Content for  TS 23.009  Word version:  18.0.0

The successful operation of the procedure is as follows. When the RNS (RNS-A), currently supporting the UE/MS, determines that the UE/MS requires to be handed over to GSM it will send an IU-RELOCATION-REQUIRED message to the 3G_MSC (3G_MSC-A). The IU-RELOCATION-REQUIRED message shall contain a single cell, to which the UE/MS can be handed over. When the 3G_MSC-A receives the IU-RELOCATION-REQUIRED message it shall begin the process of handing over the UE/MS to a BSS (BSS-B). The 3G_MSC-A shall generate an A-HANDOVER-REQUEST message to the selected BSS (BSS-B). When BSS-B receives the A HANDOVER-REQUEST message it shall take the necessary action to allow the UE/MS to access the radio resource of BSS-B, this is detailed in TS 48.058 and in TS 45.008. The switching of the radio resource through the necessary terrestrial resources is detailed in TS 24.008 and 3GPP TS 08.08 [5]. Once resource allocation has been completed by BSS-B it shall return an A-HANDOVER-REQUEST-ACK. to 3G_MSC-A. When this message is received by 3G_MSC-A it shall begin the process of instructing the UE/MS to tune to a new dedicated radio resource. An IU-RELOCATION-COMMAND will be sent by the 3G_MSC-A to RNS-A. On receipt of the IU-RELOCATION-COMMAND message RNS-A will send the radio resource control message RRC-HANDOVER-COMMAND, containing a Handover Reference number previously allocated by BSS-B, to the UE/MS. The UE/MS will then access the new radio resource using the Handover Reference number contained in the RI-HANDOVER-ACCESS message. The number will be checked by BSS-B to ensure it is as expected and the correct UE/MS has been captured. If this is the correct UE/MS then the BSS-B shall send an A-HANDOVER-DETECT to 3G_MSC-A. When the UE/MS is successfully communicating with the BSS-B a RI-HANDOVER-COMPLETE message will be sent by the UE/MS to BSS-B. The BSS-B will then send an A-HANDOVER-COMPLETE message to 3G_MSC-A. After 3G_MSC-A has received the A-HANDOVER-COMPLETE message from BSS-B it shall begin to release the resources allocated on RNS-A. In Figure 8 the resource is released by using the IU-RELEASE-COMMAND sequence. If a failure occurs during the handover attempt, for example A-HANDOVER-FAILURE returned from BSS-B, then 3G_MSC-A will terminate the handover to BSS-B and send an IU-RELOCATION-PREPARATION-FAILURE message to RNS-A. If RNS-A has decided to cancel the handover, it sends IU-RELOCATION-CANCEL message to 3G_MSC-A. The 3G_MSC-A will then terminate the handover towards BSS-B (if initiated) and send IU-RELOCATION-CANCEL- ACKNOWLEDGE message to RNS-A. In all cases the existing connection to the UE/MS shall not be cleared except in the case of expiry of the timer for receipt of A-HANDOVER-COMPLETE. During the period that the UE/MS is not in communication with the network 3G_MSC-A shall queue all appropriate messages. All messages shall be delivered to the UE/MS once communication is resumed. In the case of an Intra-3G_MSC handover from UMTS to GSM on 3G_MSC-B then the messages shall be queued by 3G_MSC-B. For the case of subsequent Inter-system UMTS to GSM Intra-3G_MSC-B handover the following applies: If handover to an A over IP capable BSS-B is performed, 3G_MSC-B includes a Codec List (MSC preferred) in the A-HANDOVER-REQUEST message to BSS-B. 3G_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/3G_MSC-A in the MAP-PREPARE-HANDOVER request. For a detailed description of the handling of these codec lists by MSC-A/3G_MSC-A and 3G_MSC-B see TS 23.153. If the AoIP-Supported Codecs List (Anchor) was not provided or 3G_MSC-B does not support the selection of codecs from the AoIP-Supported Codecs List(Anchor), then 3G_MSC-B shall create the Codec List (MSC preferred) using the channel type information received from MSC-A/3G_MSC-A in the A HANDOVER-REQUEST message included in the MAP-PREPARE-HANDOVER request. After successful completion of the Inter-system UMTS to GSM Intra-3G_MSC-B handover, if 3G_MSC-B received the AoIP-Supported Codecs List (Anchor), MSC-B/3G_MSC-B may send the new AoIP-Selected Codec (Target) and AoIP-Available Codecs List (MAP) to MSC-A/3G_MSC-A in the MAP-PROCESS-ACCESS-SIGNALLING request transporting the A HANDOVER-PERFORMED message, if the following conditions are fulfilled: 3G_MSC-B created a Codec List (MSC preferred) from the AoIP-Supported Codecs List (Anchor), the target BSS-B uses A interface over IP and BSS-B does not insert a transcoder.

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 6.2.1.1. 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, 3G_MSC-A generates an A-HO-REQUEST message for the selected bearer to BSS-B. When an A-HO-REQUEST-ACK is received from BSS-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 A-HO-COMPLETE message from BSS-B, 3G_MSC-A shall release calls via BSS-B, which has been carried by the bearers not to be handed over, and then sends IU-RELEASE-COMMAND to RNS-A.