This annex highlights the limitations of the applicability of the in-band TFO solutions, some of these limitations could be overcome, however they imply modifications of equipments that are not in the scope of the specifications.
When analogue lines are used TFO either in-band or out-if-band can not apply.
The main limitation of TFO in-band is the digital transparency of the PCM lonks behind the MSCs that cannot be guaranteed in all the configurations of Mobile-to-Mobile calls.
When digital lines are used the non-transparency is mainly due to the use of In Path Equipments (IPE). These IPEs consist mainly in Echo Cancellers and DCMEs.
The Network Echo Cancellers intend to remove the echo due to the unbalanced hybrids present in the PSTN when going from four wires to two wires in the subscriber local loops. These IPEs are useful in the mobile-to-PSTN calls. These Echo Cancellers should not be used in mobile-to-mobile calls since both terminals are digital. The enabling and disabling of these equipments are properly managed in some countries and not in other countries. Therefore TFO can apply to inter-PLMN calls in the countries where the network echo cancellers are properly configured. The main issue is that there's no easy internationally recognised way to identify mobile-to-mobile calls. They are usually correctly configured for Mobile-to-Mobile calls within a PLMN.
Another limitation that should apply to a very small number of mobile-to-mobile calls is the use of A/μ Laws converters in some international calls.
In some networks Acoustical Echo Cancellers are used. It would be preferable to keep them enabled in Tandem Free calls. This implies that this be taken into account by the Acoustical Echo Cancellers.
The DCMEs they are mainly used in the long distance calls although some operators use them in their PLMN. The only known way to disable the DCMEs is to ask for a transparent 64 kbit/s UDI connection. This option is not reasonable since such connections are more expensive than the speech calls and would imply modifications in the MSCs. Therefore when a DCME is used in-band TFO cannot be established.
In order to get TFO working for the vast majority of the Mobile-to-Mobile calls it is desirable that the IPEs be modified in order to allow the in-band TFO cross them without modification.
TFO implies that compatible speech codec configurations are being used in both MSs. In-band TFO can be established only after the call set-up has been made and thus there can be different speech codecs used in the MSs. The TFO standard shall include ways to solving the codec mismatch otherwise in-band TFO won't apply to these Mobile-to-Mobile calls.