6.3 Handover

3GPP43.055Dual Transfer Mode (DTM)Release 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.3.1 General

Another group of procedures that are affected by the definition a new GPRS class A mode of operation are those related to the change of the serving cell when the mobile station is in dual transfer mode. The term handover in this document refers to the network initiated change of serving cell for both domains, unless explicit reference to the CS domain is made. The handover and the cell change of the CS and PS domains respectively need to be performed at the same time. As 3GPP TS 45.008 [7] states, the serving cell for a class A mobile station while it is in dedicated mode "is determined by the network according to the handover procedures", irrespective of the Network Control measuring report mode (NC).

The Handover Command message sent from the network to the mobile station shall describe the CS resources in the target cell.

The RAI, Cell Identity and information whether DTM is allowed or not shall be sent to a DTM capable mobile station after handover in SI6 and/or the DTM Information message. The RAI needs also to be included in the SI 6 message sent to a DTM capable mobile station that is not in DTM so that it can detect a change of the RA.

Handover failure cases are determined only from the CS timeslot. In the event of a handover failure, the mobile station shall return to the CS resource in the old cell and send a Handover Failure message on the main DCCH. If the mobile station is in GMM Ready state, it shall then perform the Cell Update procedure in order to notify the SGSN that downlink data flows can be continued in the source cell. The mobile station shall assume that all the packet resources were released during the handover and it shall try to re-establish the uplink resources if there is uplink data ready to be sent.

Once the main DCCH is established in the cell, the network sends the DTM Information message. This message contains:

– the RAI and Cell Identity of the new cell: to detect changes of RA or cell without waiting for the SI 6 message;

– the length limitation for the use of the main DCCH.

Then the mobile station or the network may re-establish the packet resource(s).

6.3.1a DTM Handover – General

The term DTM Handover in this document always refers to the network initiated change of serving cell for both CS and PS domains, using DTM Handover procedures.

The source and target cells may be managed by either the same BSS (Intra-BSS) or different BSSs (Inter-BSS) within the same MSC (Intra-MSC) and the same SGSN (Intra SGSN) or different SGSNs (Inter SGSN) or different MSCs (Inter-MSC) and the same or different SGSNs.The DTM Handover in A/Gb mode makes use of existing CS handover procedures and PS Handover procedures. Unless explicitly stated in the present document, the behaviour of the core network entities is as specified for the respective handover procedures. The DTM Handover procedure is controlled by the RR protocol.

The DTM Handover procedure is divided into:

– a preparation phase including the allocation of CS and PS resources in the target cell, consisting of parallel CS handover preparation phase as described in 3GPP TS 23.009 [14] and PS handover preparation phase as described in 3GPP TS 43.129 [13]; and

– an execution phase which includes the sending of the (RR) DTM HANDOVER COMMAND message from the network to the mobile station on PACCH. The (RR) DTM HANDOVER COMMAND message shall describe both the CS and the PS resources in the target cell.

In the case of a DTM Handover into a network sharing supporting target cell by a MS supporting network sharing, the DTM HANDOVER COMMAND shall include PLMN index information which, upon arriving in the target cell, the MS uses as described in 3GPP TS 44.018 [5].

6.3.2 Internal handover

The network may send a Handover Command message requesting the mobile station to switch to a different cell parented by the same BSC. Prior to that, the BSC shall activate the channels in the target cell. At the receipt of the Handover Command message the mobile station shall abandon the packet session and initiate the access on the target cell, obeying the handover time requirements of 3GPP TS 45.010 [8] clause 6 and 3GPP TS 44.013 [3] clause 5.2.6.

The re-establishment of the CS connection shall continue as a CS only handover. When concluded, the BSC shall release the channels in the old cell.

The network immediately sends the DTM Information message, with information needed to resume the GPRS operation in the new cell. Once the mobile station has the necessary information, it shall perform a cell update or RA update procedure.

If the mobile station also needs to (re-)establish an uplink packet session in the new cell, the GMM signalling procedure shall take precedence and shall be performed first. Once the update procedure is performed, the (re-)establishment of the packet session may continue.

Figure 11 shows the exchange of messages in a successful internal handover.

