In-band signaling

Main article:Signaling (telecommunications)

Intelecommunications,in-band signalingis the sending ofcontrol informationwithin the same band or channel used for data such as voice or video. This is in contrast toout-of-band signalingwhich 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, andmetadataand/or instructions for how to process the literal data.

Telephony

When dialing from a land-linetelephone,thetelephone numberis encoded and transmitted across the telephone line in form ofdual-tone multi-frequency signaling(DTMF). The tones control the telephone system by instructing thetelephone switchwhere to route the call. These control tones are sent over the samechannel,the copper wire, and in the frequency range (300 Hz to 3.4 kHz) as the audio of the telephone call. In-band signaling is also used on older telephone carrier systems to provideinter-exchangeinformation for routing calls. Examples of this kind of in-band signaling system are theSignaling System No. 5(SS5) and its predecessors, andR2 signalling.

Separating the control signals, also referred to as the control plane, from the data, if abit-transparentconnection is desired, is usually done byescapingthe 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 amodem.

In-band signaling is insecure because it exposes control signals, protocols and management systems toend users,which may result infalsing.In the 1960s and 1970s, so-calledphone phreaksusedblue boxesfor 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 aguard tonemay be employed to prevent this.

Voice over IP

Invoice 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 asμ-laworA-lawpulse-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 theReal-time Transport Protocol(RTP) voice stream and encode them separately as a digital information payload, often termednamed telephone events(NTE), according to RFC 4733. Such DTMF frames are transmit in-band with all other RTP packets on the identical network path.

In contrast to in-band transmission of DTMF, VoIP signaling protocols also implementout-of-bandmethod of DTMF transmission. For example, theSession Initiation Protocol(SIP), as well as theMedia 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 bycable televisionbroadcastersto indicate the start and stop times oflocal insertionpoints during station breaks for the benefit of cable companies. Until better,out-of-band signalingequipment 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 theuplinksatellite 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:

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 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" todead air,and the cost of human-imperceptible signaling technologies decreased.

In-band signaling applies only tochannel-associated signaling(CAS). Incommon 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 ofmetadatadirectly within regular data. These uses have similar tradeoffs as in telecommunications, such as opening anattack surfacevs. simplifying processing. A few of many examples:

• Embedding amagic numberat 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).
• Embeddingmarkupwithin text, whether to categorize parts of the text, provide processing or formatting instructions, or for other purposes.
• Reserving some characters inregular expressions,such as "*", to have special processing meanings, rather than representing literals.
• Embeddingcontrol codesincomputer terminalinput as a means of device control, allowing command-line users to issue single-character commands directly, e.g.Ctrl+dissues a^Dcode, causing command-line programs to expect no further input from the user, and therefore to quit.

Whenout-of-bandcommunication is unavailable, one of two techniques may be used to preserve networktransparency.

• Encapsulation:The bundling of the control data in thepacket'sheaderand 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 orescape charactersto 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)