C.9 Location of the TFO Decision Algorithm

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The TFO Decision Algorithm as described in clause 11 and 12 shall always be located within the TRAU. Optionally it may in addition be located in the BTS (for Codec Configuration Optimisation) and the BSC (for Codec Type and Codec Configuration Optimisation).

C.9.1 Immediate TFO Set-up

The TFO Decision Algorithm shall always be within the TRAU. This is important and sufficient for Immediate TFO_Setup. It might be available also within the BTS, but that is not essential.

The TRAU shall inform the BTS with TFO_Soon, that Immediate TFO is possible (TFO_BTS into TFO_MAYBE).
The TRAU shall inform the BTS with CMR =< RCi about the allowed Rate Control.
The TRAU may send a Dis_Req to the BTS with the available distant configuration parameters, or, alternatively, with the Optimal Configuration Parameters.

Important is that the BTS shall acknowledge the TFO_Soon with TFO_Soon.
The TRAU shall wait in State WAIT_RC until the BTS has acknowledged. Then it shall start to send TFO_TRANS and TFO Frames.

When informed with TFO_Soon that Immediate TFO Setup is ongoing, the BTS shall not change the ACS on the air interface, but wait at least until in State TFO_YES.
The BTS shall restrict the rate adaptation within the limits given by the TRAU within the downlink CMR.
The TRAU shall release the rate control when in state "Operation" to the rates within the common ACS.

C.9.2 Codec Configuration Optimisation

The TFO Decision Algorithm shall always be within the TRAU. The TRAU shall inform the BTS either about the distant Codec Configuration or, alternatively the optimal Codec Configuration (defined by the OD parameter).

In the first case the BTS shall also run the TFO Decision Algorithm (again) to determine the optimal Configuration.
In the second case the TFO decision Algorithm is not needed within the BTS.

If authorised so by the BSC the BTS shall perform Codec Configuration Modification by RATSCCH in State TFO_NO (for Mismatch Resolution) or in State TFO_YES (for Optimisation). The BTS shall inform the BSC hereafter.

If not authorised by the BSC, or if the Codec Type has to be modified in addition, the BTS shall not perform any modifications, but only inform the BSC.

C.9.3 Codec Type Optimisation

In the first case the BTS shall also run the TFO Decision Algorithm (again) to determine the optimal Configuration.
In the second case the TFO decision Algorithm is not needed within the BTS.

If the Codec Type has to be modified, the BTS shall not perform any modifications, but only inform the BSC, either by sending the distant Configuration or, alternatively the optimal Configuration.

In the first case the BSC has to run the TFO Decision Algorithm (again), in the second case the TFO Decision Algorithm is not needed within the BSC.

The BSC shall perform a necessary Codec Type Modification or Codec Configuration Modification, when it had set the Configuration parameters accordingly (Codec_List contains more than the Active Codec Type, the Optimisation_Mode is set to "Change").

Annex D (normative):
Tandem Free Operation in 3G including AMR-WB