8.2 Handover GSM to UMTS
23.0093GPPHandover proceduresRelease 17TS
The following clauses describe two options for the Basic and Subsequent GSM to UMTS Handover procedures. The first, as described in clauses 8.2.1 and 8.2.3 respectively, provides for a circuit connection between (3G_)MSC‑A and (3G_)MSC‑B. The second, as described in clauses 8.2.2 and 8.2.4 respectively, provides for a Basic and Subsequent Handover without the provision of a circuit connection between (3G_)MSC‑A and (3G_)MSC‑B. In all the above mentioned clauses, the following principles apply:
a) during the handover resource allocation, except for the messages explicitly indicated in b and c below, only the handover related messages that are part of the applicable BSSAP subset – as defined in 3GPP TS 49.008 [7] – shall be transferred on the E-interface;
b) the trace related messages that are part of the applicable BSSAP subset – as defined in 3GPP TS 49.008 [7] – can be sent by the MSC‑A on the E-interface after successful handover resource allocation. In clauses 8.2.1 and 8.2.2, it is however allowed at basic handover initiation on the E-Interface to transfer one trace related message that is part of the applicable BSSAP subset – as defined in 3GPP TS 49.008 [7] – together with the applicable handover related message. The applicable handover related message shall always appear as the first message;
c) during the handover resource allocation for subsequent inter-MSC inter-system handover according to clauses 8.2.3 and 8.2.4, it is allowed to transfer either DTAP or RANAP Direct Transfer messages on the E-Interface between MSC-A and 3G_MSC-B. RANAP Direct Transfer messages shall be used for this purpose if and only if the basic handover procedure was an inter MSC SRNS relocation;
d) If 3G_MSC-B or 3G-MSC-B’ supports location reporting at change of Service Area, 3G_MSC-B or 3G_MSC-B’ shall always initiate the Location Reporting Control procedure at change of Service Area towards the target RNS since no request for Location Reporting can be received from MSC-A. In that case, the Location Reporting Control procedure shall be initiated by 3G_MSC-B or 3G-MSC-B’ after the Relocation Resource Allocation procedure has been executed successfully. The change of Service Area shall be reported to MSC-A within an A-HANDOVER-PERFORMED message;
e) during the handover execution, i.e. while the UE/MS is not in communication with the network, the MSC‑A shall queue all outgoing BSSAP or RANAP messages until the communication with the UE/MS is resumed;
f) during the execution of a basic inter-system inter-MSC handover to 3G_MSC-B or a subsequent inter-system inter-MSC handover to a third 3G-MSC-B’, only the handover related messages and the A-Clear-Request message that are part of the applicable BSSAP subset – as defined in 3GPP TS 49.008 [7] – may be sent by the target MSC on the E-interface;
g) during a subsequent inter-system inter-MSC handover back to 3G_MSC-A or to a third 3G_MSC-B’, 3G_MSC-B may initiate either an Iu-Release-Request procedure or an A-Clear-Request procedure on the E-interface. An Iu-Release-Request procedure shall be initiated only if the basic handover procedure was an inter-MSC SRNS relocation;
h) finally, during supervision, i.e. while the UE/MS is not in the area of MSC‑A after a successful Inter-3G_MSC GSM to UMTS handover, the subset of BSSAP procedures and their related messages – as defined in 3GPP TS 49.008 [7] – shall apply on the E-Interface. As the only exception to this rule, in case of a subsequent inter-MSC SRNS relocation back to 3G_MSC-A or to a third 3G_MSC-B’, during the relocation resource allocation, the relocation and trace related messages that are part of the applicable RANAP subset – as defined in 3GPP TS 29.108 [15] – shall be transferred on the E-interface (see clause 8.3, a and b).
If a subsequent inter-MSC handover/relocation back to 3G_MSC-A or to a third 3G_MSC-B’ is cancelled, then the supervision continues, and BSSAP procedures and their related messages shall apply on the E-interface;
i) during the intra-3G_MSC‑B GSM to UMTS handover execution, if any, the 3G_MSC‑B shall queue all outgoing BSSAP or RANAP messages until the communication with the UE/MS is resumed.
