08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
The time alignment procedures for the downlink TRAU Frames, as specified in GSM 08.60 (full rate traffic) and GSM 08.61 (half rate traffic) on the Abis/Ater interface, are not affected by the TFO procedures on the A interface. The relative TRAU Frame phase positions of the two TRAUs using TFO across the A interface are arbitrary and depend on the local timing structure of the relevant BTSs. This is not changed by the TFO Protocol.
TFO Frames and embedded TFO Messages are always exactly aligned with each other and follow the uplink TRAU Frames with a small, neglegible, constant delay (Tultfo: some PCM samples).
7.1 Time Alignment of TFO Messages
At start up of the TFO Protocol the first regular TFO Message is aligned to an uplink TRAU Frame in the same way as a TFO Frame, respectively an embedded TFO Message would be aligned (see subclause 7.2). Then, after that, all regular TFO Messages follow contiguously, without any phase shift in time alignment, until the first TFO Frame needs to be sent (in general after the TFO_TRANS Message). Then the necessary number of T_Bits (if any) is inserted before the first TFO Frame, see subclause 7.2. Consequently all following, embedded TFO Messages are always aligned with the TFO Frames in a way, that the first bit of any TFO Messages is placed into the LSB of the first sample of a TFO Frame. Due to this definition, embedded TFO Messages only modify some of the synchronisation bits of the TFO Frames and control bit C5.
7.2 Time Alignment of TFO Frames
The contents of the Uplink TRAU Frame, received from the BTS via the Abis/Ater Interface, undergo the small, constant delay (Tultfo) required to perform the modifications of the C5 and Sync bits, before being forwarded to the other TRAU over the A Interface as TFO Frame. Since this delay is substantially smaller than the delay for the decoded speech signal, the TFO Frames preceed the corresponding speech samples. Figure 4 shows the relations. Please note that no exact delay value for Tultfo is defined or need to be defined.
Figure 4: Uplink TFO Frame Time Alignment
On the transition between the sending of regular TFO Messages and the first TFO Frame on the A interface, a sufficient number (up to a maximum of 159) of Time Alignment Bits, also called "T_Bits", are inserted into the LSBs of the PCM samples to align the TFO Frame as described above.
This insertion of Time Alignment Bits (if necessary) is started exactly with the 16th PCM sample after the last bit of the last regular TFO Message (i.e. the TFO_TRANS Message).
Whenever, in a later stage, the phase of the uplink TRAU Frame changes, then again T_Bits need to be inserted between two consecutive TFO Frames or deleted from the tail of the last TFO Frame to ensure proper alignment.
The insertion of T_Bits as a result of timing changes shall occur between TFO Frames and not within TFO Frames.
If the time alignment is necessary while a TFO Message is embedded into a series of TFO Frames, then the TFO Message may be cut into two parts with the T_Bits in between. Therefore, whenever an adjustment of the phase of the TFO Frames is necessary, then one additional TFO Message shall be embedded into the next TFO Frames (after the possibly ongoing TFO Message). If nothing else is to be transmitted, then the TFO_FILL Message shall be used. One TFO_TRANS Message is always embedded into the first TFO Frames. See the following Figure 5:
Figure 5: Time Alignment by inserting T_Bits and embedding one TFO_TRANS Message
7.3 Time Alignment of TFO Frames to Downlink TRAU Frames
The phase position of the downlink TRAU Frames is not affected by the TFO Protocol.
The phase difference between the received TFO Frames and the downlink TRAU Frames is in general constant, but arbitrary between 0 and 159 PCM samples. The time alignment of the TFO Frames to the downlink TRAU Frames must therefore be managed by buffering the TFO Frames within the receiving downlink TRAU. This can be done in one of two methods:
The received TFO Frame is buffered for a period between 0 to 159 PCM samples in addition to the processing delay (Tbfh) required to perform a suitable Bad Frame Handling on parameter level. Transmission of the downlink TRAU Frame may in this case begin prior to receipt of the complete TFO Frame.
NOTE 1: In this first method the overall one way signal delay will be between 30 ms and 10 ms lower than the delay in normal tandem connections.
Alternatively the received TFO Frame is buffered for a period between 160 to 319 PCM samples in addition to the processing delay required to perform a suitable Bad Frame Handling on parameter level (Tbfh). Transmission of the downlink TRAU Frame will in this case always begin after the receipt of the complete TFO Frame.
NOTE 2: In this second method the overall one way signal delay will always be up to 10ms lower or up to 10 ms higher than the delay in normal tandem connections.
NOTE 3: The two methods differ in one way signal delay always by exactly 20 ms. Figure 6 highlights the relations for an arbitrarily selected relative phase difference between TFO and TRAU Frames of 80 samples (10 ms). Tbfh is in the order of some PCM samples only, if error concealment is done "in advance" based on the parameters of the previous TFO Frame, before the actual TFO Frame is even received.
Figure 6: Downlink Time Alignment of TFO Frames