Every communications system transmits data and control signals. The data are the reason for the transmission in the first place, and the control signals start, stop and manage the session.
When the data and control signals are transmitted within the same channel or frequency, the signaling is said to be "in-band." For example, an analog modem transmits control signals and data in the same human voice frequency band (see Hayes Smartmodem
When control signals reside in a channel separate from the data, they are "out-of-band" signals. For example, in ISDN service, the D channel is a dedicated channel for control signals, and the B channels carry the data (see CCS
). The traditional SS7 telephone system uses an entirely separate network for control signals (see AIN