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Content for  TS 23.009  Word version:  18.0.0

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6 General description of the procedures for intra - MSC handovers  6.1 Procedure for Intra-MSC Handovers 
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6  General description of the procedures for intra - MSC handovers  p. 29

This clause gives a brief overview of the procedures that shall be followed when performing Intra-MSC handovers. Detailed explanation of these procedures can be found in TS 48.008 and TS 24.008.
There are three types of GSM handover that involve a single BSS and a single MSC. These are "Internal Handover", "BSS Internal Handover with MSC Support" and" External Handover".
An "Internal Handover" takes place between channels on a cell or cells controlled by a single BSS, without reference to the MSC, although the MSC maybe informed of its occurrence after completion. This typical case can be used by the BSS e.g. if the A-Interface User Plane is not to be modified. This "Internal Handover" may take place with AoTDM or with AoIP and is not considered in the present document.
A "BSS Internal Handover with MSC Support" shall only be used if AoIP is supported by both MSC and BSS and if the A-Interface User Plane has to be modified. In that case the BSS or the MSC may initiate a "BSS Internal Handover with MSC Support" procedure as described in detail in subclause 6.3 in this document.
Handovers between channels on the same cell or between cells on the same BSS which are controlled by the MSC (as defined prior to the introduction of AoIP) are termed "External Handovers" and use identical procedures to those for Inter-BSS-Intra-MSC handovers. "External Handovers" are also specified with AoIP User Plane transport, for example the handover from speech to data services.Handovers from a BSS to an RNS controlled by the same 3G_MSC are intra-3G_MSC GSM to UMTS handovers. Handovers from an RNS to a BSS controlled by the same 3G_MSC are intra-3G_MSC UMTS to GSM handovers.
There are two types of handover in UMTS: soft handover and hard handover. The first one is fully performed within UTRAN, without involving the core network. The second one may be also performed within UTRAN or GERAN, or between GERAN and UTRAN, or the core network may be involved if the Iur or Iur-g interface between RNSs does not exist. This case of hard handover involving the core network is covered in the present document, together with SRNS relocation with Iur or Iur-g interface.
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6.1  Procedure for Intra-MSC Handovers  p. 29

The procedure for a successful External Intra-MSC handover is shown in Figure 7. It is assumed that selection of a candidate MS has already taken place within the BSS based upon the criteria presented in clause 5. The exact algorithm, in the BSS, for determining a candidate MS is not addressed in the present document. The procedures discussed do not make use of the Mobile Application Part (MAP), represented by signalling function 4 in Figure 2 and Figure 3. The procedure described in this clause covers case i).
Copy of original 3GPP image for 3GPP TS 23.009, Fig. 7: Basic External Intra-MSC Handover Procedure
Figure 7: Basic External Intra-MSC Handover Procedure
(⇒ copy of original 3GPP image)
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The successful operation of the procedure is as follows. When the BSS (BSS-A), currently supporting the MS, determines that the MS requires to be handed over it will send an A HANDOVER-REQUIRED message to the MSC (MSC-A). The A-HANDOVER-REQUIRED message shall contain a list of cells, or a single cell, to which the MS can be handed over. The list of cells shall be given in order of preference based upon operator determined criteria (These criteria are not addressed within the present document and are operator dependent). When the MSC-A receives the A-HANDOVER-REQUIRED message it shall begin the process of handing over the MS to a new BSS (BSS-B). (NOTE: BSS-A and BSS-B maybe the same BSS). The 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 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 TS 48.008.
Once resource allocation has been completed by BSS-B it shall return an A-HANDOVER-REQUEST-ACK. to MSC-A. When this message is received by MSC-A it shall begin the process of instructing the MS to tune to a new dedicated radio resource. An A-HANDOVER-COMMAND will be sent by the MSC-A to BSS-A. On receipt of the A-HANDOVER-COMMAND message BSS-A will send the radio interface message RI-HANDOVER-COMMAND, containing a Handover Reference number previously allocated by BSS-B, 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-B to ensure it is as expected and the correct MS has been captured. If this is the correct MS then the BSS-B shall send an A-HANDOVER-DETECT to MSC-A. When the MS is successfully communicating with the BSS-B a RI-HANDOVER-COMPLETE message will be sent by the MS to BSS-B. The BSS-B will then send an A-HANDOVER-COMPLETE message to MSC-A.
After MSC-A has received the A-HANDOVER-COMPLETE message from BSS-B it shall begin to release the resources allocated on BSS-A. In Figure 7 the resource is released by using the A CLEAR-COMMAND sequence.
In the case of ongoing GSM voice group calls the clearing of resources on BSS-A shall not be used if the resources are still be used on the down link.
If a failure occurs during the handover attempt, for example A-HANDOVER-FAILURE returned from BSS-A or BSS-B, then MSC-A will terminate the handover to BSS-B. Under these conditions MSC-A may optionally take one of a number of actions:
  1. retry the handover to the same cell;
  2. select the next cell from the list contained in the A-HANDOVER-REQUIRED message and attempt a handover to the new cell;
  3. await the next A-HANDOVER-REQUIRED message;
  4. send an A-HANDOVER-REQUIRED-REJECT to BSS-A, if an A-HANDOVER-COMMAND has not already been sent.
The exact action taken is dependent on whether the failure occurs before or after the A-HANDOVER-COMMAND has been sent.
In all cases the existing connection to the 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 MS is not in communication with the network MSC-A shall queue all appropriate messages. All messages shall be delivered to the MS once communication is resumed . In the case of an Intra-MSC handover on MSC-B then the messages shall be queued by MSC-B.
In the case of ongoing GSM voice group calls if a failure occurs when handing over a user on a dedicated channel then the procedures described above may optionally be applied.
For the case of subsequent Inter-BSS Intra-MSC-B or Inter-BSS Intra-3G_MSC-B handover the following applies:
If handover to an A over IP capable BSS-B is performed, MSC-B/3G_MSC-B includes a Codec List (MSC preferred) in the A-HANDOVER-REQUEST message to BSS-B. MSC-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 MSC-B/3G_MSC-B see TS 23.153. If the AoIP-Supported Codecs List (Anchor) was not provided or MSC-B/3G_MSC-B does not support the selection of codecs from the AoIP-Supported Codecs List (Anchor), then MSC-B/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 Intra-MSC-B handover or Intra-3G_MSC-B 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 target BSS-B uses A interface over IP and BSS-B does not insert a transcoder.
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