Figure 11: Successful internal, dual handover procedure

6.3.2a Intra-BSS DTM Handover

6.3.2a.1 General

The Intra-BSS DTM Handover procedure covers the scenarios where the source and target cells are managed by the same BSS, the same MSC and the same SGSN. The Intra-BSS with MSC involved or MSC not involved DTM Handover procedures may use the optimized PS Handover procedure (see 3GPP TS 43.129) if allowed.

The Intra-BSS inter-cell DTM Handover using optimized PS Handover procedure and MSC non-involved case and Intra-BSS inter-cell DTM Handover using non-optimized PS Handover procedure and MSC involved case are described in sub-clauses 6.3.2a.2, 6.3.2a.3 and 6.3.2a.4.

The Intra-BSS inter-cell DTM Handover using optimized PS Handover procedure and MSC involved case and Intra-BSS inter-cell DTM Handover using non-optimized PS Handover procedure and MSC not involved case are not explicitly described in the current document as they are considered to be implicitly covered by the cases that are described.

6.3.2a.2 Preparation Phase – using Optimized PS Handover procedure and without MSC involved

The preparation phase consists of the BSS allocating CS and PS resources in the target cell. The BSS shall select the same unique target cell for both the CS and the PS domains. The BSS shall activate the DTM channel configurations (CS channel and PS resources) in the target cell.

6.3.2a.3 Preparation Phase – using Non-Optimized PS Handover procedure and with MSC involved

The preparation phase consists of the BSS sending a (BSSMAP) HANDOVER REQUIRED message to the MSC and a (BSSGP) PS HANDOVER REQUIRED PDU to the SGSN and allocating CS and PS resources in the target cell upon receiving a (BSSMAP) HANDOVER REQUEST message from the MSC and a (BSSGP) PS HANDOVER REQUEST PDU from the SGSN. The BSS shall select the same unique target cell for both the CS and the PS domains. The handling of the messages between BSS and CN is largely based on the Inter-BSS DTM Handover as described in section 6.3.3a.

6.3.2a.4 Execution Phase

In case of Intra-BSS DTM Handover using the optimised PS Handover procedure and without the MSC involved, the BSS may start the execution phase by sending a (RLC/MAC) DTM HANDOVER COMMAND message to the mobile station on the PACCH requesting it to perform a DTM Handover and switch to a different cell managed by the same BSS.

In case of Intra-BSS DTM Handover using the non-optimised PS Handover procedure and with the MSC involved, the BSS may start the execution phase after receiving the (BSSMAP) HANDOVER COMMAND message from the MSC and the (BSSGP) PS HANDOVER REQUIRED ACK PDU from the SGSN.

At the receipt of the (RLC/MAC) DTM HANDOVER COMMAND message the mobile station switches to the new configuration and initiates the access on the target cell using the existing CS handover access procedures.

After successful establishment of the main signalling link in the target cell, the mobile station sends the (RR) HANDOVER COMPLETE message to the BSS which in turns sends either a (BSSMAP) HANDOVER PERFORMED message (in case of MSC not involved) or a (BSSMAP) HANDOVER COMPLETE message (in case of MSC involved) to the MSC and a (BSSGP) PS HANDOVER COMPLETE PDU to the SGSN (regardless of whether or not optimised PS Handover was used) to indicate the completion of the Intra-BSS DTM Handover. Upon successful completion of the Intra-BSS DTM Handover, the BSS releases the DTM channel configurations (i.e. CS channel and PS resources) in the old cell.

During Intra-BSS with MSC involved DTM Handover using non-optimized PS Handover procedures, the mobile station shall start the Cell/RA Update procedure immediately after sending the (RR) HANDOVER COMPLETE message to the network.

If the mobile station is not able to act on or decode the (RLC/MAC) DTM HANDOVER COMMAND message, it sends a (RR) HANDOVER FAILURE message to the network on the main DCCH of the source cell.

If the mobile station acts on the (RLC/MAC) DTM HANDOVER COMMAND message but fails to establish the main signalling link in the target cell, the MS returns to the old channels in the source cell and sends a (RR) HANDOVER FAILURE message to the network on the main DCCH.

