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((Short description|Sending control flow information in the same channel as data))
((main|Signaling (telecommunications)))
((multiple issues|
((More citations needed|date=May 2017))
((expert needed|Telecommunications|date=April 2017|reason=Too telecomm-specific, need broader theoretical perspectives.))
))

In [[telecommunications]], '''in-band signaling''' is the sending of [[control signal|control information]] within the same band or channel used for data such as voice or video. This is in contrast to [[out-of-band signal]]ing 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.<ref>((Cite web|title=Definition of signaling in/out-of-band|url=https://www.pcmag.com/encyclopedia/term/signaling-inout-of-band|access-date=2021-10-17|website=PCMAG|language=en))</ref> 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.

==Telephony==
When dialing from a land-line [[telephone]], the [[telephone number]] is encoded and transmitted across the telephone line in form of [[dual-tone multi-frequency signaling]] (DTMF). The tones control the telephone system by instructing the [[telephone switch]] where to route the call. These control tones are sent over the same [[channel (communications)|channel]], the copper wire, and in the frequency range (300&nbsp;Hz to 3.4&nbsp;kHz) as the audio of the telephone call. In-band signaling is also used on older telephone carrier systems to provide [[Interexchange carrier|inter-exchange]] information for routing calls. Examples of this kind of in-band signaling system are the [[Signaling System No. 5]] (SS5) and its predecessors, and [[R2 signalling]].<ref>((Cite book |url=https://books.google.com/books?id=YH_LBQAAQBAJ&q=signaling%2520system%2520ss4&pg=PA844 |title=The Telecommunications Illustrated Dictionary, Second Edition |last=Petersen |first=J. K. |date=2002-05-29 |publisher=CRC Press |isbn=9781420040678 |pages=844 |language=en))</ref><ref>((Cite book |url=https://books.google.com/books?id=VfPST6NEggEC&q=ccitt%2520ss1&pg=SA4-PA8 |title=Telecommunication Switching Systems and Networks |last=V.S.Bagad |date=2009 |publisher=Technical Publications |isbn=9788184315905 |pages=388 |language=en))</ref><ref>((Cite web |url=https://www.cisco.com/c/en/us/support/docs/voice/digital-cas/5717-e1-r2-sig.html |title=E1 R2 Signaling Theory |website=Cisco |language=en |access-date=2018-01-05))</ref>

Separating the control signals, also referred to as the control plane, from the data, if a [[Transparency (telecommunication)|bit-transparent]] connection is desired, is usually done by [[Escape sequence|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 [[end user]]s, which may result in [[falsing]].  In the 1960s and 1970s, so-called ''[[Phreaking|phone phreaks]]'' used [[blue box]]es for deliberate falsing, in which the appropriate tones for routing were intentionally generated, enabling the caller to abuse functions intended for testing and administrative use and to make free long-distance calls.

Modems may also interfere with in-band signaling, in which case a [[guard tone]] may be employed to prevent this.

==Voice over IP==
In [[voice over IP]] (VoIP), DTMF signals are transmitted in-band by two methods. When transmitted as audio tones in the voice stream, voice encoding must use a lossless coder, such as [[μ-law]] or [[A-law]] [[pulse-code modulation]], to preserve the integrity of frequency signals. Still, this method proved often unreliable and was subject to interference from other audio sources.  The standard method is to digitally remove DTMF tones from the audio at the source and from the [[Real-time Transport Protocol]] (RTP) voice stream and encode them separately as a digital information payload, often termed ''named telephone events'' (NTE), according to RFC 4733. Such DTMF frames are transmit in-band with all other RTP packets on the identical network path.<ref>RFC 4733, ''RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals'', Schulzrinne, Tayler (2006)</ref>

In contrast to in-band transmission of DTMF, VoIP signaling protocols also implement [[out-of-band]] method of DTMF transmission. For example, the [[Session Initiation Protocol]] (SIP), as well as the [[Media Gateway Control Protocol]] (MGCP) define special message types for the transmission of digits.

==Other applications==
As a method of in-band signaling, DTMF tones were also used by [[cable television]] [[Broadcasting|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-broadcast commercials. The following is an example of such a sequence by a cable company that communicated the following to the cable company's broadcast equipment:

<blockquote>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"</blockquote>

DTMF signaling in the cable industry was discontinued because it was distracting to viewers, and was susceptible to interference when DTMF tones were sounded by characters in television shows. For example, a character dialing a Touch-Tone telephone in a television show could cause 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 data, the term refers to embedding any kind of [[metadata]] directly within regular data. These uses have similar tradeoffs as in telecommunications, such as opening an [[attack surface]] vs. simplifying processing. A few of many examples:

* Embedding a [[magic number (programming)|magic number]] at the very start of files, to signal the format or language of the following data. 
* Embedding a NULL character as in C strings, to signal the end of the string (as opposed to keeping that information outside the string).
* Embedding [[Markup language|markup]] within text, whether to categorize parts of the text, provide processing or formatting instructions, or for other purposes.
* Reserving some characters in [[regular expressions]], such as "*", to have special processing meanings, rather than representing literals.
* Embedding [[C0 and C1 control codes|control codes]] in [[computer terminal]] input as a means of device control, allowing command-line users to issue single-character commands directly, e.g. ((Key press|Ctrl|d)) issues a <code>[[End-of-transmission character|^D]]</code> code, causing command-line programs to expect no further input from the user, and therefore to quit.

When [[Out-of-band data|out-of-band]] communication is unavailable, one of two techniques may be used to preserve network [[transparency (telecommunication)|transparency]].
* [[Encapsulation (networking)|Encapsulation]]: The bundling of the control data in the [[packet (information technology)|packet]]'s [[header (computing)|header]] 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 stuffing]]: The insertion of non-information or [[escape character]]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 character]]
* [[Escape sequence]]
* [[In-band control]]
* [[Line signaling]]
* [[Out-of-band control]]
* [[Quindar tones]]
* [[+++ (modem)]]

==References==
((reflist))

[[Category:Network management]]
[[Category:Telephony signals]]

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