Background noise is the inherent noise of the system, and it occurs when no useful signal is reproduced. Background noise is not due to noise from space, but is generated by stochastic processes of current flow in active and passive components.
Background noise is uncoordinated temperature-induced electron movements in the passive and active components in the equipment. In acoustic equipment, in amplifier systems, radios and telephone networks, background noise is audible; in television transmissions, it is visible; the television picture is affected by interfering bright-dark points (snow).
In telephone systems and in mobile phone networks, a synthetically generated comfort noise is inserted during pauses in speech when no noise can be heard. This background noise, which is generated by a Comfort Noise Generator( CNG), is intended to prevent the person making the call from being irritated by the absolute silence in the transmission line and assuming that the connection has been interrupted. The transmission technique is called Discontinuous Transmission( DTX).