The following clauses describe two options for the Basic and Subsequent UMTS to GSM Handover procedures. The first, as described in subclauses 8.1.1 and 8.1.3 respectively, provides for a circuit connection between 3G_MSC-A and 3G_MSC-B. The second, as described in subclauses 8.1.2 and 8.1.4 respectively, provides for a Basic and Subsequent Handover without the provision of a circuit connection between 3G_MSC-A and 3G_MSC-B. 3G_MSC can also be a pure GSM MSC.
In all the above mentioned clauses, the following principles apply:
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 TS 49.008 - shall be transferred on the E-interface;
the trace related messages that are part of the applicable BSSAP subset - as defined in TS 49.008- can be sent by the 3G_MSC-A on the E-interface after successful handover resource allocation. In the subclauses 8.1.1 and 8.1.2, 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 TS 49.008 - together with the applicable handover related message. The applicable handover related message shall always appear as the first message;
during the handover resource allocation for subsequent inter-MSC inter-system handover according to subclauses 8.1.3 and 8.1.4, it is allowed to transfer either DTAP or RANAP Direct Transfer messages on the E-Interface between 3G_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;
during the handover execution, i.e. while the UE/MS is not in communication with the network, the 3G_MSC-A shall queue all outgoing BSSAP or RANAP messages until the communication with the UE/MS is resumed;
during the execution of a basic inter-system inter-MSC handover to MSC-B or a subsequent inter-system inter-MSC handover to a third MSC-B', only the handover related messages and the A-Clear-Request message that are part of the applicable BSSAP subset - as defined in TS 49.008 - may be sent by the target MSC on the E-interface;
during a subsequent inter-system inter-MSC handover back to 3G_MSC-A or to a third 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;
finally, during supervision, i.e. while the UE/MS is not in the area of 3G_MSC-A after a successful Inter-3G_MSC handover, the subset of BSSAP procedures and their related messages - as defined in TS 49.008 - 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 TS 29.108 - shall be transferred on the E-interface (see subclause 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.
during the intra-3G_MSC -B 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.
The procedure used for successful Inter-3G_MSC UMTS to GSM Handover is shown in Figure 18. Initiation of the UMTS to GSM handover procedure is described in clause 5. The procedure described in this clause makes use of messages from the TS 49.008 and of the transport mechanism from the Mobile Application Part (MAP) (TS 29.002). After an Inter-3G_MSC relocation/handover, Intra-3G_MSC UMTS to GSM handover may occur on 3G_MSC -B, this handover will follow the procedures specified in a previous clause.
The UMTS to GSM handover is initiated as described in subclause 6.2.1. (This is represented by Iu-RELOCATION-REQUIRED in Figure 18). Upon receipt of the Iu-RELOCATION-REQUIRED from RNS-A, 3G_MSC-A shall send a MAP-PREPARE-HANDOVER request to MSC-B including a complete A-HO-REQUEST message.
The MAP-PREPARE-HANDOVER request shall carry in the A HO-REQUEST all information needed by MSC-B for allocating a radio channel, see 3GPP TS 08.08. For compatibility reasons, the MAP-PREPARE-HANDOVER request will also identify the cell to which the call is to be handed over. For speech calls, 3G_MSC-A shall also include the Iu Supported Codecs List to be used by MSC-B for subsequent intra-MSC-B intersystem handover to UMTS and intra-MSC-B SRNS relocation.
If 3G_MSC-A supports A interface over IP, then for speech calls 3G_MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request. If handover to an A over IP capable BSS-B is performed, MSC-B shall include a Codec List (MSC preferred) in the A-HO-REQUEST message to BSS-B. MSC-B may select the codecs for the Codec List (MSC preferred) from the channel type information and the AoIP-Supported Codecs List (Anchor), if this list was provided by 3G_MSC-A in the MAP-PREPARE-HANDOVER request. For a detailed description of the handling of these codec lists by 3G_MSC-A and MSC-B see TS 23.153. If the AoIP-Supported Codecs List (Anchor) was not provided or MSC-B does not support the selection of codecs from the AoIP-Supported Codecs List, then MSC-B shall create the Codec List (MSC preferred) using the channel type information received from 3G_MSC-A in the A-HO-REQUEST message included in the MAP-PREPARE-HANDOVER request.
