In
telecommunication
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, signaling is the use of
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
s for controlling
communication
Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
s. This may constitute an information exchange concerning the establishment and control of a
telecommunication circuit
A telecommunication circuit is a path in a telecommunications network used to transmit information. Circuits have evolved over time from generally being built on physical connections between individual hardware cables, as in an analog phone swit ...
and the management of the network.
Classification
Signaling systems may be classified based on several principal characteristics.
In-band and out-of-band signaling
In the
public switched telephone network
The public switched telephone network (PSTN) provides Communications infrastructure, infrastructure and services for public Telecommunications, telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that ...
(PSTN),
in-band signaling
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 ov ...
is the exchange of call control information within the same physical channel, or within the same frequency band, that the telephone call itself is using. An example is
dual-tone multi-frequency signaling
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 ...
(DTMF), which is used on most
telephone line
A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...
s to customer premises.
Out-of-band signaling is telecommunication signaling on a dedicated
channel
Channel, channels, channeling, etc., may refer to:
Geography
* Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water.
Australia
* Channel Country, region of outback Austral ...
separate from that used for the telephone call. 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
Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, 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 perfo ...
(SS7) in 1980 which became the standard for signaling among exchanges ever since.
Line versus register signaling
Line 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 pa ...
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
Ringing may mean:
Vibrations
* Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts
* Vibration of a harmonic oscillator
** Bell ringing
* Ringing (telephony), the sound of a telephone bell
* Ringing (medicine), a ri ...
, and
hook flash 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.
The term "register-recall" in Europe refers to sending a discrete signal to alert t ...
. In the middle 20th century, supervision signals on
long-distance
Long distance or Long-distance may refer to:
*Long-distance calling
* Long-distance operator
* Long-distance relationship
* Long-distance train
*Long-distance anchor pylon, see dead-end tower
Footpaths
* Long-distance trail
*European long-d ...
trunks in North America were primarily inband, for example at
2600 Hz, necessitating a
notch filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a ...
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
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) or ...
dedicated to 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
Line signaling is a class of telecomm ...
is concerned with conveying addressing information, such as the calling and/or called
telephone number
A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...
. In the early days of
telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
, 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
A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number ...
, which rapidly breaks the line current into
pulses
In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...
, with the number of pulses conveying the address. Finally, starting in the second half of the century, address signaling is by
DTMF
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 ...
.
Channel-associated versus common-channel signaling
Channel-associated signaling Channel-associated signaling (CAS), also known as ''per-trunk signaling'' (PTS), 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 i ...
(CAS) employs a signaling channel that is dedicated to a specific
bearer channel
A bearer channel is a DS-0 that carries call content i.e. one that does not carry signaling.
In the common-channel signaling scheme for telecommunications, signaling is sent out-of-band, while all other traffic rides bearer channels. When a call ...
.
Common-channel signaling
In telecommunication, 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 ...
(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 signaling
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.[R2 signaling 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 e ...]
), while R1
multi-frequency signaling 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 ( telephone number) transport and supervision signaling on trunk li ...
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 versus trunk signaling
Subscriber signaling refers to the signaling between the telephone and the
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
. Trunk signaling is the signaling between exchanges.
Examples
Every signaling system can be characterized along each of the above axes of classification. A few examples:
*
DTMF
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 ...
is an in-band, channel-associated register signaling system. It is not compelled.
*
SS7 (e.g., TUP or ISUP) is an out-of-band, common-channel signaling system that incorporates both line and register signaling.
*
Metering pulses (depending on the country, these are 50 Hz, 12 kHz or 16 kHz pulses sent by the exchange to
payphones
A payphone (alternative spelling: pay phone) is typically a coin-operated public telephone, often located in a telephone booth or in high-traffic outdoor areas, with prepayment by inserting money (usually coins) or by billing a credit or debit ...
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 signaling E and M signaling 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. Various mnemonic names have been us ...
(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 System
The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...
.
*
Loop start
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 ...
,
ground start {{refimprove, date=January 2008
In telephony, ground start is a method of signaling from a terminal of a subscriber local loop to a telephone exchange, where one side of a cable pair is temporarily grounded to request dial tone. Most middle 20th-c ...
, 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.
See also
*
Control character
In computing and telecommunication, a control Character (computing), character or non-printing character (NPC) is a code point (a number) in a character encoding, character set, that does not represent a written symbol. They are used as in-band ...
*
In-band control {{Unreferenced, date=June 2019, bot=noref (GreenC bot)
In-band control is a characteristic of network protocols with which data control is regulated. In-band control passes control data on the same connection as main data. Protocols that use in-band ...
*
Metadata
Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including:
* Descriptive metadata – the descriptive ...
*
Out-of-band control
Out-of-band control is a characteristic of network protocols with which data control is regulated. Out-of-band control passes control data on a separate connection from main data. Protocols such as FTP use out-of-band control.
FTP sends its contr ...
*
Signaling protocol A signaling protocol is a type of communications protocol for encapsulating the signaling between communication endpoints and switching systems to establish or terminate a connection and to identify the state of connection.
The following is a list ...
References
*
{{DEFAULTSORT:Signaling (Telecommunications)