Content for  TS 23.009  Word version:  18.0.0

If the A-Interface User Plane is carried over IP (or shall be handed over to IP) and one or more of the A-Interface User Plane parameters need to be modified, for example the Codec Type, or the Codec Configuration (BSS determines that no compatible Codec Type or Codec Configuration exists for the target cell), or the IP Transport Layer Address, or the UDP Port, or the CSData Redundancy Level, or the A-Interface Type itself (e.g. from TDM to IP or vice versa), then a "BSS Internal Handover with MSC support" shall be performed (see subclause 3.1.5c.1 of TS 48.008). The "BSS Internal Handover with MSC support" for AoIP is performed by the MSC that is currently serving the connected BSS (in the following just termed "serving MSC"); it may be either MSC-A, MSC-B, 3G_MSC-A or 3G_MSC-B. The "BSS Internal Handover with MSC support" applies only if both BSS and Core Network support the AoIP procedures and messages, and an A-Interface User Plane connection has been established beforehand. The procedures and messages for this "BSS Internal Handover with MSC support" are described in TS 48.008. The "BSS Internal Handover with MSC Support" can be initiated either:

1. by the BSS, by sending the A-INTERNAL-HANDOVER-REQUIRED message; or:
2. by the serving MSC, by sending the A-INTERNAL-HANDOVER-ENQUIRY message.

The BSS-initiated "BSS Internal Handover with MSC Support" starts with an A-INTERNAL-HANDOVER-REQUIRED message from the BSS to the serving MSC, for further details see subclause 3.1.5c of TS 48.008. An example sequence is shown in Figure 6.3.2.1

The A-INTERNAL-HANDOVER-REQUIRED message contains a reason for the required handover and the currently valid Codec List (BSS Supported). It shall also contain an AoIP Transport Layer Address and UDP Port, if the BSS requires an IP-based target User Plane. The Codec List (BSS supported) contains the key requirements from the BSS, like target Codec Type(s), target Codec Configuration(s) and target A-interface Type(s) (TDM and/or IP), and may contain the required Redundancy Level for CSData, etc. When sending the A-INTERNAL-HANDOVER-REQUIRED message the BSS starts a timer "T25" (TS 48.008) and it expects an answer from the serving MSC within that timer period. If "T25" (TS 48.008) expires before the MSC has answered, then the BSS ignores any subsequent (late) answer from the serving MSC after expiry of timer "T25" (TS 48.008). The BSS will not send any new A-INTERNAL-HANDOVER-REQUIRED message before timer "T25" (TS 48.008) has expired or before the Internal Handover Preparation is terminated by other reasons. When the serving MSC receives the A-INTERNAL-HANDOVER-REQUIRED message it shall start timer T105 (see subclause 9.3A). The serving MSC shall not send any answer to the BSS after timer T105 has expired. Both timers ("T25" - TS 48.008 and T105) shall be configured (by O&M) to minimise the likelihood that the answer from serving MSC to BSS crosses with a new or repeated A-INTERNAL-HANDOVER-REQUIRED message from the BSS to the serving MSC, i.e. the timer T105 shall always expire before "T25" (TS 48.008) expires. If the serving MSC is able to fulfil the required "BSS Internal Handover with MSC Support", then it shall generate and send an A-INTERNAL-HANDOVER-COMMAND message to the BSS and stop timer T105. This answer shall contain the exact new A-Interface User Plane parameters, e.g. Codec Type, Codec Configuration, A-Interface Type, either TDM Circuit Identity Code or IP Transport Layer Address and UDP Port (see TS 48.008). While T25 is still running the BSS can either accept or reject the A-INTERNAL-HANDOVER-COMMAND. When the BSS receives the A-INTERNAL-HANDOVER-COMMAND message it takes the necessary action to allow the MS to access the radio resource of the new cell in BSS, 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 44.018 and TS 48.008. On receipt of the A-INTERNAL-HANDOVER-COMMAND message the BSS will send e.g. the radio interface message RI-HANDOVER-COMMAND, containing a Handover Reference number previously allocated to the MS. The 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 to ensure it is as expected and the correct MS has been captured. As BSS and MS proceed with the handover the BSS may send an A-HANDOVER-DETECT message to the serving MSC to enable fast User Plane switching on the Core Network side. As soon as the MS and BSS have completed the handover the BSS send an A-HANDOVER-COMPLETE message to serving MSC. Both BSS and serving MSC will then release the no longer needed BSS and Core Network resources. If the serving MSC is unable to support the required Internal Handover due to whatever reason then it shall send an A-INTERNAL-HANDOVER-REQUIRED-REJECT message to the BSS (if T105 has not expired already). The serving MSC shall not send an A-INTERNAL-HANDOVER-REQUIRED-REJECT message after an A-INTERNAL-HANDOVER-COMMAND has been sent to the BSS. If a failure occurs during the handover attempt and the BSS sends an A-HANDOVER-FAILURE message, then the serving MSC shall terminate the handover and shall revert back to using the resources used before the handover attempt was made. The serving MSC shall supervise the "BSS Internal Handover with MSC Support" procedure after sending the A-INTERNAL-HANDOVER-COMMAND using the same timer (T102) as used for Intra-MSC handover, see subclauses 9.3 and 11.3. In all cases the existing connection to the MS shall not be cleared, except in the case of expiry of the timer T102 before receipt of A-HANDOVER-COMPLETE. Whilst the MS is not in communication with the Core Network (i.e. in the time span between sending of A-INTERNAL-HANDOVER-COMMAND and the reception of A-HANDOVER-COMPLETE or an A-HANDOVER FAILURE) the serving MSC shall queue all appropriate messages towards the MS. All these messages shall be delivered to the MS once the communication is resumed. For the case of subsequent Intra-BSS handover with support from MSC-B or 3G_MSC-B the following applies: After successful completion of the Intra-BSS handover, if MSC-B/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: MSC-B/3G_MSC-B created a Codec List (MSC preferred) from the AoIP-Supported Codecs List (Anchor) received from MSC-A/3G_MSC-A, the BSS uses A interface over IP and the BSS does not insert a transcoder.

During a call the MSC may request to modify the A-Interface User Plane, for example to change the Codec Type or Codec Configuration on the A-Interface to optimise end-to-end speech quality by avoiding transcoding. The serving MSC may initiate a "BSS Internal Handover with MSC Support" by sending an A-INTERNAL-HANDOVER-ENQUIRY message to the BSS containing, within the Speech Codec (MSC Chosen) IE, the serving MSC's preferred speech Codec Type and Codec Configuration and A-Interface Type. If accepted by the BSS, the BSS responds with an A-INTERNAL-HANDOVER-REQUIRED message, as described in subclause 6.3.2, with reason "Response to an INTERNAL HANDOVER ENQUIRY". Then the "BSS Internal Handover with MSC Support" may start. If the BSS does not accept the A-INTERNAL-HANDOVER-ENQUIRY message, then it returns an A-HANDOVER-FAILURE message to the serving MSC.