23.1533GPPOut of band transcoder controlRelease 17Stage 2TS
5.4.1 Framing Protocol Initialisation
For TrFO calls the compressed speech is carried end to end (RNC to RNC or between RNC and other compressed voice terminal). In 3GPP Core Networks compressed voice framing protocol shall be specified by the Nb User Plane specification. The specification for Iu interface is defined in , the specification for the Nb interface is defined in . The framing protocol for these interfaces is the same, Iu framing and is thus described as such, for both the Iu interface and the Nb interface. For compressed voice only the support mode is used, thus for TrFO the UP Initialisation procedure defined for the Nb UP shall be supported by the CN, when a CN MGW is required to establish a connection.
When negotiating TrFO OoB, a given serving MSC server shall consider the capabilities of the RNCs and MGWs, which are candidates to handle the TrFO call and which are controlled by this MSC server via an Iu/Mc interface. For TrFO, the selected RNC and MGW have to be able to support at least one Iu/Nb UP version with TrFO capabilities. Each MGW and RNC that supports TrFO shall support Iu/Nb UP version 2. If an RNC only supports Iu UP version 1 without TrFO capabilities, the MSC server shall insert a transcoder at the MGW that is connected to this RNC. For a TrFO call, each MSC server shall indicate in the "RAB assignment"/"Add request" only UP versions with TrFO capabilities. In the inband UP framing protocol version negotiation during framing protocol initialisation, the informed RNCs/MGW shall only offer and/or accept UP version listed in the "RAB assignment"/"Add request".
The Iu framing Protocol is established through the CN in a forward direction, independently of the bearer establishment direction. The Notify message to indicate bearer establishment shall not be sent until the Iu framing has been initialised. The continuity message (COT) shall not be sent forward until the Notify message has been received from the MGW and also the COT from the previous server has been received. The sequences for mobile originated calls are shown in figures 5.4/1 and 5.4/2 for forward bearer and backward bearer establishment, respectively. The parameters in the Add Request messages in the Figures are described in further detail in clause 5.4.5.
Figure 5.4.1/1: Iu Framing Protocol Establishment, Forward Bearer
Figure 5.4.1/2: Iu Framing Protocol Establishment, backward bearer.
The transport independent call control procedures in  shall support a continuity mechanism, as described above.
5.4.2 RFCI Storage
The RNC shall allocate RAB Subflow Combination Indicators to the SDU formats (SDU formats sent to the RNC by the MSC in the RAB Assignment). This allocation is then sent in the Iu Framing Initialisation PDU by the RNC in the User Plane. For further details see  and .
During the TrFO call establishment each MGW linked into the call shall store the RFCIs received from Iu Framing PDU Type 14. The first subflow combination in the initialisation frame corresponds to an Initial Rate Control, i.e. indicates the highest rate for the first speech mode to be used in the direction of the Initialisation Acknowledgement frame.
After the out of band codec negotiation has been performed, if the originating side is a UTRAN, then on request from the MSC for a RAB Assignment, it shall initiate the Iu user plane. If the originating side is a network that does not support Iu Framing then the Iu Framing initialisation is initiated by the GMSC, as described in detail in Clause 6.7. An Initialisation Protocol Data Unit (PDU) shall be sent to the first MGW in the call connection. Each initialisation leg is acknowledged per TrFO Link, i.e. per MGW-MGW interface. The subsequent initialisation is performed using the same RFCI set as received from the preceding node, independently of the Stream mode directions (i.e. if the terminations are not through connected).
This is shown figure 5.4.2/1.
Figure 5.4.2/1: RFCI Storage and subsequent initialisation in MGW
When the MGW terminations are through-connected and the RFCIs at both terminations are matching, then the MGW may revert to transparent mode; the RNCs shall not perform any subsequent Iu Framing initialisations without explicit request by the serving MSCs.
All succeeding MGWs in the path shall behave in a similar way as described above.
