06.513GPPGSM Enhanced full rate speech processing functions: General descriptionTS
The Enhanced Full Rate speech frame substitution and muting function is described in GSM 06.61 .
In the receiver, frames may be lost due to transmission errors or frame stealing. GSM 06.61  describes the actions to be taken in these cases, both for lost speech frames and for lost SID frames in DTX operation.
In order to mask the effect of an isolated lost frame, the lost speech frame is substituted by a predicted frame based on previous frames. Insertion of silence frames is not allowed. For several subsequent lost frames, a muting technique shall be used to indicate to the listener that transmission has been interrupted.