Out-of-band signal

In telecommunication, signaling (signalling in Commonwealth English) has the following meanings:

Signaling systems may be classified based on several principal characteristics.

In the public switched telephone network (PSTN), in-band signaling is the exchange of call control information within the same channel that the telephone call itself is using. An example is dual-tone multi-frequency signaling (DTMF), which is used on most telephone lines to customer premises.

Out-of-band signaling is telecommunication signaling on a dedicated channel separate from that used for the telephone call. Out-of-band signaling has been used since Signaling System No. 6 (SS6) was introduced in the 1970s, and also in Signalling System No. 7 (SS7) in 1980 which became the standard for signaling among exchanges ever since.

Line signaling is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (answer supervision and disconnect supervision, together referred to as supervision), ringing current (alerting), and recall. In the middle 20th century, supervision signals on long-distance trunks in North America were usually inband, for example at 2600 Hz, necessitating a notch filter to prevent interference. Late in the century, all supervisory signals were out of band. With the advent of digital trunks, supervision signals are carried by robbed bits or other bits in the E1-carrier dedicated to signaling.

...
Wikipedia