5.4.3 RFCI Value Correction
At the terminating end of a TrFO connection with Iu Framing initialised to the terminating MGW, the originating RFCI allocation is stored. The terminating RNC is then requested to perform a RAB Assignment towards the terminating MGW. This results in an Iu Framing initialisation, where the allocation of the RFCI values is independent from the Originating RNC’s allocation. These values may then be different to the originating RNC’s set.
The terminating MGW shall acknowledge the Iu Framing Initialisation and compare the RFCI values stored from the originating side. If the allocated index values do not match, then the MGW shall perform one of the following procedures:
1) initiate an Iu Framing Initialisation PDU towards the terminating RNC with the RFCI allocation as defined by the preceding node (previously stored in the MGW. This behavior is shown in figure 5.4.3/1 and termed "RFCI value correction") As the first Subflow combination received from the terminating RNC corresponds to an initial (maximum) rate control the MGW shall send a Rate Control PDUindicating this maximum speech mode back to the preceeding node in the core-network.
2) map the RFCI indices of the incoming side to the corresponding RFCI indices at the outgoing side for all SDUs passed between the Iu Framing protocol terminations. As the first Subflow combination in the IuUP initialisation corresponds to an initial rate control, i.e. indicates maximum rate for the mode to be used (in direction of Initialisation acknowledgement frame) it is treated as the initial maximum rate control (see ) the MGW shall initiate a Rate Control PDU indicating this maximum speech mode toward the terminating RNC. Similarly as the first Subflow combination received from the terminating RNC corresponds to an initial (maximum) rate control the MGW shall send a Rate Control PDUindicating this maximum speech mode back to the preceding node in the core-network. For further details on the rate control see clause 5.7.
Figure 5.4.3/1:RFCI Value Correction
Further details of the TrFO call establishment are described in clause 6.
This resolution handling is required also during RNC relocation; further details are described in clause 6.
5.4.4 TrFO Break
The event and procedure when a TrFO connection must be interrupted at a certain point in the path, e.g. due to a supplementary service invocation or for handover/relocation, is termed "TrFO Break". A TrFO Break may occur at a MGW as a consequence of a command directed by the associated Server. During this period the Iu User Plane protocol is terminated by this MGW, in general at both sides of the MGW. This means that it must respond to new Initialisation PDUs and Inband Rate Control PDUs. The MGW inserts a TrFO Break Function, which then makes use of the stored RFCI values, in order to perform the required Iu Framing protocol functions and interpret the payload. Further call scenarios for specific services that incur a TrFO break are described in clause 6..
5.4.5 TrFO Break Recovery
During the TrFO break situation the individual connections are free to change, the RFCIs may have changed and also the rate control (maximum rate, current rate). After the service that caused the TrFO break is complete, the MGW shall check if TrFO.can be re-established. If the coding schemes are matching but the RFCI’s have changed then RFCI value correction can be performed at the RNC side. In order to correct any changes in rate control between two RNCs, the MGW shall send a rate control request from each Termination, with the current rate and maximum rate applied at the other Termination. This will then result in the Distributed Rate Decision between the two RNCs in the call.
5.4.6 MGW Control Protocol Iu Framing Package properties
The following is a summary of the Iu Framing H.248 requirements; the procedures are described in  and are valid for Iu Framing in Support Mode:
Additional Package Properties:
Iu Framing Termination Type: Values – Iu-RAN (Iu Framing Protocol on Iu Interface)
– Iu-CN (Iu Framing Protocol on Nb Interface)
Iu Framing Initialisation Procedure: Values – Incoming (For Iu-CN: the Iu Framing Protocol initialisation is received by the media gateway and used for subsequent initialisation from this MGW. For Iu-RAN this indicates the originating RNC interface).
– Outgoing (For Iu-CN the Iu Framing Protocol is generated by the core network MGW, i.e. initialised on the Nb Interface. For the Iu-RAN interface this specifies the terminating RNC Interface)