06.213GPPHalf rate speechSubstitution and muting of lost frames for half rate speech traffic channelsTS
The RSS indicates lost speech or SID frames by setting the Bad Frame Indication (BFI) flag and unreliably decoded speech or SID frames by setting the Unreliable Frame Indication (UFI) flag.
If the BFI flag is set, the speech decoder performs frame substitution and muting of the speech output. The purpose of frame substitution is to conceal the effect of lost frames. The purpose of muting the speech output in case of several lost frames is to indicate the breakdown of the channel to the Mobile Station (MS) user in a way that avoids excessively unpleasant sounds.
If the UFI flag is set, the speech decoder performs a plausibility analysis of the received frame parameters and of the output signal aiming at the detection and concealment of erroneous frames which are not marked with the BFI flag.