23.1533GPPOut of band transcoder controlRelease 17Stage 2TS
The IuFP must be initialised sequentially from one end to the other in order to store new RFCIs in each node to allow TrFO to resume. The IuFP shall be initialised in the backward direction with respect to the Codec Modification/Modify To Selected Codec message as shown in Figure 5.8.4/1.
Figure 5.8.4/1: Successful Codec Modification including IuFP
A MGW receiving a Modify Bearer Characteristics procedure shall be prepared to receive an incoming modify bearer procedure, this may be to increase the bandwidth prior to IuUP Initialisation or to reduce the bandwidth after the IuUP Intialisation. The new codec indicated in the Modify Bearer Characteristics procedure shall always result in the MGW being prepared to receive an Iu UP initialisation for the new codec, even if the SDU format is unchanged. The MSC shall send the RAB Parameters IE and NAS Synchronisation Indicator IE to the RNC to indicate that the codec has changed and IuUP Initialisation shall be generated..
Each termination receiving a Reserve_Char will initiate bearer level modification to the preceeding node if needed – i.e. if the bandwidth needs to be increased to support the new IuUP. No IuUP initialisation occurs at this point in time. If the Codec Modification Request is terminated by a MGW the IuUP init through the core-network is triggered by the setting of the 3GUP package property "initialisation direction" to "OUT" in either the Reserve_Char or the Confirm_Char procedure; the MGW shall then start the IuUP Initialisation out from that Termination. If the node terminating the modification is an RNC then it will generate a new IuUP Initialisation toward its access MGW, each Termination shall have the initialisation direction set to "IN". Each MGW shall in turn acknowledge the IuUP Init to the succeeding node (with respect to the modification request) and forward the RFCIs in an IuUP Initialisation to the preceding MGW (as for call set-up). After completing the Iu UP initialisation and receiving the"Confirm Characteristics" procedure, the MGW may decrease the bandwidth of the corresponding bearer performing the "Modify Bearer" procedure (if needed) – no bearer bandwidth reduction shall be initiated while the UP is still initialised for the old codec.
An example call sequence is shown in Figures 5.8.4/2 and 5.8.4/3.
Figure 5.8.4/2: Mid Call Codec Negotiation Call Sequence. Call Flow Part 1
Figure 5.8.4/3: Mid Call Codec Negotiation Call Sequence. Call Flow Part 2