8.2.1 Basic Handover procedure requiring a circuit connection between MSC‑A and 3G_MSC‑B
The procedure used for successful Inter-3G_MSC Handover from GSM to UMTS is shown in figure 24. Initiation of the GSM to UMTS handover procedure is described in clause 5. The procedure described in this clause makes use of messages from the 3GPP TS 48.008 [5], 3GPP TS 25.413 [11] and of the transport mechanism from the Mobile Application Part (MAP) (3GPP TS 29.002 [12]). After an Inter-3G_MSC handover further Intra-3G_MSC handovers may occur on 3G_MSC‑B, these handovers will follow the procedures specified in the previous clauses.
NOTE: Can be sent at any time after the reception of IAM.
Figure 24: Basic GSM to UMTS Handover Procedure requiring a circuit connection
The GSM to UMTS handover is initiated as described in clause 6.2.2. (This is represented by A-HO-REQUIRED in figure 24). Upon receipt of the A-HO-REQUIRED from BSS-A, MSC‑A shall send a MAP-PREPARE-HANDOVER request to 3G_MSC‑B including a complete A-HO-REQUEST message.
NOTE: MSC‑A shall not send further MAP-PREPARE-HANDOVER requests while a MAP-PREPARE-HANDOVER response is pending or before any timeouts.
The MAP-PREPARE-HANDOVER request shall carry in the A‑HO-REQUEST all information needed by 3G_MSC‑B for allocating radio resources in RNS-B, see 3GPP TS 48.008 [5].
The MAP-PREPARE-HANDOVER request shall also carry the identity of the target RNS to which the call is to be handed over, see 3GPP TS 29.002 [12].
If MSC-A 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-A shall check the CSG membership of the UE for the target cell as described in clause 4.1.1 before generating the MAP-PREPARE-HANDOVER request. If the UE fails the CSG membership check and the target cell is a CSG cell, MSC-A shall send an A-HANDOVER-REQUIRED-REJECT to BSS-A.
If MSC-A supports inter-system handover to a CSG cell, the target cell belongs to the registered PLMN or an equivalent PLMN, and the HLR or the CSS provided CSG subscription data, MSC-A shall include the CSG subscription data for the registered PLMN and, if available, for the equivalent PLMNs in the MAP-PREPARE-HANDOVER request.
3G_MSC‑B will return the MAP-PREPARE-HANDOVER response after having retrieved a Handover Number from its associated VLR (exchange of the messages MAP-allocate-handover-number request and MAP-send-handover-report request). The Handover Number shall be used for routing the connection of the call from MSC‑A to 3G_MSC‑B.
For speech calls, if 3G_MSC-B supports the selection of codec based on the Iu-Supported Codecs List, 3G_MSC‑B shall select an Iu Selected codec from the Iu Supported Codecs List, generate associated RAB parameters and connect a transcoder. If the Iu Supported Codecs List was not received or 3G_MSC-B does not support the selection of codec based on the Iu-Supported Codecs List, 3G_MSC‑B shall select the appropriate default speech codec.
For handover to UTRAN Iu mode, 3G_MSC-B shall also generate a NAS Synch Indicator for the Iu-RELOCATION-REQUEST message. If the Iu Supported Codecs List was received by 3G_MSC-B and 3G_MSC-B supports the selection of codec based on the Iu-Supported Codecs List, then the Iu Selected codec shall be indicated in the MAP-PREPARE-HANDOVER response, sent from 3G_MSC‑B to MSC‑A.
If A over IP is supported by MSC-A, then for speech calls MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request to be used by 3G_MSC-B for subsequent intra-MSC-B intersystem handover to A over IP capable BSC. For a detailed description of the handling of this codec list by MSC‑A and 3G_MSC-B see 3GPP TS 23.153 [25].
If radio resources are available in RNS-B the MAP-PREPARE-HANDOVER response will contain the complete A-HO-REQUEST-ACK message generated from the Iu-RELOCATION-REQUEST-ACK received from RNS-B, containing the radio resources definition to be sent by BSS-A to the UE/MS. If the radio resource allocation is not possible, the MAP-PREPARE-HANDOVER response containing an A-HO-FAILURE will be sent to MSC‑A. 3G_MSC‑B will do the same if a fault is detected on the identity of the cell where the call has to be handed over. 3G_MSC‑B simply reports the events related to the dialogue. It is up to MSC‑A to decide the action to perform if it receives negative responses or the operation fails due to the expiry of the MAP-PREPARE-HANDOVER timer.
