D.2 Overview

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

Tandem Free Operation in 3G-3G calls and 3G-GSM calls implies that the different entities of the Core Network and Radio Access Networks collaborate. Figures D.2-1a and D.2-1b provide an overview of the nodes involved in Tandem Free Operation and the interfaces between these nodes.

The interfaces as shown in figures D.2-1a and D.2-1b are:

– MSC-MSC Interfaces: The ISUP protocol is not influenced by TFO. Optionally the OoBTC protocol (not shown) should take the Optimal Codec Type and Configuration and the Distant Codec List into account.
If this feature is not desired then the Optimisation Mode shall be set to "No Change".
This feature is mandatory when the Optimisation Mode has been set to "Change".

– RANAP: This Interface between MSC and RNC is not influenced by TFO.

– Iu Interface: This interface between MGW and RNC is not influenced by TFO.

– H.248: This interface between MGW and MSC Server has to provide the configuration data to the TC. In the minimal version this shall contain the Local Codec Type and the Configuration, with the Optimisation Mode set to "No Change". The Local Codec List is optional.
If the Optimisation Mode has been set to "Change" then the TC shall send (after the TFO Negotiation has taken place) the Optimal Codec Type and Optimal Configuration, as well as the Distant Codec List back to the MSC Server.

– Nb Interface: This interface carries (in case of TFO) the PCM samples and embedded in these the TFO Messages and TFO Frames.

Figure D.2-1a: Nodes and Interfaces for TFO in UMTS-UMTS Calls

Figure D.2-1b: Nodes and Interfaces for TFO in UMTS-GSM Calls

TFO in UMTS involves the following two processes:

TFO_TC: within the MGW, for all Speech Codec Types;
handles the TFO_Protocol and its four sub-processes,
including Codec Optimisation and Mismatch Resolution and Rate Control

TFO_MSC: within the MSC_Server; for all Speech Codec Types;
handles Initialisation and optionally Codec Modification

The RNC handles the Rate Control, required for the AMR and AMR-WB Speech Codec Types, but this procedure is not impacted by TFO.

These different processes and the inter-processes dialogues are described in the following clauses.

D.2.1 TFO_TC

Tandem Free Operation is essentially managed by the TC. In the simplest implementation version (Optimisation Mode set to "No Change") the TC can establish and maintain TFO fully on its own (within certain limits) as described below.

For all Codec Types the TC is responsible for the inband TFO Protocol, i.e. the TFO negotiation, TFO setup and the fast fall back to normal operation, if necessary. The TC has to monitor the ongoing call permanently for fast reaction, if required.
In all cases the TC has to perform the TFO Decision algorithm (see clauses 11 and 12). This TFO decision algorithm takes all known local and distant configuration parameters into account and identifies whether TFO is possible and what are the optimal call configuration parameters (Optimal Codec Type and Codec Configuration) in the given situation.
If the Optimisation Mode is set to "Change" then the TC has the responsibility to inform the MSC Server about any change in the distant call configuration, especially the distant alternative Codec List. It is then mandatory for the MSC Server to evaluate this information.

If the Optimisation Mode has been set by the MSC Server to "Change", then the TC shall provide to the MSC Server the optimal call configuration parameters resulting from the TFO Decision algorithm. It is then mandatory for the MSC Server to evaluate these parameters and to perform the necessary Codec Modification.

In case of the AMR Codec Types the TC is responsible for the TFO relevant Rate Control. It shall limit the maximally allowed Rate (Codec Mode) in a way that it is always within the Common Active Codec Set of both sides. During TFO Konnect the TC is responsible to steer the uplink rate down to the TFO Setup Mode and release it as soon as TFO is in Operation.

D.2.2 TFO_MSC

The Call Control Layer has the overall responsibility, especially for all resources, on the Radio Access Network (RAN) and the Core Network (CN). For all Codec Types it is responsible for Call Setup, Handover and Supplementary Services. The Call Control Layer should take care that the call configuration is not modified during handover unless absolutely necessary, because in TFO (TrFO) every local change has direct influence on the distant side. The Call Control Layer is responsible that TFO is properly terminated before handover, if the call configuration after handover is not longer TFO compatible.

The MSC Server is responsible for the interaction between Call Control Layer and the inband TFO signalling. It shall support of the TC with the necessary configuration parameters (Codec Type, Codec Configuration, Optimisation Mode, optional the alternative Codec List, etc). The MSC Server is responsible to enable or disable TFO.

The MSC Server is responsible for the change of the Codec Type and/or Codec Configuration, e.g. for Mismatch Resolution and Optimisation for TFO, if this is required or better for Tandem Free Operation and requested by the TC. This feature is optional for the MSC Server unless the Optimisation Mode is set to "Change".