In-band signaling
Encyclopedia
In telecommunications, in-band signaling is the sending of metadata
and control information in the same band or channel used for data.
, the telephone number
is encoded and transmitted across the telephone line as Dual-Tone Multi-Frequency (DTMF) tones. The tones "control" the telephone system by instructing the telephone company's equipment where to route the call to. These control tones are sent over the same channel
and in the same band (300Hz to 3.4kHz) as the voice and other sounds of the telephone call. In-band signaling is also used on older telephone carrier systems to provide inter-exchange
information on how to route calls. Examples of this kind of in-band signaling system are SS5 and R2
.
Separating the control signals, also referred to as the control plane, from the data (if a bit-transparent
connection is desired) is usually done by escaping
the control instructions. Occasionally, however, networks are designed so that data is (to a varying degree) garbled by the signaling. Allowing data to become garbled is usually acceptable when transmitting sounds between humans, since the users rarely notice the slight degradation, but this leads to problems when sending data that has very low error tolerance, such as information transmitted using a modem
.
In-band signaling is insecure because it exposes control signals, protocols and management systems to the user(s), which may result in falsing
. In the case of the blue box
es that were popular in the 1960s and 1970s, such falsing was deliberate. By using blue boxes to generate the appropriate tones, a caller could abuse functions intended for testing and administrative use to make free long-distance calls.
Modems may also interfere with in-band signaling, so in some countries, a guard tone
is employed to prevent this.
broadcasters
to indicate the start and stop times of local insertion
points during station breaks for the benefit of cable companies. Until better, out-of-band signaling equipment was developed in the 1990s, fast, unacknowledged, and loud DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere.
These DTMF sequences were sent by the originating cable network's equipment at the uplink
satellite facility, and were decoded by equipment at local cable companies. A specific tone sequence indicated the exact time that the feeds should be switched to and away from the master control feed, to locally-broadcasted commercials.
An example of a cable company DTMF sequence code would communicate the following to the cable company's broadcast equipment:
DTMF signaling in the cable industry went away because it was distracting to viewers, it was susceptible to interference when DTMF tones were sounded by characters in television shows (a character dialing a Touch-Tone telephone in a television show might convince the cable company computers to switch away from a "hot feed" to dead air
), and the cost of human-imperceptible signaling technologies decreased.
In-band signaling applies only to Channel Associated Signaling
(CAS). In Common Channel Signaling
(CCS) separate channels are used for control and data, as opposed to the shared channel in CAS, so all control is out-of-band by definition.
In computer programming, an example of in-band signaling are magic number
s, used for signaling of file formats.
When out-of-band
communication is unavailable, one of two techniques may be used to preserve network transparency
.
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
and control information in the same band or channel used for data.
Telephone
For example, when dialing a modern telephoneTelephone
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...
, the 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...
is encoded and transmitted across the telephone line as Dual-Tone Multi-Frequency (DTMF) tones. The tones "control" the telephone system by instructing the telephone company's equipment where to route the call to. These control tones are sent over the same channel
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...
and in the same band (300Hz to 3.4kHz) as the voice and other sounds of the telephone call. In-band signaling is also used on older telephone carrier systems to provide inter-exchange
Interexchange carrier
An Interexchange Carrier is a U.S. legal and regulatory term for a telecommunications company, commonly called a long-distance telephone company, such as MCI , Sprint and the former AT&T in the United States...
information on how to route calls. Examples of this kind of in-band signaling system are SS5 and R2
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....
.
Separating the control signals, also referred to as the control plane, from the data (if a bit-transparent
Transparency (telecommunication)
In telecommunications, transparency can refer to:#The property of an entity that allows another entity to pass through it without altering either of the entities....
connection is desired) is usually done by escaping
Escape sequence
An escape sequence is a series of characters used to change the state of computers and their attached peripheral devices. These are also known as control sequences, reflecting their use in device control. Some control sequences are special characters that always have the same meaning...
the control instructions. Occasionally, however, networks are designed so that data is (to a varying degree) garbled by the signaling. Allowing data to become garbled is usually acceptable when transmitting sounds between humans, since the users rarely notice the slight degradation, but this leads to problems when sending data that has very low error tolerance, such as information transmitted using a modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
.
In-band signaling is insecure because it exposes control signals, protocols and management systems to the user(s), which may result in falsing
Falsing
In telecommunications, falsing describes a decoder assuming that it is detecting a valid input when one is not present. This is also known as a false decode...
