Comfort Noise is 'realistic' background acoustic noise that is inserted at a receiver during periods that there is no signal received. Comfort noise is used in association with discontinious transmission (DTX).
DTX means that a transmitter is switched off during silent periods. Therefore, the background acoustic noise abruptly disappears at the receiving end. This can be very annoying for the receiving party. The receiving party might even think that the line is dead if the silent period is rather long. To overcome these problems 'comfort noise' is generated at the receiving end whenever the transmission is switched off. The comfort noise is generated by a Comfort Noise Generator (CNG). To generate 'realistic' acoustic background noise the DTX module sends Silence Insertion Descriptor (SID) frames periodically to the CNG module at the receiving end.
If the comfort noise is well matched to that of the transmitted background acoustic noise during speech periods, the gaps between speech periods can be filled in such a way that the receiving party does not notice the switching during the conversation. Since the noise constantly changes, the comfort noise generator should be updated regularly.