In-band signaling
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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 carry the voice and other sound of the telephone call.
Separating the signaling from the data (if a bit-transparent connection is desired), is usually solved by escaping the control instructions, but occasionally the network is designed so that data is (to a varying degree) garbled by the signaling. Letting the data get garbled is usually acceptable when transmitting voice between humans, since the users rarely notice the slight degradation of sound, but it leads to problems when transmitting binary data. An example of this kind of in-band signalling systems is SS5.
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Security Issues
In-band signaling may be insecure because it exposes the systems that control everything to the user. A blue box was a device that let people make free calls in the 1960s and 1970s. It manipulated the telephone company's long distance equipment because this equipment was controlled using in-band signaling. Phone phreaks simply played tones into the telephones receiver that could hang up lines, seize a long distance trunk line, and dial a number anywhere in the world.
Other uses
In computer programming, magic numbers are used for in-band signaling of file formats.