If an error related to the TCAP dialogue or to the MAP-PREPARE-HANDOVER request is returned from 3G_MSC‑B, this will be indicated to MSC‑A and MSC‑A will terminate the handover attempt. MSC‑A shall reject the handover attempt towards BSS-A. The existing connection to the UE/MS shall not be cleared.
When the A-HO-REQUEST-ACK has been received, MSC‑A shall establish a circuit between MSC‑A and 3G_MSC‑B by signalling procedures supported by the network. In figure 24 this is illustrated by the messages IAM (Initial Address Message) and ACM (Address Complete Message) of Signalling System no 7. 3G_MSC‑B awaits the capturing of the UE/MS (clause 6.2.2) on the radio path when the ACM is sent and MSC‑A initiates the handover execution when ACM is received (illustrated by the A-HO-COMMAND and described in clause 6.2.2).
If the BSS-A was connected via an A interface over IP and no transcoding performed in the BSS then MSC-A shall remove the transcoder between the MSC and the other party.
3G_MSC‑B transfers to MSC‑A the acknowledgement received from the correct UE/MS (A-HO-DETECT/A-HO-COMPLETE). The Iu-RELOCATION-DETECT, if received, is converted to A-HO-DETECT and transferred to MSC‑A using the MAP-PROCESS-ACCESS-SIGNALLING request. The Iu-RELOCATION-COMPLETE, when received from the correct UE/MS, is converted to A-HO-COMPLETE and included in the MAP-SEND-END-SIGNAL request and sent back to MSC‑A. The circuit is through-connected in MSC‑A when the A-HO-DETECT or the A‑HO‑COMPLETE is received from 3G_MSC‑B. The old radio channel is released when the A-HO-COMPLETE message is received from 3G_MSC‑B. The sending of the MAP-SEND-END-SIGNAL request starts the MAP supervision timer for the MAP dialogue between MSC‑A and 3G_MSC‑B. When the MAP-SEND-END-SIGNAL request including the A-HO-COMPLETE message is received in MSC‑A the resources in BSS-A shall be cleared.
In order not to conflict with the PSTN/ISDN signalling system(s) used between MSC‑A and 3G_MSC‑B, 3G_MSC‑B must generate an answer signal when Iu-RELOCATION-DETECT/COMPLETE is received.
3G_MSC‑B shall release the Handover Number when the circuit between MSC‑A and 3G_MSC‑B has been established.
If the circuit between MSC‑A and 3G_MSC‑B cannot be established (e.g. an unsuccessful backward message is received instead of ACM). MSC‑A terminates the inter3G_MSC handover attempt by sending an appropriate MAP message, for example an ABORT.
MSC‑A shall retain overall call control until the call is cleared by the fixed subscriber or the UE/MS and there is no further call control functions to be performed (e.g. servicing waiting calls, echo cancellers).
When MSC‑A clears the call to the UE/MS it also clears the call control functions in MSC‑A and sends the MAP-SEND-END-SIGNAL response to release the MAP resources in 3G_MSC‑B.
MSC‑A may terminate the procedure at any time by sending an appropriate MAP message to 3G_MSC‑B. If establishment of the circuit between MSC‑A and 3G_MSC‑B has been initiated, the circuit must also be cleared.
The GSM to UMTS handover will be aborted by MSC‑A if it detects clearing or interruption of the radio path before the call has been established on 3G_MSC‑B.
8.2.2 Basic GSM to UMTS Handover procedure not requiring the establishment of a circuit connection between MSC‑A and 3G_MSC‑B
The basic GSM to UMTS handover procedures to be used when no circuit connection is required by MSC‑A are similar to those described in clause 8.2.1 for circuit switched calls. The main differences to the procedures described in clause 8.2.1 relate to the establishment of circuits between the network entities and the Handover Number allocation.
