The following clauses describe two options for the Basic and Subsequent Handover procedures. The first, as described in subclauses 7.1 and 7.3 respectively, provides for a circuit connection between MSC-A and MSC-B. The second, as described in subclauses 7.2 and 7.4 respectively, provides for a Basic and Subsequent Handover without the provision of a circuit connection between MSC-A and MSC-B.
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 MSC-A on the E-interface after successful handover resource allocation. In subclauses 7.1 and 7.2, 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 handover according to subclauses 7.3 and 7.4, it is allowed to transfer either DTAP or RANAP Direct Transfer messages on the E-Interface between MSC-A and 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, ie while the MS is not in communication with the network, the MSC-A shall queue all outgoing BSSAP or RANAP messages until the communication with the MS is resumed;
during the execution of a basic inter-MSC handover to MSC-B or a subsequent 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-MSC handover back to MSC-A or to a third MSC-B', 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, ie while the MS is not in the area of MSC-A after a successful Inter-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-MSC-B handover execution, if any, the MSC-B shall queue all outgoing BSSAP messages until the communication with the MS is resumed.
The procedure used for successful Inter-MSC Handover is shown in Figure 12. Initiation of the handover procedure is described in clause 5. The procedure described in this clause makes use of messages from the TS 48.008 and of the transport mechanism from the Mobile Application Part (MAP) (TS 29.002). After an Inter-MSC handover further Intra-MSC handovers may occur on MSC-B, these handovers will follow the procedures specified in the previous clause.
The handover is initiated as described in subclause 6.1. (This is represented by A-HO-REQUIRED in Figure 12. Upon receipt of the A-HO-REQUIRED from BSS-A, 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 TS 48.008. 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, 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 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. 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 MSC-A in the MAP-PREPARE-HANDOVER request. For a detailed description of the handling of these codec lists by MSC-A and MSC-B see TS 23.153. If the AoIP-Supported Codecs List was not provided or MSC-B does not support the selection of codecs from the AoIP-Supported Codecs List (Anchor), then MSC-B shall create the Codec List (MSC preferred) using the channel type information received from MSC-A in the A-HO-REQUEST message included in the MAP-PREPARE-HANDOVER request.
If MSC-A supports 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.
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 MSC-B. If a traffic channel is available in MSC-B the MAP-PREPARE-HANDOVER response, sent to MSC-A will contain the complete A HO REQUEST-ACKNOWLEDGE message received from BSS-B, containing the radio resources definition to be sent by BSS-A to the 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 optionaly be sent back to MSC-A. The further traffic channel allocation result (A-HO-REQUEST-ACK or A-HO-FAILURE) will be transferred to 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 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 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 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 MSC-A in the MAP-PREPARE-HANDOVER response.
If BSS-B does not support A interface over IP or 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 AoIP-Selected Codec (Target) and AoIP Available Codecs List (MAP) from MSC-B with the MAP-PREPARE-HANDOVER response indicates to 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 MSC-A and MSC-A will terminate the handover attempt. MSC-A may retry the handover attempt using the cell identity list, if provided, or may reject the handover attempt towards BSS-A. The existing connection to the MS shall not be cleared.
When the A-HO-REQUEST-ACKNOWLEDGE has been received, MSC-A shall establish a circuit between MSC-A and MSC-B by signalling procedures supported by the network. In Figure 12 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 MS (subclause 6.1) 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 the subclause 6.1.
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.
MSC-B transfers to MSC-A the acknowledgement received from the correct MS (A-HO-DETECT/A-HO-COMPLETE). The A-HO-DETECT, if received, is transferred to MSC-A using the MAP-PROCESS-ACCESS-SIGNALLING request. The A-HO-COMPLETE, when received from the correct MS, is 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 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 MSC-A and 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 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 MSC-A and MSC-B has been established.
If the circuit between MSC-A and MSC-B cannot be established (e.g. an unsuccessful backward message is received instead of ACM). MSC-A terminates the inter-MSC handover attempt by sending an appropriate MAP message, for example an ABORT. MSC-A may retry the handover at this point, see subclause 6.1.
MSC-A shall retain overall call control until the call is cleared by the fixed subscriber or the 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 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 MSC-B.
MSC-A may terminate the procedure at any time by sending an appropriate MAP message to MSC-B. If establishment of the circuit between MSC-A and MSC-B has been initiated, the circuit must also be cleared.
The handover will be aborted by MSC-A if it detects clearing or interruption of the radio path before the call has been established on MSC-B.