In case the responses received by the BSS do not consist of the combination of both a (BSSMAP) HANDOVER COMMAND message and a (BSSGP) PS HANDOVER REQUIRED ACK PDU, the Intra-BSS DTM Handover using non-optimized PS handover and with MSC involved fails and the BSS and the mobile station proceed as per the Inter-BSS DTM Handover described in section 6.3.3a.3

Figure 11a shows the exchange of messages in a successful inter-cell Intra-BSS DTM Handover using optimized PS Handover procedure and MSC not involved case.

Figure 11b and Figure 11c show the exchange of messages in a successful inter-cell Intra-BSS DTM Handover using non-optimized PS Handover procedure and MSC involved scenario.

Figure 11a: Intra-BSS with MSC not involved DTM Handover using optimized PS Handover procedures

Figure 11b: Intra-BSS with MSC involved DTM Handover using non-optimized PS Handover procedures, preparation phase

Figure 11c: Intra-BSS with MSC involved DTM Handover using non-optimized PS Handover procedures, execution phase

6.3.3 External handover

In the case of an external handover, the target BSS:

– shall be provided with the IMSI of the mobile station;

– shall be provided with information about the nature of the packet resources in the serving cell, so that the CS resource is compatible with the packet resources that are going to be requested in the new cell (e.g. transceiver supporting AMR or EDGE, timeslot with a free, adjacent one). This information is conveyed in the Old BSS to New BSS Information IE.

NOTE: This indication that the MS is in DTM in the source cell is also included in the handover to GERAN from another RAT when the MS has resouces in the source cell allocated towards the CS and PS domains simultaneously.

No changes are foreseen for an inter-MSC handover. Current implementations are expected to be able to carry the extended Old BSS to New BSS Information IE without modifications to 3GPP TS 49.008.

No changes are foreseen for an inter-SGSN handover. The mobile shall perform a Routing Area Update procedure in the new cell. This may be as a result of the SI 6 contents (RAC is now added) or caused by information contained in the DTM INFORMATION message.

6.3.3a Inter-BSS DTM Handover

6.3.3a.1 General

The Inter-BSS DTM Handover procedure covers the scenarios where the source and target cells are managed by different BSSs. All four cases of Intra-/Inter-MSC and Intra-/Inter-SGSN handovers use these procedures.

The Inter-BSS DTM Handover is initiated by the source BSS sending a (BSSMAP) HANDOVER REQUIRED message and a (BSSGP) PS HANDOVER REQUIRED PDU to the MSC and the SGSN respectively. The Inter-BSS DTM Handover requires synchronization of the handovers in the CS and the PS domains in both the source BSS and the target BSS through:

– Selection of the same unique target cell for both the CS and the PS domains. In the preparation phase the source BSS selects the same target cell ID for both the CS and the PS domains and indicates it to the MSC and the SGSN in the (BSSMAP) HANDOVER REQUIRED message and (BSSGP) PS HANDOVER REQUIRED PDU respectively.

– Indications to the target BSS that the CS (respectively PS) handover is ongoing at the same time as the PS (respectively CS) handover for the same mobile station (see Figure 11d). These indications require the target BSS to wait for both the (BSSMAP) HANDOVER REQUEST message and (BSSGP) PS HANDOVER REQUEST PDU.

– PS Indication IE is sent in the (BSSMAP) HANDOVER REQUIRED message and (BSSMAP) HANDOVER REQUEST message within the Old BSS to New BSS information.

– CS Indication IE is sent in the (BSSGP) PS HANDOVER REQUIRED PDU and (BSSGP) PS HANDOVER REQUEST PDU within the Source BSS to Target BSS transparent container.

– Management of synchronization timers in both the source BSS and the target BSS that ensure the target receives both PS and CS domain resource allocation requests and the source BSS receives both PS and CS domain resource allocation responses from the target BSS before proceeding with the Inter-BSS DTM Handover.

6.3.3a.2 Preparation Phase

The Inter-BSS DTM Handover is initiated by the source BSS by sending a (BSSMAP) HANDOVER REQUIRED message and a (BSSGP) PS HANDOVER REQUIRED PDU to the MSC and the SGSN respectively.

