08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
In Transparent_Mode, i.e. after properly receiving an IS_TRANS Message, all IPEs shall monitor the bypassing bit stream for the Keep_Open_Indication (definition see below). If this Keep_Open_Indication is not seen for some time, then the IPEs shall fall automatically back into normal operation, i.e. the mode of operation before the IS_TRANS Message.
This automatic fall back shall have the same effect as the IS_NORMAL Message would have.
By definition the Keep_Open_Indication is a continuous bit stream of one "0"-Bit in the LSB of every 160th PCM sample, i.e. every 20 ms. At least one "1"-Bit must be present within the LSBs of the other 159 PCM samples. See Figure 24.
Figure 24: Keep_Open_Indication
The "0"-Bit stream of the Keep_Open_Indication shall always be present as long as the IPEs need to be in Transparent_Mode.
The Keep_Open_Indication shall be in phase with the preceding IS Messages., i.e. the first bit of the Keep_Open_Indication shall be in the position of the first bit of the (hypothetical) next IS Message. In fact, the IS Messages themselves contain this Keep_Open_Indication by definition.
In case of a known phase shift of the Keep_Open_Indication, the IS_Sender has to send at least one IS Message, which defines the new phase position of the Keep_Open_Indication. If no other IS Message is to be sent, then the IS_FILL Message shall be used. If an IS Message longer than 160 ms is scheduled for transmission, then an IS_FILL Message should be inserted before, to guarantee fast resynchronization of the IPEs.