8.3.3 Subsequent Inter-MSC GSM to UMTS Handover back to the Anchor MSC

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

The following handling shall be applied for a call that started as UMTS call. The procedures specified in 3GPP TS 23.009 [8] for "Subsequent Inter-MSC GSM to UMTS Handover back to the Anchor MSC" shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

8.3.3.1 MSC-A

8.3.3.1.1 Handover Request

When the MSC-A server receives a MAP Prepare Subsequent Handover request containing a Handover Request message, it requests the 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 channel 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 to the RNC-B containing the bearer address and binding reference (bullet 1 in figure 8.18/1).

For Handover towards GERAN Iu-mode, the MSC-A Server shall select a service according to the Channel Type received with the Handover Request message and the capabilities of the GERAN target cell, if the GERAN classmark was received. For speech calls, the MSC-A server shall additionally take into account the selected codec and the currently used codec received with MAP Prepare Subsequent Handover request, and the list of supported codecs.

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.

8.3.3.1.2 Handover Command/Relocation Detect

When the MSC-A server sends the MAP Prepare Subsequent Handover response message or alternatively if it receives a Relocation Detect message, the MSC-A server uses the Change Flow Direction procedure to requests the MGW-A to set the Handover Device to intermediate state (bullet 2 in figure 8.18/2).

8.3.3.1.3 Relocation Complete

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

8.3.3.1.4 Interworking function

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

8.3.3.1.5 Codec handling

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

8.3.3.1.6 Voice Processing function

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

8.3.3.1.7 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 been already 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.

8.3.3.2 MSC-B / MGW-B

8.3.3.2.1 Handover Required

For Handover towards GERAN Iu mode the GERAN capabilities of the target cell will be indicated to the MSC-B server within the Handover 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 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.

8.3.3.2.2 Handover Complete

When the MSC-B server receives the Handover Complete message, it releases the A-interface line towards the BSC-A and requests the MGW-B to remove the bearer termination towards the BSC-A using the Release Termination procedure (bullet 3 in figure 8.18/2).

8.3.3.2.3 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.

8.3.3.2.4 Example

Figure 8.17 shows the network model for Subsequent Inter-MSC GSM to UMTS Handover back to the Anchor MSC. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling (not applicable in case of GSM access) and the bearer. In the MGW the bearer termination T1 is used for the bearer towards RNC-B, the bearer termination T3 is used for the bearer towards MSC-A server, 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 BSC-A, bearer termination T5 is used for the bearer towards MGW-A.

Before Handover

During Handover

After Handover

Figure 8.17: Subsequent Inter-MSC GSM to UMTS Handover back to the Anchor MSC
(network model)

Figure 8.18 shows the message sequence example for the Subsequent Inter-MSC GSM to UMTS Handover 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.

In the example the MSC-A server requests MGW-A to seize RNC-B side bearer termination with specific flow directions. 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 Handover Complete indication from RNC-B it transfers this indication to MSC-B server. MSC-B server releases the A-interface line towards the BSC-A. MSC-A server initiates call clearing towards MSC-B server.

Figure 8.18/1: Subsequent Inter-MSC GSM to UMTS Handover back to the Anchor MSC
(message sequence chart)

Figure 8.18/2: Subsequent Inter-MSC GSM to UMTS Handover back to the Anchor MSC
(message sequence chart)