8.1.3 Subsequent Inter-MSC SRNS/SBSS Relocation back to the Anchor MSC

23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The procedures specified in 3GPP TS 23.009 [8] for "Subsequent Relocation from 3G_MSC‑B to 3G_MSC‑A requiring a Circuit Connection between 3G_MSC‑A and 3G_MSC‑B" shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

8.1.3.1 MSC-A/MGW-A

8.1.3.1.1 Relocation Request

When the MSC-A server receives the MAP Prepare Subsequent Handover request containing a Relocation Request message, it requests MGW-A to provide a binding reference and a bearer address using the Prepare Bearer procedure. For speech calls, the MSC-A server shall provide the MGW-A with the speech coding information for the bearer. For non-speech calls the MSC-A server shall provide MGW-A with the same PLMN Bearer Capability [4] as was provided at the last access bearer assignment. The MSC-A server uses the Change Flow Direction procedure to request the MGW-A to set the Handover Device to initial state. The MSC-A server sends the Relocation Request message, containing the bearer address and the binding reference, to RNC-B (bullet 1 in figure 8.6/1).

For Relocation towards GERAN Iu mode, if the selected service is speech and the MSC-A server cannot provide the codec requested by the MSC-B server, the MSC-A server shall select another codec according to the received GERAN capabilities of the target cell and the list of supported codecs.

8.1.3.1.2 Relocation Command/Relocation Detect

When the MSC-A server sends the MAP Prepare Subsequent Handover response message or alternatively if it receives the Relocation Detect message, the MSC-A server uses the Change Flow Direction procedure to requests MGW-A to set the Handover Device to intermediate state (bullet 2 in figure 8.6/1).

8.1.3.1.3 Relocation Complete

When the MSC-A server receives the Relocation Complete message, it informs the MSC-B server about reception of this message. The MSC-A server then initiates call clearing towards the MSC-B server as described in clause 7.3.

8.1.3.1.4 Interworking function

The interworking function used by MGW-A before relocation will also be used after relocation.

8.1.3.1.5 Codec handling

The MGW may include a speech transcoder based upon the speech coding information provided to each bearer termination.

8.1.3.1.6 Voice Processing function

Voice processing function(s) provided by MGW-A and MGW-B before relocation, may be continued or modified by MGW-A after relocation.

8.1.3.1.7 Handling of multiple bearers (multicall)

If the UE is engaged with multiple bearers all procedures related to the handling of bearers and terminations described for the relocation of a single bearer shall be repeated for each bearer.

8.1.3.1.8 Failure Handling in MSC server

When a procedure between the MSC-A server and the MGW fails the MSC-A server shall handle the failure as an internal error in accordance with 3GPP TS 23.009 [8] and 3GPP TS 29.010 [23]. If MGW resources have already been seized at the target access side then the resources shall be released using the Release Termination procedure. If the call is to be cleared, then it shall be handled as described in clause 7.3.

8.1.3.2 MSC-B/MGW-B

8.1.3.2.1 Relocation Required

For Relocation towards GERAN Iu mode the GERAN capabilities of the target cell will be indicated to the MSC-B server within the RANAP RELOCATION REQUIRED message if the target cell provides different capabilities than the current cell. If no information about the GERAN capabilities of the target cell are received within this message, the MSC-B server shall assume that the GERAN target cell will provide the same capabilities as the current cell (for details see [29]). The different interworking scenarios (e.g. interworking to pre-Rel5 UTRAN) are described in [29].

The MSC-B server shall indicate to the MSC-A server the GERAN capabilities of the target cell, if available, with the MAP Prepare Subsequent Handover request. For speech calls, the MSC-B server shall additionally indicate to the MSC-A server the selected codec and the currently used codec.

Furthermore, the MSC-B server shall indicate to the GERAN the selected service within the RANAP RELOCATION REQUEST message and shall set the RAB parameters within the RANAP RELOCATION REQUEST message according to the selected service. The MSC server shall not set codec information in the NAS Synchronisation Indicator (see [4]). Instead it shall set codec information in the GERAN BSC container…

8.1.3.2.2 Relocation Complete

When the MSC-B server receives the Relocation Complete message, it requests RNC-A to release the IU. The MSC-B server requests MGW-B to remove the bearer termination towards RNC-A using the Release Termination procedure (bullet 3 in figure 8.6/2).

8.1.3.2.3 Release of bearer towards MGW-A

When the MSC-B server receives a call clearing indication from the MSC-A server, the MSC-B server handles it as described in clause 7.2.

8.1.3.2.4 Handling of multiple bearers (multicall)

If the UE is engaged with multiple bearers all procedures related to the handling of bearers and terminations described for the relocation of a single bearer shall be repeated for each bearer.

8.1.3.2.5 Example

Figure 8.5 shows the network model for the Subsequent Inter-MSC SRNS Relocation back to the Anchor MSC. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling and the bearer. In MGW-A the bearer termination T6 is used for the bearer towards RNC-B, bearer termination T3 is used for the bearer towards MGW-B, and the bearer termination T2 is used for the bearer towards the succeeding/preceding MGW. In MGW-B the bearer termination T4 is used for the bearer towards RNC-A, bearer termination T5 is used for the bearer towards MGW-A.

Before Relocation:

During Relocation:

After Relocation:

Figure 8.5: Subsequent Inter-MSC SRNS Relocation back to the Anchor MSC (network model)

Figure 8.6 shows the message sequence example for the Subsequent Inter-MSC SRNS Relocation back to the Anchor MSC. It is assumed that the Handover Device is located in the MGW (MGW-A) selected for the call establishment by the MSC server (MSC-A server) which controls the call and the mobility management. Also assumed that only one bearer has been established towards RNC-A. In the example the MSC-A server requests MGW-A to seize RNC-B side bearer termination with specific flow directions. The MSC server orders the establishment of the bearer towards RNC-B by sending Relocation Request. When the relocation is detected in RNC-B the MSC-A server requests to change the flow directions between the terminations within the context in MGW-A. When the MSC-A server receives a Relocation Complete indication from RNC-B it transfers this indication to MSC-B server. MSC-B server orders RNC-A to release the IU. This action causes release of the bearer between RNC-A and the MGW-B. MSC-A server initiates call clearing towards MSC-B server.

Figure 8.6/1: Subsequent Inter-MSC SRNS Relocation back to the Anchor MSC
(message sequence chart)

Figure 8.6/2: Subsequent Inter-MSC SRNS Relocation back to the Anchor MSC
(message sequence chart)