. In the case of the blue box
Blue box
An early phreaking tool, the blue box is an electronic device that simulates a telephone operator's dialing console. It functioned by replicating the tones used to switch long-distance calls and using them to route the user's own call, bypassing the normal switching mechanism...
es that were popular in the 1960s and 1970s, such falsing was deliberate. By using blue boxes to generate the appropriate tones, a caller could abuse functions intended for testing and administrative use to make free long-distance calls.
Modems may also interfere with in-band signaling, so in some countries, a guard tone
Guard tone
Guard tone is a feature of wireline modems.The guard tone is sent by the answering modem after it has sent the answer tone. It is a single continuous tone with a frequency of either 1800 Hz or 550 Hz, sent at a level of -6dB or -3dB, respectively, below the level of the transmitted data...
is employed to prevent this.
Non telephone
As a method of in-band signaling, DTMF tones were also used by cable televisionCable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
broadcasters
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
to indicate the start and stop times of local insertion
Local insertion
In broadcasting, local insertion is the act or capability of a broadcast television station, radio station, or cable TV system to insert or replace part of a broadcast network feed with content unique to the local station or system...
points during station breaks for the benefit of cable companies. Until better, out-of-band signaling equipment was developed in the 1990s, fast, unacknowledged, and loud DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere.
These DTMF sequences were sent by the originating cable network's equipment at the uplink
Uplink
A telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites to connect two points on earth.-Uplink:...
satellite facility, and were decoded by equipment at local cable companies. A specific tone sequence indicated the exact time that the feeds should be switched to and away from the master control feed, to locally-broadcasted commercials.
An example of a cable company DTMF sequence code would communicate the following to the cable company's broadcast equipment:
SWITCH TO LOCAL NOW - SWITCH TO LOCAL NOW - PREPARE TO SWITCH BACK - PREPARE TO SWITCH BACK - SWITCH BACK TO NATIONAL NOW - SWITCH BACK TO NATIONAL NOW - "IF YOU HAVEN'T SWITCHED BACK TO NATIONAL NOW, DO SO IMMEDIATELY"
DTMF signaling in the cable industry went away because it was distracting to viewers, it was susceptible to interference when DTMF tones were sounded by characters in television shows (a character dialing a Touch-Tone telephone in a television show might convince the cable company computers to switch away from a "hot feed" to dead air
Dead air
Dead air is an unintended interruption in a radio broadcast during which no sound is transmitted.The term is most often used in cases where program material comes to an unexpected halt, either through operator error or for technical reasons, although it is also used in cases where a broadcaster...
), and the cost of human-imperceptible signaling technologies decreased.
In-band signaling applies only to Channel Associated Signaling
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...
(CAS). In 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) separate channels are used for control and data, as opposed to the shared channel in CAS, so all control is out-of-band by definition.
In computer programming, an example of in-band signaling are magic number
Magic number (programming)
In computer programming, the term magic number has multiple meanings. It could refer to one or more of the following:* A constant numerical or text value used to identify a file format or protocol; for files, see List of file signatures...
s, used for signaling of file formats.
When out-of-band
Out-of-band
The term out-of-band has different uses in communications and telecommunication. In case of out-of-band control signaling, signaling bits are sent in special order in a dedicated signaling frame...
communication is unavailable, one of two techniques may be used to preserve network transparency
Transparency (telecommunication)
In telecommunications, transparency can refer to:#The property of an entity that allows another entity to pass through it without altering either of the entities....
.
- EncapsulationEncapsulation (networking)In computer networking, encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects....
: The bundling of the control data in the packet's headerHeader (information technology)In information technology, header refers to supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the header are sometimes called the payload or body....
and then removing the header (and/or footer) of the packet at the far end, restoring the data to be the same as the original. - Bit StuffingBit stuffingIn data transmission and telecommunication, bit stuffing is the insertion of noninformation bits into data...
: The insertion of noninformation or escape characterEscape characterIn computing and telecommunication, an escape character is a character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacters...
s to modify, synchronize and justify the data so it never looks like signaling information (and remove the stuffed bits and escape codes at the far end, restoring the data to be the same as the original).
See also
- Control characterControl characterIn computing and telecommunication, a control character or non-printing character is a code point in a character set, that does not in itself represent a written symbol.It is in-band signaling in the context of character encoding....
- Escape sequenceEscape sequenceAn escape sequence is a series of characters used to change the state of computers and their attached peripheral devices. These are also known as control sequences, reflecting their use in device control. Some control sequences are special characters that always have the same meaning...
- Out-of-band signaling
- Line signalingLine signalingLine 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...
- Quindar tonesQuindar tonesQuindar Tones, most often referred to as the "beeps" that were heard during the American Apollo space missions, were a means by which remote transmitters on Earth were turned on and off so that the Capsule communicator could communicate with the crews of the spacecraft...