The target BSS, upon reception of a (BSSMAP) HANDOVER REQUEST message (respectively (BSSGP) PS HANDOVER REQUEST PDU) containing an indication of an ongoing PS handover (respectively CS handover) as described in sub-clause 6.3.3a.1, waits for reception of the corresponding (BSSGP) PS HANDOVER REQUEST PDU (respectively (BSSMAP) HANDOVER REQUEST message).

If the target BSS receives a (BSSMAP) HANDOVER REQUEST message containing an indication of an ongoing PS handover, but does not receive a corresponding (BSSGP) PS HANDOVER REQUEST PDU within the expected time frame, the target BSS may proceed with allocating a CS resource only, in which case it returns a (BSSMAP) HANDOVER REQUEST ACKNOWLEDGE message containing a (RR) HANDOVER COMMAND message (within the L3 Information IE). Otherwise, if target BSS decides not to continue with the handover of a CS resource it returns a (BSSMAP) HANDOVER FAILURE message to the MSC. If the target BSS receives the corresponding (BSSGP) PS HANDOVER REQUEST PDU containing an indication of an ongoing CS handover after the expected time frame, it shall return a (BSSGP) PS HANDOVER REQUEST NACK PDU to the SGSN.

If the target BSS receives a (BSSGP) PS HANDOVER REQUEST PDU containing an indication of an ongoing CS handover, but does not receive a corresponding (BSSMAP) HANDOVER REQUEST message within the expected time frame, it shall return a (BSSGP) PS HANDOVER REQUEST NACK PDU to the SGSN and abort the DTM Handover. If the target BSS receives the corresponding (BSSMAP) HANDOVER REQUEST message containing an indication of an ongoing PS handover after the expected time frame, it shall return a (BSSMAP) HANDOVER FAILURE message to the MSC.

If the target BSS receives both a (BSSGP) PS HANDOVER REQUEST PDU and a (BSSMAP) HANDOVER REQUEST message within the expected time frame and is able to allocate both the CS and PS resources, it returns a (BSSMAP) HANDOVER REQUEST ACKNOWLEDGE message to the MSC and a (BSSGP) PS HANDOVER REQUEST ACK PDU to the SGSN. Both these messages contain the same (RLC/MAC) DTM HANDOVER COMMAND message.

If the target BSS receives both a (BSSGP) PS HANDOVER REQUEST PDU and a (BSSMAP) HANDOVER REQUEST message within the expected time frame and allocates a CS resource but is unable to or chooses not to allocate any of the corresponding PS resources, it returns a (BSSGP) PS HANDOVER REQUEST NACK PDU to the SGSN. The target BSS may continue with the CS handover in which case it returns a (BSSMAP) HANDOVER REQUEST ACKNOWLEDGE message containing a (RR) HANDOVER COMMAND message (within the L3 Information IE). Otherwise, if the target BSS decides not to continue with the CS handover it returns a (BSSMAP) HANDOVER FAILURE message to the MSC.

If the target BSS receives both a (BSSGP) PS HANDOVER REQUEST PDU and a (BSSMAP) HANDOVER REQUEST message within the expected time frame but is unable to or chooses not to allocate a CS resource, it shall not allocate the corresponding PS resources. The target BSS returns a (BSSMAP) HANDOVER FAILURE message and a (BSSGP) PS HANDOVER REQUEST NACK PDU to the MSC and the SGSN respectively.

Figure 11d shows the exchange of messages in a successful Inter-BSS Intra-MSC Intra-SGSN DTM Handover, preparation phase. The same procedures are also used for an Inter-BSS Inter-MSC handover and/or Inter-SGSN DTM Handover.

Figure 11d: Inter-BSS DTM Handover, preparation phase

6.3.3a.3 Execution Phase

If the source BSS receives both a (BSSGP) PS HANDOVER REQUIRED ACK PDU and a (BSSMAP) HANDOVER COMMAND message within the expected time frame, it sends the (RLC/MAC) DTM HANDOVER COMMAND message to the mobile station.

The source BSS may instead send the (RLC/MAC) DTM HANDOVER COMMAND message to the mobile station as soon as it has received a (BSSMAP) HANDOVER COMMAND message which contains the (RLC/MAC) DTM HANDOVER COMMAND message.

