F.2 Earliest possible TFO Establishment

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

Since speech quality is improved by TFO, it is important to establish TFO as soon as possible. This can be achieved by reducing / simplifying the TFO negotiation.

This leads to two categories of guidelines:

1) Immediate TFO establishment without Codec Mode Optimisation (TFO is established with the current ACS, or with a subset of this ACS).

2) Immediate TFO establishment with Codec Mode Optimisation (after TFO establishment the ACS may be changed by a) Intra Cell Hand-over, b) Mode Modify or c) RATSCCH).

F.2.1 Avoidance of Codec Mode Optimisation

Guideline 2:

If the operator wants to avoid Codec Mode Optimisation after TFO establishment with AMR, then he shall set the "Optimisation Mode" to "No_Change".

Guideline 3:

All operators should configure all AMR Codec Types, except UMTS_AMR, in their networks with preferred configuration 1 (see Table 7.11.3.1.3-2, Config-NB-code 1). For UMTS_AMR the ACS should include only mode 12,20 kbps. By these configurations the chances for TFO/TrFO between systems (GSM-UMTS) and between different PLNMs are substantially enhanced.

This configuration 1 fulfils automatically also Guidelines 2, 4 and 5.

Other ACSs for FR_AMR, UMTS_AMR, UMTS_AMR_2, OHR_AMR and HR_AMR are possible. They should include as many as possible common Codec Modes in the lower, contiguous subsets. Also for UMTS_AMR other ACSs are possible. In that case Inter-PLNM TFO is not as obvious and may need other inter-operator agreements.

Guideline 4:

The operator should configure AMR so that the ACSs are homogeneous within the whole PLMN (same ACS used in all BSS of a given PLMN for a given Codec Type: UMTS_AMR_2, FR_AMR, OHR_AMR, HR_AMR). The ACSs of different Codec Types of the AMR Family should contain as many as possible Codec Modes within the common, lower, contiguous subset. Also UMTS_AMR should be configured homogeneous within the whole PLMN, but since it is not compatible to the other AMR codec types, another configuration is possible (see Guideline 3).

Guideline 5:

If the network is heterogeneous, the operator should choose ACSs so that all resulting Common ACSs are acceptable (see clause 12), with as many as possible Codec Modes within the common, lower, contiguous subset.

Guideline 6:

If for AMR-WB an optimisation shall be restricted to certain configurations, this can be acchieved by choosing a suitable configuration with OM=F(orbidden) in a combination with Maximum Rate Control.
Example: If configuration 4 and 5 shall not be possible, then configuration 2 shall be indicated. Then the effective ACS A and B (see Table 12.8-1) are possible, but not ACS C.

F.2.2 Immediate TFO establishment with Codec Mode Optimisation

Guideline 6:

The operator should choose the ACSs in a way that all resulting immediate Common ACSs are acceptable and CACSs are subsets of Optimised ACSs (see clause 12).

Then TFO will most of the times establish immediately (with the obvious benefits in speech quality) and the Codec Mode Optimisation may be achieved with Mode Modify or RATSCCH, i.e. without the problematic Intra-Cell hand-over.

Remark: This guideline is not easy to fulfil since it is of course in general not possible to foresee all possible ACS constellations, especially not for inter-PLMN calls.