E&M line signaling is however usually paired with DTMF register signaling. - By contrast, the L1 signaling system (which typically employs a 2280 Hz tone of various durations) is an in-band channel-associated signaling system as was the SF 2600 hertz system formerly used in the Bell SystemBell SystemThe 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...
. - Loop startLoop startIn telecommunications, a loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit, commonly referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC...
, Ground startGround startIn telephony, a ground start or GST is a method of signaling from a terminal or subscriber local loop to a telephone exchange, in which method a cable pair is temporarily grounded to request dial tone...
, Reverse Battery and Revertive Pulse systems are all DC, thus out of band, and all are channel-associated, since the DC currents are on the talking wires.
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.