If 3G_MSC-A supports handover/relocation 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, 3G_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.
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 3G_MSC-A to MSC-B. If a traffic channel is available in MSC-B the MAP-PREPARE-HANDOVER response, sent to 3G_MSC-A will contain the complete A-HO-REQUEST-ACKNOWLEDGE message received from BSS-B, containing the radio resources definition to be sent by RNS-A to the UE/MS and possible extra BSSMAP information, amended by MSC-B due to the possible interworking between the BSSMAP protocol carried on the E-interface and the BSSMAP protocol used on the A-interface. If the traffic channel allocation is queued by BSS-B, the A-QUEUING-INDICATION may optionally be sent back to 3G_MSC-A. The further traffic channel allocation result (A-HO-REQUEST-ACK or A-HO-FAILURE) will be transferred to 3G_MSC-A using the MAP-PROCESS-ACCESS-SIGNALLING request. If the traffic channel allocation is not possible, the MAP-PREPARE-HANDOVER response containing an A-HO-FAILURE will be sent to 3G_MSC-A. 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. MSC-B simply reports the events related to the dialogue. It is up to 3G_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 A interface over IP is supported, then for speech calls via an A over IP capable BSS-B the selection of the speech codec shall be as described in TS 48.008, and if no transcoder is inserted in the BSS-B then MSC-B shall insert a transcoder.
If 3G_MSC-A provided an AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request and MSC-B selected the codecs for the Codec List (MSC preferred) from the AoIP-Supported Codecs List (Anchor), MSC-B may send the AoIP-Selected Codec (Target) and AoIP-Available Codecs List (MAP) to 3G_MSC-A in the MAP-PREPARE-HANDOVER response.
If BSS-B does not support A interface over IP or 3G_MSC-A did not include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE HANDOVER request, then MSC-B shall not include the AoIP-Selected Codec (Target) and AoIP-Available Codecs List (MAP) in the MAP-PREPARE-HANDOVER response. Reception of the AoIP-Selected Codec (Target) and AoIP Available Codecs List (MAP) from MSC-B with the MAP-PREPARE-HANDOVER response indicates to 3G_MSC-A that the target access supports A interface over IP.
If an error related to the TCAP dialogue or to the MAP-PREPARE-HANDOVER request is returned from MSC-B, this will be indicated to 3G_MSC-A and 3G_MSC-A will terminate the handover attempt. 3G_MSC-A rejects the handover attempt towards RNS-A. The existing connection to the UE/MS shall not be cleared.
When the A-HO-REQUEST-ACKNOWLEDGE has been received, 3G_MSC-A shall establish a circuit between 3G_MSC-A and MSC-B by signalling procedures supported by the network. In Figure 18 this is illustrated by the messages IAM (Initial Address Message) and ACM (Address Complete Message) of Signalling System no 7. MSC-B awaits the capturing of the UE/MS (subclause 6.2.1) on the radio path when the ACM is sent and 3G_MSC-A initiates the UMTS to GSM handover execution when ACM is received (illustrated by the Iu-RELOCATION-COMMAND and described in subclause 6.2.1). 3G_MSC-A removes the transcoder from the path to the other party.
MSC-B transfers to 3G_MSC-A the acknowledgement received from the correct UE/MS (A-HO-DETECT/A-HO-COMPLETE). The A-HO-DETECT, if received, is transferred to 3G_MSC-A using the MAP-PROCESS-ACCESS-SIGNALLING request. The A-HO-COMPLETE, when received from the correct UE/MS, is included in the MAP-SEND-END-SIGNAL request and sent back to 3G_MSC-A. The circuit is through connected in 3G_MSC-A when the A-HO-DETECT or the A-HO-COMPLETE is received from MSC-B. The old radio channel is released when the A-HO-COMPLETE message is received from MSC-B. The sending of the MAP-SEND-END-SIGNAL request starts the MAP supervision timer for the MAP dialogue between 3G_MSC-A and MSC-B. When the MAP-SEND-END-SIGNAL request including the A-HO-COMPLETE message is received in 3G_MSC-A, the resources in RNS-A shall be cleared.
