8.2.2 Basic Inter-MSC UMTS to GSM Handover

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

The procedures specified in 3GPP TS 23.009 [8] for "Basic Handover Procedure Requiring a Circuit Connection between 3G_MSC-A and MSC‑B" shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

8.2.2.1 MSC-A/ MGW-A

8.2.2.1.1 Bearer establishment between MGW-A and MGW-B

The handling of the bearer establishment between MGW-A and MGW-B is as described for a Basic Mobile Originating Call, using either forward or backward bearer establishment. For non-speech calls the MSC-A server shall provide MGW-A with the GSM Channel coding properties and 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 8.10/1).

8.2.2.1.2 Relocation Command/Handover Detect

When the MSC-A server sends the Relocation Command message or alternatively if it receives the Handover Detect message, the MSC-A server uses the Change Flow Direction procedure to requests the MGW to set the Handover Device to intermediate state (bullet 2 in figure 8.10/1).

8.2.2.1.3 Handover Complete

When the MSC-A server receives the Relocation Complete message, it requests RNC-A to release the IU. The MSC-A 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 3 in figure 8.10/1).

8.2.2.1.4 Interworking function

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

8.2.2.1.5 Voice Processing function

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

8.2.2.1.6 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 bearers that have not been selected will be cleared after the reception of Handover Complete message, as described in clause 7.3.

8.2.2.1.7 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 MGW-A resources have been already seized for the bearer towards MGW-B then the resources shall be released using the Release Termination procedure. 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.

8.2.2.2 MSC-B/MGW-B

8.2.2.2.1 MGW selection

The MSC-B server selects an MGW when it receives Prepare Handover Request message (bullet 1 in figure 8.10/1).

8.2.2.2.2 Bearer establishment towards BSC-B

When the MSC-B server has selected MGW-B it requests MGW-B to seize a TDM circuit, using the Reserve Circuit procedure. The MSC-B server sends the Handover Request message to the BSC-B containing the CIC (bullet 2 in figure 8.10/1).

8.2.2.2.3 Bearer establishment between MGW-A and MGW-B

The handling of the bearer establishment between MGW-A and MGW-B is as described for a Basic Mobile Terminating Call, using either forward or backward bearer establishment.

8.2.2.2.4 Voice Processing function

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

8.2.2.2.5 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 already been 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.

8.2.2.2.6 Example

Figure 8.9 shows the network model for the Basic Inter-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. 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 BSC-B, bearer termination T5 is used for the bearer towards MGW-A.

Before Handover

During Handover

After Handover

Figure 8.9: Basic Inter-MSC UMTS to GSM Handover (network model)

Figure 8.10 shows the message sequence example for the Basic Inter-MSC UMTS to GSM Handover.
It is assumed that the Handover Device is located in the MGW (MGW-A) which has been selected for the call establishment by the MSC server (MSC-A server). The MSC server 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 BSC-B side bearer termination. The call is established between MSC-A server and MSC-B server, and the bearer is established between MGW-A and MGW-B. When the handover is detected in BSC-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 MSC-B server it orders RNC-A to release the IU. Finally MSC-A server requests MGW-A to remove RNC-A side bearer termination.

Figure 8.10/1: Basic Inter-MSC UMTS to GSM Handover (message sequence chart)

Figure 8.10/2: Basic Inter-MSC UMTS to GSM Handover (message sequence chart)

8.2.2.3 Basic Inter-MSC UMTS to GSM Handover for A Interface over IP

If AoIP is supported by the MSC server, the Core Network side procedures described earlier in 8.2.2 shall apply. For the access side termination, the exchange of IP addresses via call control procedures is described in this clause.

When the MSC-B server receives the MAP Prepare Handover Request., it requests the MGW-B to reserve an RTP bearer termination using the Reserve RTP Connection Point procedure. The MSC-B server requests the MGW-B to reserve an IP address and UDP port.

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

The sequence is shown in Figure 8.2.2.3/1.

Figure 8.2.2.3/1: Basic Inter-MSC UMTS to GSM Handover for AoIP (message sequence chart)