there are different parameters (vendor dependent, i believe) that can control this behaviour.
The MS has a certain physical capability to handle FR and/or HR. This is known by the BSC. If a call is in HR and the MS can support both FR and HR, there is no restriction in 3GPP : a HO can be made to a HR timeslot or a FR timeslot.
In alcatel, a MS in HR that does a HO will be sent to FR or HR depending on current load conditions in the target cell, and depending on TFO activation or not.
TFO is “Tandem Free Operation”.
It permits the adaptation of the UL codec of the caller with the DL codec of the called and vice versa. In this case the transcoding on TRAU Client can be avoided. See 3GPP 28.062 specification.
yes, the same codec is used from one MS to the other MS, therefore there is no “transcoding” performed in the TC. For instance, 2 MS in EFR (12.2kb/s) –> the 12.2kb/s are not coded into 64kb/s and then back to 12.2kb/s.
The TFO can be used only during MS-MS call.
Benefit: comparing one codec with or without TFO, you’ll notice a better voice quality with TFO.
Constraint: specific parameters shall be tuned, in order to manage the strategy of TFO vs. AMR and TFO vs. HR codecs.