In-band signaling

In telecommunications, in-band signaling is the sending of control information within the same band or channel used for voice or video. This is in contrast to out-of-band signaling 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.

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, the copper wire, and in the frequency range (300Hz to 3.4kHz) as the audio of the telephone call. In-band signaling is also used on older telephone carrier systems to provide inter-exchange information for routing calls. Examples of this kind of in-band signaling system are the Signaling System No. 5 (SS5) and R2 signalling.

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 end users, which may result in falsing. In the 1960s and 1970s, so-called phone phreaks used blue boxes 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.

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