The present document specifies the stage 1 description for the Tandem Free Operation (TFO) feature which provides the capability to avoid tandeming speech codecs in mobile to mobile speech calls. The primary aim is to realise improvements in speech quality. The TFO mode of operation could also be used to reduce inter-Mobile services Switching Center (MSC) transmission bandwidth requirements.
In analogy with ITU-T Recommendations I.130 , Stage 1 is an overall service description, from the service subscriber's and user's standpoints, that views the network as a single entity which provides services to the user.
The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
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TFO shall be applicable to all mobile to mobile speech calls where both parties use the same GSM speech coding standard. TFO shall be supported for all of the GSM speech coding standards (i.e. Full Rate, Half Rate and Enhanced Full Rate). It would also be desirable to extend the TFO approach to realise quality enhancements when different speech codec standards are in use, e.g. by handing over one of the links to provide common codec types and then applying TFO.
TFO shall not require any modifications to existing or future mobile stations. There shall be no additional user intervention required to enable or invoke the feature. Correspondingly, it shall not be possible for a user to disable the feature.
TFO shall be capable of being applied for mobile to mobile calls regardless of the serving networks of the A or B parties. Thus, the A and B parties may be registered on different PLMNs or on the same network.
Provision of the TFO feature shall be determined by the network operator on a network (or sub-network) wide basis, where equipment to support the feature has been deployed. It shall not be necessary for the service capability to be rolled out throughout an entire network before being made operational.
TFO between networks shall be applied if appropriate between supporting (sub) networks.
In the case where a TFO call is handed over between two parts of a network, but the new part is not provisioned with TFO, the call shall revert to normal operation.
In the case where a normal call is handed over between two parts of a network, and the new part is provisioned with TFO, the call will switch to TFO if appropriate.
The speech quality and timing requirements depend on the phase of the call and are defined in table 1. In all cases the duration and degradation should be minimised.
The timing requirements for the application and removal of TFO are derived from the typical user's perception of the feature in the context of typical call set-up times together with handover operation. There is a much stricter requirement on the time taken to revert to normal operation than to apply TFO because when TFO is enabled only on one side of a call, the speech will be unintelligible. Likewise, failures in the transmission of the coded speech without (significant) errors (e.g. due to routing via an ADPCM link) should be detected rapidly as the speech quality may also be seriously degraded.
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.