D.5 TFO_MSC

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

The MSC Server in UMTS, which controls the Media Gate Way (MGW) and the Transcoder (TC) within the MGW, is responsible for the interaction between the "Out-of-Band-Transcoder Control" (OoBTC) and the "inband TFO" control. The communication between this Control Layer and the Transport Layer is performed e.g. via a "vertical" interface using the H.248 protocol .

The MSC Server provides the necessary configuration parameters to the TC at call setup:

– Used Codec Type (mandatory)

– Codec Configuration (mandatory)

– Optimisation Mode (mandatory)

– Alternative Codec List (optional)

– TFO_Enable / TFO_Disable. (mandatory)

These parameters may be changed during the call ("Codec Modification").

It is up to the MSC Server, which Codec Types and Codec Configuration parameters it provides to the TC. But once it has provided them, the MSC Server commits to perform In_Call_Modifications, in case the TFO Protocol decides that another Codec Type or Configuration is preferred.

After call setup the TFO_Protocol performs the inband negotiation and may determine a better, optimal Codec Type with optimal Configuration for TFO. This optimal Codec Type with Optimal Configuration parameters is reported to the MSC Server via the same vertical interface. The MSC Server has the duty to perform "Codec_Modification", if it has offered these options, via the OoBTC.

In addition the TC provides the MSC Server with the distant Codec List, as received via the TFO interface. The TC has the duty to update the MSC Server with these parameters whenever a modification on the distant side becomes available.

When the MSC Server got notice that TFO is ongoing it shall try to avoid changes of the Codec Type or Configuration.

D.5.1 Status of the Connection

The TC shall inform the MSC Server of its status with two messages:

– TFO_Off TFO is not established.

– TFO_On TFO is established and ongoing.

D.5.2 Change of Codec Type or Configuration

If TFO is ongoing the MSC Server shall try to keep the Codec Type and Configuration as far as possible during the call.
If this is not possible, e.g. due to handover or supplementary services, then the MSC Server shall disable TFO before changing to a new Codec Type or to a new Configuration that is not TFO compatible.

The new Codec Type may be selected taking the Codec List of the distant TFO partner into account.
TFO may be enabled again by the MSC Server after the change has been performed.