The procedure for successful GSM to UMTS handover from MSC-B back to 3G_MSC-A is shown in Figure 26
The procedure is as follows.
If MSC-B supports inter-system handover to a CSG cell, 3G_MSC-A provided CSG subscription data during the basic inter-MSC handover, and BSS-A includes a CSG ID for the target cell in the A-HANDOVER-REQUIRED message, then MSC-B shall check the CSG membership of the UE for the target cell as described in subclause 4.2.1
before generating the MAP-PREPARE- SUBSEQUENT-HANDOVER request. If the UE fails the CSG membership check and the target cell is a CSG cell, MSC-B shall send an A-HANDOVER-REQUIRED-REJECT to BSS-A.
MSC-B sends the MAP-PREPARE-SUBSEQUENT-HANDOVER request to 3G_MSC-A indicating the new MSC number (3G_MSC-A number), indicating also the identity of the target RNS where the call has to be handed over and including a complete A-HO-REQUEST message. (NOTE: MSC-B shall not send further MAP-PREPARE-SUBSEQUENT-HANDOVER requests while a handover attempt is pending or before any timeouts). Since 3G_MSC-A is the call controlling MSC, this MSC needs no Handover Number for routing purposes; 3G_MSC-A can immediately initiate the search for free radio resources. 3G_MSC-A then inserts a transcoder between it's RNS and the connection to the other party.
When radio resources can be assigned, 3G_MSC-A shall return in the MAP-PREPARE-SUBSEQUENT-HANDOVER response the complete A-HO-REQUEST-ACK message generated from the Iu-RELOCATION-REQUEST-ACK received from the RNS-B and possible extra BSSMAP information, amended by 3G_MSC-A due to the possible interworking between the BSSMAP protocol carried on the E-interface and the RANAP protocol used on the Iu-interface. If radio resources cannot be assigned or if a fault is detected on the target cell identity, or the target cell identity in the A-HO-REQUEST is not consistent with the target MSC number, the MAP-PREPARE-SUBSEQUENT-HANDOVER response containing an A-HO-FAILURE message shall be given to MSC-B, in addition MSC-B shall maintain the connection with the UE/MS.
If the procedure in 3G_MSC-A is successful then MSC-B can request the UE/MS to retune to the new RNS-B on 3G_MSC-A. This is illustrated in Figure 26
by the A-HO-COMMAND message. The operation is successfully completed when 3G_MSC-A receives the Iu-RELOCATION-COMPLETE message.
After GSM to UMTS handover 3G_MSC-A shall release the circuit to MSC-B.
3G_MSC-A must also terminate the MAP procedure for the basic handover between 3G_MSC-A and MSC-B by sending an appropriate MAP message. MSC-B will clear the resources in BSS-A when the MAP-SEND-END-SIGNAL response is received.
The procedure for successful GSM to UMTS handover from MSC-B to 3G_MSC-B' is shown in Figure 27
The procedure consists of two parts:
a subsequent handover from MSC-B back to MSC-A as described in subclause 7.3.1 (the same procedures apply if MSC-A is replaced by 3G_MSC-A); and
a basic GSM to UMTS handover from MSC-A to 3G_MSC-B' as described in subclause 8.2.1.
If MSC-B supports inter-system handover to a CSG cell and BSS-A includes a CSG ID for the target cell in the A-HANDOVER-REQUIRED message, then MSC-B shall check the CSG membership of the UE for the target cell as described in subclause 22.214.171.124
before initiating the procedure, and reject the handover if necessary.
MSC-B sends the MAP-PREPARE-SUBSEQUENT-HANDOVER request to MSC-A indicating a new MSC number (which is the identity of 3G_MSC-B'), indicating also the identity of the target RNS where the call has to be handed over and including a complete A-HO-REQUEST, MSC-A then starts a basic handover procedure towards 3G_MSC-B'.
If MSC-A supports A interface over IP, then for speech calls MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request towards 3G_MSC-B'. For a detailed description of the handling of this codec list by MSC-A and 3G_MSC-B' see TS 23.153
When MSC-A receives the ACM from 3G_MSC-B', MSC-A informs MSC-B that 3G_MSC-B' has successfully allocated the radio resources on RNS-B' side by sending the MAP-PREPARE-SUBSEQUENT-HANDOVER response containing the complete A-HO-REQUEST-ACK generated from the RELOCATION-REQUEST-ACK received from RNS-B' and possible extra BSSMAP information, amended by MSC-A due to the possible interworking between the BSSMAP protocol carried on the E-interface between MSC-A and 3G_MSC-B' and the BSSMAP protocol carried on the E-interface between MSC-A and MSC-B. Now MSC-B can start the procedure on the radio path.
For MSC-A the handover is completed when it has received the MAP-SEND-END-SIGNAL REQUEST from 3G_MSC-B' containing the A-HO-COMPLETE generated from Iu-RECOLATION COMPLETE received from the RNS-B'. The circuit between MSC-A and MSC-B is released. MSC-A also sends the MAP-SEND-END-SIGNAL response to MSC-B in order to terminate the original MAP dialogue between MSC-A and MSC-B. MSC-B releases the radio resources when it receives this message.
If no radio resources can be allocated by 3G_MSC-B' or no circuit between MSC-A and 3G_MSC-B' can be established or a fault is detected on the target cell identity or the target cell identity in the A-HO-REQUEST is not consistent with the target MSC number, MSC-A informs MSC-B by using the A-HO-FAILURE message included in the MAP-PREPARE-SUBSEQUENT-HANDOVER response. MSC-B shall maintain the existing connection with the UE/MS.
When the subsequent GSM to UMTS handover is completed, 3G_MSC-B' is considered as 3G_MSC-B. Any further inter-MSC handover is handled as described above for a subsequent handover.