In telecommunications, in-band signaling is the sending of metadata and control information in the same channel used for data.

For example, when dialing a modern telephone, the telephone number is encoded and transmitted across the telephone line as 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 that carries the voice and other sounds of the telephone call. In-band signalling 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 signalling system are SS5 and R2.

Separating the signaling 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 may be insecure because it can expose control systems to the user, as in the case of the blue boxes that were popular in the 1960s and 1970s. 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.

In computer programming, magic numbers are used for in-band signaling of file formats.

See also

• Out-of-band signaling

External links

• InBandSignal from the Portland Pattern Repository

Telecommunications