F.3 Usage of AMR Tandem compared to TFO with GSM_EFR, GSM_FR, or GSM_HR

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

Guideline 7:

If an AMR is the Active Codec Type and the operator prefers a tandeming connection with this AMR Codec on one side to a tandem free connection with GSM_EFR, GSM_FR or GSM_HR, then he should not include GSM_EFR, GSM_FR or GSM_HR within the supported Codec list.

Reason: The TFO algorithm will always try to establish TFO with the best available common Codec Type, which could be GSM_EFR, GSM_FR or GSM_HR. But often a Tandem Connection including one AMR Codec Type may be preferable in terms of speech quality.