NOTE: In this case, it is possible that the PS resources in the target cell which are described in the (RLC/MAC) DTM HANDOVER COMMAND message will not be valid when the MS performs access in the target cell.

If the source BSS receives from the MSC a (BSSMAP) HANDOVER COMMAND message containing a (RR) HANDOVER COMMAND message it may choose to proceed with the CS handover immediately by stopping the synchronisation timer and sending the contents of the L3 Information IE (i.e. the (RR) HANDOVER COMMAND message) to the mobile station. If the source BSS chooses not to proceed with the CS handover, it sends a (BSSMAP) HANDOVER FAILURE message to the MSC and nothing is sent to the mobile station.

If the source BSS receives a (BSSMAP) HANDOVER COMMAND message containing a (RLC/MAC) DTM HANDOVER COMMAND message and a (BSSGP) PS HANDOVER REQUIRED NACK PDU within the expected time frame, and the failure cause in (BSSGP) PS HANDOVER REQUIRED NACK PDU indicates an SGSN failure, the source BSS shall abort both the CS and PS handovers and send a (BSSMAP) HANDOVER FAILURE message to the MSC and nothing is sent to the mobile station.

If the source BSS receives a (BSSMAP) HANDOVER REQUIRED REJECT message and a (BSSGP) PS HANDOVER REQUIRED ACK PDU within the expected time frame, it shall abort the PS handover by sending a (BSSGP) PS HANDOVER CANCEL PDU to the SGSN and nothing is sent to the mobile station.

If the source BSS receives both a (BSSMAP) HANDOVER REQUIRED REJECT message and a (BSSGP) PS HANDOVER REQUIRED NACK PDU then the DTM Handover fails and nothing is sent to the mobile station.

On the receipt of the (RLC/MAC) DTM HANDOVER COMMAND message the mobile station switches to the new configuration and initiates the access on the target cell using the existing CS handover access procedures. After successful establishment of the main signalling link in the target cell, the mobile station sends the (RR) HANDOVER COMPLETE message to the target BSS which in turns sends both a (BSSMAP) HANDOVER COMPLETE message to the MSC and a (BSSGP) PS HANDOVER COMPLETE PDU to the SGSN to indicate the completion of the handover. Thereafter the release of the old DTM channel configurations (CS channel and PS resources) is initiated by the MSC and the SGSN respectively.

The mobile station shall start the Cell/RA Update procedure immediately after sending the (RR) HANDOVER COMPLETE message to the network.

If the mobile station is not able to act on or decode the (RLC/MAC) DTM HANDOVER COMMAND message, it sends a (RR) HANDOVER FAILURE message to the network on the main DCCH of the source cell.

If the mobile station fails to establish the main signalling link in the target cell, the MS returns to the old channels in the source cell and sends a (RR) HANDOVER FAILURE message to the network on the main DCCH.

If the source BSS receives a (RR) HANDOVER FAILURE message from the mobile station, it cancels the current DTM Handover by sending a (BSSMAP) HANDOVER FAILURE message and a (BSSGP) PS HANDOVER CANCEL PDU to the MSC and SGSN respectively.

Figure 11e shows the exchange of messages in a successful Inter-BSS Intra-MSC Intra-SGSN DTM Handover, execution phase. The same procedures are also used for an inter-MSC handover and/or inter-SGSN DTM Handover.

Figure 11e: Inter-BSS DTM Handover, execution phase

6.3.4 Inter-RAT DTM Handover

6.3.4.1 General

The Inter-RAT DTM Handover refers only to the DTM Handover performed between GERAN A/Gb mode and UTRAN. The inter-RAT DTM handover from GERAN A/Gb mode to E-UTRAN according to the DTM handover principles is not feasible as there is no CS domain support in E-UTRAN. The handover from E-UTRAN to GERAN DTM is described in 3GPP TS23.216 [19].

The source and target cells may be managed by either the GERAN BSS or the UTRAN RNC. The BSS and RNC may be within the same MSC (Intra-MSC) and the same SGSN (Intra SGSN) or different SGSNs (Inter SGSN) or different MSCs (Inter-MSC).

