Generally, any signaling transmission that takes place within a range of frequencies that are normally used only for data transmission. Instead of using separate control and data channels, control information is transmitted using a portion of the data channel. If a separate control channel is used, the service is called out-of-band signaling.
For example, in-band signaling is used in switched 56 services, in which a 64-Kbps digital communication link has 8 Kbps set aside for control signaling. This is sometimes referred to as “robbed-bit signaling” because the 8-Kbps bandwidth is “robbed” from the data channel for handling control functions such as wide area network (WAN) link synchronization. T1 lines that use switched channels also use in-band signaling techniques.
Graphic I-1. Comparison with out-of-band signaling.