8.4.3 Subsequent Inter-MSC GSM to GSM Handover back to the Anchor MSC

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

The procedures specified in 3GPP TS 23.009 [8] for "Subsequent Handover from MSC‑B to MSC‑A 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.

8.4.3.1 MSC-A / MGW-A

When the MSC-A server receives a MAP Prepare Subsequent Handover request containing a Handover Request message, it requests MGW-A to seize a TDM circuit, using the Reserve Circuit procedure. 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 first access bearer assignment The MSC-A server uses the Change Flow Direction Procedure to request MGW-A to set the Handover Device to initial state. The MSC-A server sends the Handover Request message to the BSC-B containing the CIC (bullet 1 in figure 8.24/1).

8.4.3.1.1 Handover Request Acknowledge

For non-speech calls after receiving Handover Request Acknowledge message if the assigned GSM Channel coding properties differ from the previously provided ones the MSC-A server provides the MGW-A with the assigned GSM Channel coding properties using the Modify Bearer Characteristics procedure (bullet 2 in figure 8.24/2).

8.4.3.1.2 Handover Command/Handover Detect

When the MSC-A server sends the MAP Prepare Subsequent Handover response message or alternatively if it receives the Handover Detect message, the MSC-A server uses the Change Flow Direction procedure to request MGW-A to set the Handover Device to intermediate state (bullet 3 in figure 8.24/2).

8.4.3.1.3 Handover Complete

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

8.4.3.1.4 Interworking function

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

8.4.3.1.5 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.

8.4.3.1.6 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 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.

8.4.3.2 MSC-B/MGW-B

8.4.3.2.1 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 4 in figure 8.24/2).

8.4.3.2.2 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.

8.4.3.2.3 Example

Figure 8.24 shows the network model for the Subsequent Inter-MSC GSM to GSM Handover back to the Anchor MSC. The "squared" line represents the call control signalling. The "dotted" line represents the bearer. In MGW-A the bearer termination T6 is used for the bearer towards BSC-B, 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-A, bearer termination T5 is used for the bearer towards MGW-A.

Before Handover

During Handover

After Handover

Figure 8.23: Subsequent Inter-MSC GSM to GSM Handover back to the Anchor MSC
(network model)

Figure 8.24 shows the message sequence example for the Subsequent Inter-MSC GSM to GSM 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 BSC-B side bearer termination with specific flow directions. 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 Relocation Complete indication from BSC-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 8.24/1: Subsequent Inter-MSC GSM to GSM Handover back to the Anchor MSC
(message sequence chart)

Figure 8.24/2: Subsequent Inter-MSC GSM to GSM Handover back to the Anchor MSC
(message sequence chart)

8.4.3.3 Subsequent Inter-MSC GSM to GSM Handover back to the anchor MSC for A Interface over IP

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

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

The sequence is shown in Figure 8.4.3.3/1.

Figure 8.4.3.3/1: Subsequent Inter-MSC GSM to GSM Handover back to the anchor MSC for AoIP (message sequence chart)