8.3.1 Intra-MSC GSM to UMTS Handover

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

The procedures specified in 3GPP TS 23.009 [8] for "Intra-3G_MSC GSM to UMTS Handover" shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

8.3.1.1 Handover Required

When the MSC server receives the Handover Required message, it requests the MGW to provide a binding reference and a bearer address using the Prepare Bearer procedure. For speech calls, the MSC server shall provide the MGW with the speech coding information for the bearer. For non-speech calls the MSC server shall provide the MGW with the same PLMN Bearer Capability [4] as was provided at the last channel assignment. The MSC server uses the Change Flow Direction procedure to request the MGW to set the Handover Device to initial state. The MSC server sends the Relocation Request message to the RNC-B containing the bearer address and binding reference (bullet 1 in figure 8.14).

For Relocation towards GERAN Iu mode the GERAN capabilities of the target cell will be indicated to the MSC-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-Server shall assume that the GERAN target cell will provide the same capabilities as the current cell (for details see [29]). The MSC server shall indicate to GERAN the selected services within the RANAP RELOCATION REQUEST message. 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.

8.3.1.2 Handover Command/Relocation Detect

When the MSC server sends the Handover Command message or alternatively if it receives a Relocation Detect message, the MSC server uses the Change Flow Direction procedure to requests the MGW to set the Handover Device to intermediate state (bullet 2 in figure 8.14).

8.3.1.3 Relocation Complete

When the MSC server receives the Relocation Complete message, it releases the A-interface line towards BSC-A and requests the MGW to set the Handover Device to its final state removing the bearer termination towards BSC-A, using Release Termination procedure (bullet 3 in figure 8.14).

8.3.1.4 Interworking function

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

8.3.1.5 Codec handling

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

8.3.1.6 Voice Processing function

After handover, the MGW may continue or modify voice processing function(s) provided to each bearer termination.

8.3.1.7 Failure Handling in MSC server

When a procedure between the MSC server and the MGW fails the MSC 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.

8.3.1.8 Example

Figure 8.13 shows the network model for the Intra-3G_MSC GSM to UMTS Handover. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling and the bearer. The bearer termination T1 is used for the bearer towards the BSC-A (connected through the MSC server), the bearer termination T3 is used for the bearer towards the RNC-B and the bearer termination T2 is used for the bearer towards the succeeding/preceding MGW.

Before Handover

During Handover

After Handover

Figure 8.13: Intra-3G_MSC GSM to UMTS Handover (network model)

Figure 8.14 shows the message sequence example for the Intra-MSC GSM to UMTS Handover.
It is assumed that the Handover Device is located in the MGW selected for the call establishment by the MSC server, which controls the call and the mobility management. In the example the MSC server requests seizure of RNC-B side bearer termination with specific flow directions. The MSC server starts handover execution by sending Relocation Request towards RNC-B. When the relocation is detected in RNC-B the MSC server requests to change the flow directions between the terminations within the context. When MSC server receives Relocation Complete indication from RNC-B it releases the A-interface line towards the BSC-A. Finally the MSC server requests the MGW to release BSC-A side bearer termination.

Figure 8.14: Intra-3G_MSC GSM to UMTS Handover (message sequence chart)