4 Description

22.0533GPPRelease 17Service descriptionStage 1Tandem Free Operation (TFO)TS

The TFO feature avoids the tandeming of speech codecs that occurs in mobile to mobile speech calls.

4.1 Applicability of TFO to Basic Services

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.

4.2 Support in Mobile Stations (MS)

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.

4.3 Parameters to be indicated and negotiated

The speech codec standards used on each party shall be indicated and negotiated, if applicable.

4.4 Provision of Service

4.4.1 Location Independence

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.

4.4.2 Provision of service within and between networks

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.

NOTE: TFO operation will normally require that the coded speech is transmitted transparently (i.e. without errors) between the relevant transcoders (e.g. by bit-stealing).

4.4.3 Subscription and Billing Information

This feature shall not be provisioned on a per-subscriber basis and no record of the application of TFO is necessary for billing purposes.

4.5 Quality of Service (QoS)

4.5.1 Impact on Speech Quality

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.

Table 1: TFO speech quality and timing requirements

Call phase

Reference condition

(note 1)

Maximum degradation

Speed of operation

TFO negotiation following call set-up

normal transcoding stages

"no perceptible degradation"

2 sec (note 2)

7 sec (note 3)

TFO negotiation following TFO interruption or handover

normal transcoding stages

"no perceptible degradation"

7 sec
(notes 4 and 5)

transition from normal operation to TFO

normal transcoding stages

"no annoying artefacts"

return from TFO to normal operation

TFO transcoding stages

"no annoying artefacts"

160 ms

continuous TFO

TFO transcoding stages

"no degradation"

continuous normal operation

normal transcoding stages

"no degradation"

NOTE 1: All reference conditions are defined as error free with no bit-stealing and with no transmission delay between the TRAUs "normal transcoding stages" refers to the speech codec(s) selected prior to TFO.

NOTE 2: Objective for time taken to establish TFO after call set-up.

NOTE 3: Maximum time allowed for TFO negotiation after call set-up.

NOTE 4: Objective for time taken to establish TFO after re-establishment of transparent PCM link.

NOTE 5: Once TFO has been successfully established during a call, an unlimitted number of negotiation attempts may be made after any subsequent interruptions of TFO.

Note: The timing limits and the quality degradations specified in table 1 are to some extent inter-dependent.