(Or CAS, channel associated signaling) Transmission of control signals in the same channel as data. This is commonly used in the Public Switched Telephone Network where the same pair of wires carry both voice and control signals (e.g. dialling, ringing). Another example is the use on a computer serial line of Control-S and Control-Q characters for flow control as opposed to hardware flow control which would be out-of-band signalling.
In digital communications, in-band signalling often uses "bit-robbing" where, for example, one bit in each frame is used for signalling instead of data. This is the reason why a D1 channel in the T-carrier system can only carry 56 Kbps of usable data instead of the 64 Kbps carried by the D0 channel in the E-carrier system.