The TFO/TrFO function is used to prevent tandeming codecs in the traditional UE-UE calling procedure.

The traditional UE-UE calling procedure includes two coding operations and two decoding operations. When the calling UE and the called UE use the same AMR rate set, the TrFO function enables compressed speech frames to be directly transmitted between the two UEs, without the need for the second speech coding/decoding.

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