In the case of basic GSM to UMTS handover, MSC‑A shall specify to 3G_MSC‑B that no Handover Number is required in the MAP-PREPARE-HANDOVER request (see 3GPP TS 29.002 [12]). As for the basic GSM to UMTS handover using a circuit connection, the A-HO-REQUEST is transmitted at the same time. Any subsequent Handover Number allocation procedure will not be invoked until the completion of the basic GSM to UMTS handover procedure (see clause: Subsequent Channel Assignment using a circuit connection). 3G_MSC‑B shall then perform the radio resources allocation as described in clause 8.2.1. The MAP-PREPARE-HANDOVER response shall be returned to MSC‑A including either the translated response of the radio resources allocation request received from RNS-B (A-HO-REQUEST-ACK/A-HO-FAILURE). The basic GSM to UMTS handover procedure will continue as described in clause 8.2.1 except that no circuit connection will be established towards 3G_MSC‑B.
The relevant case for the basic GSM to UMTS handover without circuit connection is shown in figure 25. As can be seen the major differences to the equivalent figure 24 are the omission of any circuit establishment messaging and the omission of handover number allocation signalling.
Figure 25: Basic GSM to UMTS Handover Procedure without circuit connection
8.2.3 Procedure for subsequent GSM to UMTS handover requiring a circuit connection
After the call has been handed over to MSC‑B, if the UE/MS leaves the GSM area of MSC‑B during the same call and enters a UTRAN area, subsequent GSM to UMTS handover is necessary in order to continue the connection.
The following cases apply:
i) the UE/MS moves back to the area of 3G_MSC‑A;
ii) the UE/MS moves into the area of a third 3G_MSC (3G_MSC‑B’).
In both cases the call is switched in 3G_MSC‑A; the circuit between 3G_MSC‑A and MSC‑B shall be released after a successful subsequent handover has been performed.
8.2.3.1 Description of subsequent GSM to UMTS handover procedure i): MSC‑B to 3G_MSC‑A
The procedure for successful GSM to UMTS handover from MSC‑B back to 3G_MSC‑A is shown in figure 26.
Figure 26: Subsequent GSM to UMTS handover procedure i): successful handover
from MSC‑B to 3G_MSC‑A using a circuit connection
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 clause 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.
8.2.3.2 Description of subsequent GSM to UMTS handover procedure ii): MSC‑B to 3G_MSC‑B’
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 clause 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 clause 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 clause 8.2.3.1 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 3GPP TS 23.153 [25].
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.
NOTE 1: Can be sent at any time after the reception of IAM.
Figure 27: Subsequent GSM to UMTS handover procedure ii): Successful handover
from MSC‑B to 3G_MSC‑B’ requiring a circuit connection
8.2.4 Procedure for subsequent GSM to UMTS handover not requiring a circuit connection
As for the subsequent GSM to UMTS handover with a circuit connection, the same two cases of subsequent handover apply:
i) the UE/MS moves back to the area of 3G_MSC‑A;
ii) the UE/MS moves into the area of a third 3G_MSC (3G_MSC‑B’).
8.2.4.1 Description of subsequent GSM to UMTS handover procedure without circuit connection i): MSC‑B to 3G_MSC‑A
The procedure for successful GSM to UMTS handover from MSC‑B back to 3G_MSC‑A without circuit connection is shown in figure 28. The only difference with the figure 26, is that no circuit release is needed between 3G_MSC‑A and MSC‑B.
Figure 28: Subsequent GSM to UMTS handover procedure i): Successful handover
from MSC‑B to 3G_MSC‑A not requiring a circuit connection
8.2.4.2 Description of subsequent GSM to UMTS handover procedure without circuit connection ii): MSC‑B to 3G_MSC‑B’
The procedure for successful GSM to UMTS handover from MSC‑B to 3G_MSC‑B’ is shown in figure 29.
The procedure consists of two parts:
– a subsequent handover from MSC‑B back to MSC‑A as described in clause 7.4.1 (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 clause 8.2.2.
The only difference to the equivalent figure 27 is the omission of the circuit and handover number allocation signallings.
Figure 29: Subsequent GSM to UMTS handover procedure ii): Successful handover
from MSC‑B to 3G_MSC‑B’ without circuit connection