The Inter-RAT DTM Handover between GERAN A/Gb mode and UTRAN makes use of existing Inter-RAT CS Handover procedures and Inter-RAT PS Handover procedures. Unless explicitly stated in the present document, the behaviour of the core network entities is as specified for the respective handover procedures. The Inter-RAT DTM Handover procedure is controlled by the RR protocol in GERAN A/Gb mode and the RRC protocol in UTRAN.

The Inter-RAT DTM Handover procedure is divided into:

– a preparation phase including the allocation of CS and PS resources in the target cell, consisting of parallel Inter-RAT CS handover preparation phase as described in 3GPP TS 23.009 [14] and Inter-RAT PS handover preparation phase as described in 3GPP TS 43.129 [13]; and

– an execution phase which includes the sending of a (RRC) HANDOVER FROM UTRAN COMMAND message containing a (RLC/MAC) DTM HANDOVER COMMAND message from the network to the mobile station for Inter-RAT DTM Handover procedure from UTRAN to GERAN A/Gb mode or the sending of a (RLC/MAC) DTM HANDOVER COMMAND message containing a (RRC) HANDOVER TO UTRAN COMMAND message for the Inter-RAT DTM Handover from GERAN A/Gb mode to UTRAN. The (RLC/MAC) DTM HANDOVER COMMAND message and the (RRC) HANDOVER TO UTRAN COMMAND message shall describe both the CS and PS resources in the target cell.

Unless explicitly stated otherwise in the present document, the Inter-RAT DTM Handover procedure follows the Inter BSS DTM Handover procedure defined in sub-clause 6.3.3a and the UTRAN-UTRAN SRNS Relocation procedure (for two signaling connections) as defined in 3GPP TS 25.413 [16]. The correspondence between the messages/PDUs/IEs used over RANA P [16] and BSSMAP [17]/BSSGP [18] is described in Tables 2a, 2b, 2c and 2d.

Table 2a. Messages/PDUs/IEs exchanged between network nodes in the direction from the source BSS/RNC to the target BSS/RNC

→		Target BSS	Target RNC	MSC	SGSN
Source BSS	CS	Old BSS to New BSS Information IE (see Note 1)	Source RNC to Target RNC Transparent Container (see Note 1)	Handover Required	N/A
Source BSS	PS	Source BSS to Target BSS Transparent Container IE (see Note 1)	Source RNC to Target RNC Transparent Container (see Note 1)	N/A	PS Handover Required
MSC	CS	Handover Request	Relocation Request	N/A	N/A
SGSN	PS	PS Handover Request	Relocation Request	N/A	N/A
Source RNC	CS	Old BSS to New BSS Information IE (see Note 1)	Source RNC to Target RNC Transparent Container (See Note 1)	Relocation Required	N/A
Source RNC	PS	Source BSS to Target BSS Transparent Container IE (see Note 1)	Source RNC to Target RNC Transparent Container (See Note 1)	N/A	Relocation Required
NOTE 1: This is an Information Element exchanged between a source BSS/RNC and a target BSS/RNC through the Core Network

Table 2b. Messages/PDUs/IEs exchanged between network nodes in the direction from the target BSS/RNC to the source BSS/RNC

→		Source BSS	Source RNC	MSC	SGSN
Target BSS	CS	Layer 3 Information IE (see Note 1)		Handover Request Acknowledge	N/A
		N/A		Handover Failure
	PS	Target BSS to Source BSS Transparent Container IE (see Note 1)		N/A	PS Handover Request ACK
		N/A			PS Handover Request NACK
MSC	CS	Handover Command	Relocation Command	N/A	N/A
		Handover Required Reject	Relocation Preparation Failure
SGSN	PS	PS Handover Required ACK	Relocation Command	N/A	N/A
		PS Handover Required NACK	Relocation Preparation Failure
Target RNC	CS	Target RNC to Source RNC Transparent Container IE (see Note 1)		Relocation Request Acknowledge	N/A
		N/A		Relocation Failure
	PS	Target RNC to Source RNC Transparent Container IE (see Note 1)		N/A	Relocation Request Acknowledge
		N/A			Relocation Failure
NOTE 1: This is an Information Element exchanged between a target BSS/RNC and a source BSS/RNC through the Core Network