In order not to conflict with the PSTN/ISDN signalling system(s) used between 3G_MSC-A and MSC-B, MSC-B must generate an answer signal when A-HO-DETECT/COMPLETE is received.
MSC-B shall release the Handover Number when the circuit between 3G_MSC-A and MSC-B has been established.
If the circuit between 3G_MSC-A and MSC-B cannot be established, (e.g. an unsuccessful backward message is received instead of ACM), 3G_MSC-A terminates the inter-3G_MSC UMTS to GSM handover attempt by sending an appropriate MAP message, for example an ABORT.
3G_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 3G_MSC-A clears the call to the UE/MS it also clears the call control functions in 3G_MSC-A and sends the MAP-SEND-END-SIGNAL response to release the MAP resources in MSC-B.
3G_MSC-A may terminate the procedure at any time by sending an appropriate MAP message to MSC-B. If establishment of the circuit between 3G_MSC-A and MSC-B has been initiated, the circuit must also be cleared.
The UMTS to GSM handover will be aborted by 3G_MSC-A if it detects clearing or interruption of the radio path before the call has been established on MSC-B.
If 3G_MSC-A supports the optional supplementary service Multicall (See TS 23.135), 3G_MSC-A shall have the following functionality additionally to the description in subclause 18.104.22.168.
Upon receipt of the IU-RELOCATION-REQUIRED from RNS-A 3G_MSC-A shall select one bearer to be handed over if the UE is engaged with multiple bearers. After that, the 3G_MSC-A generates an A-HO-REQUEST message for the selected bearer and sends it to MSC-B over MAP-PREPARE-HANDOVER request.
When MAP-PREPARE-HANDOVER response including an A-HO-REQUEST-ACK is received from MSC-B, 3G_MSC-A sends IU-RELOCATION-COMMAND, which indicates the bearers not to be handed over as bearers to be released, to RNS-A.
After 3G_MSC-A receives MAP-SEND-END-SIGNAL request from MSC-B, 3G_MSC-A shall release calls via MSC-B, which has been carried by the bearers not to be handed over, and then 3G_MSC-A sends IU-RELEASE-COMMAND to RNS-A.
The basic UMTS to GSM handover procedures to be used when no circuit connection is required by 3G_MSC-A are similar to those described in clause 8.1.1 for circuit switched calls. The main differences to the procedures described in clause 8.1.1 relate to the establishment of circuits between the network entities and the Handover Number allocation.
In the case of basic UMTS to GSM handover, 3G_MSC-A shall specify to MSC-B that no Handover Number is required in the MAP-PREPARE-HANDOVER request (see TS 29.002). As for the basic UMTS to GSM 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 UMTS to GSM handover procedure (see clause: Subsequent Channel Assignment using a circuit connection). MSC-B shall then perform the radio resources allocation as described in subclause 8.1.1. The MAP-PREPARE-HANDOVER response shall be returned to 3G_MSC-A including either the response of the radio resources allocation request received from BSS-B (A HO-REQUEST-ACKNOWLEDGE/A-HO-FAILURE with possible extra BSSMAP information. These extra information are amended by MSC-B due to the possible interworking between the BSSMAP protocol carried on the E interface and the BSSMAP protocol used on the A-interface) or potentially the A-QUEUING-INDICATION. The basic UMTS to GSM handover procedure will continue as described in subclause 8.1.1 except that no circuit connection will be established towards MSC-B.
The relevant case for the basic UMTS to GSM handover without circuit connection is shown in Figure 19. As can be seen the major differences to the equivalent Figure 18 is the omission of any circuit establishment messaging and the omission of handover number allocation signalling.