C.3 TFO_TRAU

28.0623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS

The following clauses describe the actions within the TRAU to establish and maintain Tandem Free Operation in terms of a State Machine, respectively TFO Processes, handling synchronisation and protocol. The description of the TFO Protocol does not reflect implementation details for the I/O Processes (Rx_TRAU, Tx_TRAU, Tx_TFO, and Rx_TFO), but they may need to be considered for the exact understanding of the behaviour. Only the TFO_Protocol Process is detailed, which is responsible for the handling of the TFO Protocol.

The TFO_TRAU can be regarded as consisting of five processes, which are strongly coupled to each other, which run in parallel, but phase shifted. The TFO_Protocol Process communicates with the TFO I/O processes and, optionally, with its corresponding process within the BSS (TFO_BSC and/or TFO_BTS) to resolve Codec Mismatch, see Figure C.3-1.

Under normal circumstances (exceptions occur during time alignments or octet slips) all TFO I/O Processes are triggered every 160 samples or every speech frame of 20 ms. All events and actions are quantized in time into these smallest intervals.

It can be assumed that the processing times for the TFO Processes are very short and negligible. However, it must be ensured that no timing ambiguity occurs between the Processes.

This means the processing and exchange of information between them do not overlap in time. Care must be taken especially when time alignment occurs, which may be independent in uplink and downlink.

During these time alignments the TFO Frames or TFO Messages may shift in time and consequently the triggering point for the related TFO Processes changes, too.

Figure C.3-1: The five TFO_TRAU Processes