Table 2c. Messages/PDUs exchanged between the target BSS/RNC and the core network upon successful handover

→	MSC	SGSN
Target BSS	Handover Complete	PS Handover Complete
Target RNC	Relocation Complete	Relocation Complete

Table 2d. Messages/PDUs exchanged between the source BSS/RNC and the core network upon unsuccessful handovers

→	MSC	SGSN
Source BSS	Handover Failure	PS Handover Cancel
Source RNC	Relocation Cancel	Relocation Cancel

6.3.4.2 Inter-RAT DTM Handover from GERAN A/Gb mode to UTRAN

For the Inter-RAT DTM handover from GERAN A/Gb mode to UTRAN procedure, the behaviour of the source BSS (and the MS in the source cell) is as specified for the Inter-BSS DTM Handover procedure described in sub-clause 6.3.3a and the behaviour of the target RNC is as specified for the UTRAN-UTRAN SRNS Relocation procedure (with two signaling connections) as defined in 3GPP TS 25.413 [16], using the messages/IEs/PDUs defined in tables 2a-2d above, with the following exceptions:

– Number of Iu instances IE (set equal to 2) is used to indicate to the target RNC that the CS (respectively PS) handover is ongoing at the same time as the PS (respectively CS) handover for the same mobile station.

– The (RLC/MAC) DTM HANDOVER COMMAND message sent to the MS containing the (RRC) HANDOVER TO UTRAN COMMAND message.

– If the mobile station fails to access the target cell, the MS shall return to the old channel in the source cell and send a (RR) HANDOVER FAILURE message to the network.

Figure 11f shows the exchange of messages in a successful Inter-RAT Intra-MSC Intra-SGSN DTM Handover from GERAN A/Gb mode to UTRAN, preparation phase. The same procedures are also used for an Inter-RAT Inter-MSC or Inter-SGSN DTM Handover from GERAN A/Gb mode to UTRAN. .

Figure 11f: Inter-RAT DTM Handover from GERAN A/Gb mode to UTRAN, preparation phase

Figure 11g shows the exchange of messages in a successful Inter-RAT Intra-MSC Intra-SGSN DTM Handover from GERAN A/Gb mode to UTRAN, execution phase. The same procedures are also used for an Inter-RAT Inter-MSC or Inter-SGSN DTM Handover from GERAN A/Gb mode to UTRAN.

Figure 11g: Inter-RAT DTM Handover from GERAN A/Gb mode to UTRAN, execution phase

6.3.4.3 Inter-RAT DTM Handover from UTRAN to GERAN A/Gb mode

For the Inter-RAT DTM handover from UTRAN to GERAN A/Gb mode, the behaviour of the source RNC is as specified for the UTRAN-UTRAN SRNS Relocation procedure (with two signaling connections) as defined in 3GPP TS 25.413 [16], and the behaviour of the target BSS (and MS on performing access in the target cell) is as specified for the Inter-BSS DTM Handover procedure described in sub-clause 6.3.3a, using the messages/IEs/PDUs defined in tables 2a-2d above, with the following exception:

– The (RRC) HANDOVER FROM UTRAN COMMAND message sent to the MS contains the (RLC/MAC) DTM HANDOVER COMMAND message.

– If the mobile station fails to access the target cell, the MS shall return to the old channel in the source cell and send a (RRC) HANDOVER FROM UTRAN FAILURE message to the network.

Figure 11h shows the exchange of messages in a successful Inter-RAT Intra-MSC Intra-SGSN DTM Handover, preparation phase. The same procedures are also used for an Inter-RAT Inter-MSC or Inter-SGSN DTM Handover from UTRAN to GERAN A/Gb mode.

Figure 11h: Inter-RAT DTM Handover from UTRAN to GERAN A/Gb mode, preparation phase

Figure 11i shows the exchange of messages in a successful Inter-RAT Intra-MSC Intra-SGSN DTM Handover from UTRAN to GERAN A/Gb mode, execution phase. The same procedures are also used for an Inter-RAT Inter-MSC or Inter-SGSN DTM Handover from UTRAN to GERAN A/Gb mode.

Figure 11i: Inter-RAT DTM Handover from UTRAN to GERAN A/Gb mode, execution phase