In-Band versus Out-Of-Band

In-Band versus Out-Of-Band
In the public switched telephone network, (PSTN), in-band signalling is the exchange of signalling (call control) information within the same channel that the telephone call itself is using. An example is DTMF 'Dual-Tone multi-frequency' signalling, which is used on most telephone lines to exchanges.
Out-of-band signalling is telecommunication signalling (exchange of information in order to control a telephone call) that is done on a channel that is dedicated for the purpose and separate from the channels used for the telephone call. Out-of-band signalling is used in Signalling System #7 (SS7), the standard for signalling among exchanges that has controlled most of the world's phone calls for some twenty years.