08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
The transmission delay introduced by an IPE for the speech, audio or modem signal is in general different in Normal_Mode and Transparent_Mode. Some IPEs may have several different Normal_Modes with possibly different signal delays. IS Messages are transmitted within the regular 16_PCM_Sample_Grid. It is important that this regularity is not disturbed. Therefor care must be taken at the transition between these modes.
The transmission delay of a specific IPE is in general lower for IS Messages than for speech, audio or modem signals.
B.5.1 IPE Transmission Delay in Normal_Mode
The delay for IS Messages in Normal_Mode shall be identical to the delay in that Transparent_Mode, that follows after the first IS_TRANS Message. If different Transparent_Modes with different delays could follow, then the shortest delay of all possible Transparent Modes shall be selected for IS Messages in Normal_Mode.
If an IPE in Normal_Mode has to change its transmission delay, then this shall not affect the delay of the IS Messages.
B.5.2 IPE Transmission Delay in Transparent_Mode
In the majority of all cases the IPE will keep the transmission delay for the IS Messages in Normal_Mode also in Transparent_Mode for the transmission of the commanded transparent LSBs. IPEs which do not understand the IS Protocol shall never modify the transparent bits, so they are also not allowed to change delay.
Some IPEs, which understand a specific IS Protocol, may have even different Transparent_Modes and also here the transmission delays may differ. Examples are GCMEs.
If an IPE has to change its transmission delay at the transition from Normal_Mode to Transparent_Mode, then the IPE shall readjust the phase of the Keep_Open_Indication after transition into the Transparent_Mode with higher delay by inserting the relevant number of T_Bits after the first IS_TRANS Message and before the next IS Message. If no other IS Message is following, then the IS_FILL shall be inserted, obeying all other relevant rules of the specific IS Protocol (e.g. EMBED bit C5 in TFO Frames).
If an IPE has to change from one Transparent_Mode to an other one with a different transmission delay, then the IPE shall readjust the phase of the Keep_Open_Indication after transition into the new Transparent_Mode by inserting the relevant number of T_Bits. If no other IS Message is following, then the IS_FILL shall be inserted at the new phase position to mark the new grid position of the 16_PCM_Sample_Grid and to allow other IPEs to resynchronize, obeying all other relevant rules of the specific IS Protocol (e.g. EMBED bit C5 in TFO Frames).