8.2.1 Intra-MSC UMTS to GSM Handover

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

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

8.2.1.1 Relocation Required

When the MSC server receives the Relocation Required message, it requests the MGW to seize a TDM circuit, using the Reserve Circuit procedure. For non-speech calls the MSC server shall provide the MGW with the GSM Channel coding properties and the same PLMN Bearer Capability [4] as was provided at the last access bearer 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 Handover Request message, containing the CIC, to BSC-B (bullet 1 in figure 8.8/1).

8.2.1.2 Handover Request Acknowledge

For non-speech calls after receiving the Handover Request Acknowledge message if the assigned GSM Channel coding properties differ from the previously provided ones the MSC server provides the MGW with the assigned GSM Channel coding properties using the Modify Bearer Characteristics procedure (bullet 2 in figure 8.8/1).

8.2.1.3 Relocation Command/Handover Detect

When the MSC server sends the Relocation Command message or alternatively if it receives the Handover Detect message, the MSC server uses the Change Flow Direction procedure to requests the MGW to set the Handover Device to intermediate state (bullet 3 in figure 8.8/1).

8.2.1.4 Handover Complete

When the MSC server receives the Handover Complete message, it requests RNC-A to release the IU. The MSC server also requests the MGW to set the Handover Device to its final state by removing the bearer termination towards RNC‑A, using the Release Termination procedure (bullet 4 in figure 8.8/2).

8.2.1.5 Interworking function

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

8.2.1.6 Voice Processing function

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

8.2.1.7 Handling of multiple bearers (multicall)

If the UE is engaged with multiple bearers then one bearer is selected to be handed over according to 3GPP TS 23.009 [8]. The calls carried by the bearers that have not been selected will be cleared after the reception of the Handover Complete message, as described in clause 7.3.

8.2.1.8 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 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.

8.2.1.9 Example

Figure 8.7 shows the network model for the Intra-MSC UMTS to GSM Handover. 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. The bearer termination T1 is used for the bearer towards RNC-A, bearer termination T3 is used for the bearer towards BSC-B and the bearer termination T2 is used for the bearer towards the succeeding/preceding MGW.

Before Handover

During Handover

After Handover

Figure 8.7: Intra-MSC UMTS to GSM Handover (network model)

Figure 8.8 shows the message sequence example for the Intra-MSC UMTS to GSM 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. It is also assumed that only one bearer has been established towards RNC-A and that MGW-A is capable of handling GSM access.
In the example the MSC server requests seizure of BSC-B side bearer termination with specific flow directions. The MSC server starts handover execution by sending Handover Request towards BSC-B. When the handover is detected in BSC-B the MSC server requests to change the flow directions between the terminations within the context. When MSC server receives Handover Complete indication from BSC-B it orders RNC-A to release the IU. Finally the MSC server requests the MGW to release RNC-A side bearer termination.

Figure 8.8/1: Intra-MSC UMTS to GSM Handover (message sequence chart)

Figure 8.8/2: Intra-MSC UMTS to GSM Handover (message sequence chart)

8.2.1.10 Intra-MSC UMTS to GSM Handover for A interface over IP

When the MSC server receives the RANAP Relocation Required message, it requests the MGW to reserve an RTP bearer termination using the Reserve RTP Connection Point procedure. The MSC server requests the MGW to reserve an IP address and UDP port. The MSC server uses the Change Flow Direction procedure to request the MGW to set the Handover Device to initial state.

The MGW reserves the RTP termination and indicates the IP address and UDP port number to the MSC server. The IP addresses and UDP ports of the MGW is sent to the BSC in the BSSMAP Handover Request message. When the MSC server receives the BSSMAP Handover Request-Ack message, it shall send the BSC IP address and UDP Port number to the MGW using the Configure RTP Connection Point procedure.

The sequence is shown in Figure 8.2.1.10/1.

Figure 8.2.1.10/1: Intra-MSC UMTS to GSM Handover for AoIP (message sequence chart)