08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
IS Messages replace some bits of the PCM samples and therefor cause a minimal signal distortion. Therefore IS Messages shall be used with care and not longer than necessary. The IS Protocol is kept to a minimum to avoid unnecessary complexity. One basic assumption is that only one IS Protocol is active at a time between two IS Partners.
Only specific telecommunication entities shall be allowed to initiate IS Protocols. They are called IS_Active or active IS Partners. In principle these shall only be terminal devices or their "representatives" within the network. Examples are ISDN-Terminals, Speech-Servers, TRAUs (in GSM as representatives of the MSs).
Other telecommunication entities shall only react on IS Protocols. They are called IS_Passive. Most IPEs are of this type. They bypass the IS Messages, they obey the IS_IPE Messages, but they never initiate IS Messages.
Other telecommunication entities are IS_Passive by default. But if they receive IS Protocols that they can understand, then they may become IS_Active and start to initiate IS Protocols. They thus become active IS Partners and shall take care that only one IS Protocol is active on both of their sides. They are called IS_Responsive. Examples are GCMEs.
Active IS Partners shall send
either continuous sequences of IS Messages without interruption of the 16_PCM_Sample_Grid:
– or isolated IS Messages with same message lengths;
– or isolated IS Messages with sufficient distance between them, if shorter IS Messages follow longer IS Messages.
The latter case is important, because shorter isolated IS Messages travel faster through IPEs than longer ones, see annex B.
As said above, after initialization of an IS Message sequence, no interruption of the 16_PCM_Sample_Grid shall occur within the sequence. Adjustments of the phase position of the Keep_Open_Indication shall be done only after the IS_TRANS Message by inserting the necessary number n (with 0 n 160) of "1" Bits (termed "T_Bits") into the LSBs of the PCM samples that have to be skipped. The first PCM sample for this insertion of T_Bits is the one where the next regular IS Message or next regular Keep_Open_Indication would begin. At the new phase position the next IS Message or the IS_FILL Message shall be sent, to allow IPEs to resynchronize fast. See Figure 25.
Figure 25: Phase Shift of the 16_PCM_Sample_Grid by inserting T_Bits
Similarly, the adjustment of the phase between two Keep_Open_Indications shall be done by inserting the necessary number of T_Bits and by sending an IS Message – preferably, but not necessarily – the IS_FILL.
Finally a "negative" phase adjustment between two Keep_Open_Indications shall be allowed by shortening the cycle by a maximum of 2 PCM samples and sending an IS Message (see above) at the new phase position.