06.613GPPSubstitution and muting of lost frames for encanced full rate speech traffic channelsTS
The purpose of frame substitution is to conceal the effect of lost frames. The purpose of muting the output in the case of several lost frames is to indicate the breakdown of the channel to the user and to avoid generating possible annoying sounds as a result from frame substitution procedure.
The RSS indicates lost speech or SID frames by setting its Bad Frame Indication flag (BFI) based on its 3‑bit and 8-bit CRCs and possibly other error detection mechanisms. The TRAU calculates from the CRCs inserted by the CCU in the TRAU frames one BSI_Abis flag for every sub-block of speech parameters. If either one or more of these flags is set, the speech decoder shall either perform frame substitution or subframe substitution.
The example solution provided in clause 6 applies only for bad frame handling on a complete speech frame basis. However some parts could be modified for substitution of bad sub-blocks.