8.1.2 Basic Inter-MSC SRNS/SBSS Relocation

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 "Basic Relocation Procedure 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.

8.1.2.1 MSC-A/MGW-A

8.1.2.1.1 Relocation Required

For Relocation towards GERAN Iu mode the GERAN capabilities of the target cell will be indicated to the MSC-A 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-A server shall assume that the GERAN target cell will provide the same capabilities as the current cell (for details see [29]).

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

Furthermore, the MSC-A 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..

8.1.2.1.2 Bearer establishment between MGW-A and MGW-B

The handling of the bearer establishment is as described for a Basic Mobile Originating Call, using either forward or backward bearer establishment. 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 also uses the Change Flow Direction procedure to request MGW-A to set the Handover Device to initial state (bullet 3 in figure 8.4/1).

8.1.2.1.3 Relocation Command/Relocation Detect

When the MSC-A server sends the Relocation Command 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 4 in figure 8.4/2).

8.1.2.1.4 Relocation Complete

When the MSC-A server receives the Relocation Complete message, it requests RNC-A to release the IU. The MSC-A server also requests MGW-A to set the Handover Device to its final state by removing the bearer termination towards RNC-A, using the Release Termination procedure (bullet 5 in figure 8.4/2).

8.1.2.1.5 Interworking function

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

8.1.2.1.6 Codec handling

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

8.1.2.1.7 Voice Processing function

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

8.1.2.1.8 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.

8.1.2.1.9 Failure Handling in MSC server

When a procedure between the MSC-A server and MGW-A 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 call establishment towards MSC-B has already started then the call towards MSC-B server shall be cleared as described in clause 7.3. If the original call is to be cleared, then it shall be handled as described in clause 7.3.

8.1.2.2 MSC-B/MGW-B

8.1.2.2.1 MGW selection

The MSC-B server selects an MGW when it receives Prepare Handover Request message (bullet 1 in figure 8.4/1).

8.1.2.2.2 Bearer establishment towards RNC-B

When the MSC-B server has selected MGW-B it requests MGW-B to provide a binding reference and a bearer address, using the Prepare Bearer procedure. For speech calls, the MSC-B server shall provide the MGW-B with the speech coding information for the bearer. The MSC-B server sends the Relocation Request message to the RNC-B containing the bearer addresses and binding references (bullet 2 in figure 8.4/1).

For Relocation towards GERAN Iu mode, if the selected service is speech and the MSC-B server cannot provide the codec requested by the MSC-A server, the MSC-B server shall select another codec according to the received GERAN capabilities of the target cell and the received list of supported codecs, and shall set the RAB parameters within the RANAP RELOCATION REQUEST message according to the new selected codec. Furthermore, the MSC-B server shall report the chosen codec and codec modes back to the MSC-A server with MAP Prepare Handover response.

8.1.2.2.3 Bearer establishment between MGW-A and MGW-B

The handling of the bearer establishment is as described at Basic Mobile Terminating Call, using either forward or backward bearer establishment.

8.1.2.2.4 Codec handling

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

8.1.2.2.5 Voice Processing function

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

8.1.2.2.6 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.

8.1.2.2.7 Failure Handling in MSC server

When a procedure between the MSC-B server and MGW-B fails the MSC-B 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-B resources have been already seized at the target access side then the resources shall be released using the Release Termination procedure. The call from MSC-A server shall be released as described at clause 7.1.

8.1.2.2.8 Example

Figure 8.3 shows the network model for the Basic Inter-MSC SRNS Relocation. 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 T1 is used for the bearer towards RNC-A, 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-B, bearer termination T5 is used for the bearer towards MGW-A.

Before Relocation:

During Relocation:

After Relocation:

Figure 8.3: Basic Inter-MSC SRNS Relocation (network model)

Figure 8.4 shows the message sequence example for the Basic Inter-MSC SRNS Relocation. 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. It is also assumed that only one bearer has been established towards RNC-A. In the example the MSC-B server requests MGW-B to seize an RNC-B side bearer. The MSC-B server orders the establishment of the bearer towards RNC-B by sending Relocation Request. The call is established between MSC-A and MSC-B servers, and the bearer is established between MGW-A and MGW-B. 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 MSC-A server receives Relocation Complete indication from MSC-B server it orders RNC-A to release the IU. This action causes release of the bearer between RNC-A and MGW-A. Finally MSC-A server requests MGW-A to remove RNC-A side bearer termination.

Figure 8.4/1: Basic Inter-MSC SRNS Relocation (message sequence chart)

Figure 8.4/2: Basic Inter-MSC SRNS Relocation (message sequence chart)