The procedure dates back to the time before the liberalization of the telecom market and is thus well before the time of flat rates, which makes this procedure obsolete. However, it is still listed for the sake of completeness.
The Digital Speech Interpolation method works with speech pause detection( VAD). During the pauses in speech in a telephone call, further speech data is transmitted in the DSI procedure. With this procedure, the speech capacity of a transmission line could be doubled. Digital Speech Interpolation is used in Enhanced Time Division Multiple Access( E-TDMA).