9.23 PS Handover between GAN Iu mode and GERAN/UTRAN mode

3GPP43.318Generic Access Network (GAN)Release 17Stage 2TS

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

9.23.1 PS Handover from GERAN to GAN

9.23.1.1 Preparation Phase

The description of the GERAN to GAN PS handover procedure assumes the following:

– the MS has one or more active packet flow contexts in the GERAN;

– the MS has successfully registered with a GANC, allowing the MS to obtain GAN system information;

– the GANC has directed the MS to operate in GAN Iu mode;

– the PS handover is intra-SGSN (i.e., assumes that the SGSN is capable of both 2G and 3G operation and interworking); and

– the GERAN provides information on neighbouring cells such that one of the cells in the neighbour list matches the cell associated with the GANC, as provided in the AS-related component of the system information obtained from the GANC.

Figure 67: GERAN to GAN PS Handover Preparation Phase

1. The source BSS decides to initiate a PS handover. At this point both uplink and downlink user data is transmitted via the following: TBFs between MS and source BSS, BSSGP PFCs tunnel(s) between the source BSS and 3G/2G SGSN, GTP tunnel(s) between the 3G/2G SGSN and GGSN.

2. The source BSS sends a PS Handover Required message (TLLI, Cause, Source Cell Identifier, Target RNC Identifier, Source RNC to Target RNC Transparent Container, Active PFCs List) to the SGSN.

3. The SGSN determines from the Target RNC Identifier that this is a GERAN to UTRAN handover. The SGSN constructs a Relocation Request message as described in 3GPP TS 43.129 [44] and sends the message to the target GANC.

4. One or more GAN PTCs are activated between the MS and the GANC as specified in steps 3-4 in sub-clause 9.16.2.2. Upon each GA-RRC PTC activation, the PS domain GA-RRC sublayer entity in the MS enters the PTC-ACTIVE substate and starts the PTC Timer for the PTC.

5. The GANC sends the Relocation Request Acknowledge message (Target RNC to Source RNC Transparent Container, RABs setup list, RABs failed to setup list) to the SGSN. Upon sending the Relocation Request Acknowledge message the target GANC shall be prepared to receive downlink GTP PDUs from the SGSN for the accepted RABs.

When the SGSN receives the Relocation Request Acknowledge message and it decides to proceed with the handover, the preparation phase is finished and the execution phase will follow.

9.23.1.2 Execution Phase

Figure 68: GERAN to GAN PS Handover Execution Phase

1. The SGSN continues to receive IP packets from the GGSN (via GTP) and forwards the associated PDU payload to the MS via the source BSS.

2. When receiving the Relocation Request Acknowledge message the SGSN may, based on QoS, start downlink N-PDU relay and duplication to the target GANC if a Tunnel Endpoint is available as follows:

– For PDP context, which uses LLC ADM, all new downlink N-PDUs received after completion of the PS handover preparation phase are relayed to the target GANC. All such N-PDUs are encapsulated in a GTP-PDU when transmitted to the target GANC.

– If the 3G/2G SGSN forwards downlink packets to the target GANC, the target GANC may start blind transmission of downlink user data towards the MS over the allocated PTC(s).

3. The SGSN continues the PS Handover by sending a PS Handover Required Acknowledge message (TLLI, List of Set Up PFCs, Target RNC to Source RNC Transparent Container) to the source BSS, as described in 3GPP TS 43.129 [44].

4. The SGSN shall send the Forward SRNS Context message (NSAPI, DL GTP-U number, UL GTP-U number) to the target GANC if there is at least one PDP context which requires "delivery order" to be preserved, as described in 3GPP TS 43.129 [44].

The target GANC proceed as follows:

– For RABs not requiring lossless PDCP the target GANC may, according the QoS profile of the PDP context, store the received data until it receives confirmation of MS presence in the target cell.

– The target GANC disregards PDCP sequence numbers, since lossless relocation is not supported.

5. The source BSS sends the PS Handover Command message containing the Handover to UTRAN Command message (as it is specified in 3GPP TS 25.331 [40]) to the MS, as described in 3GPP TS 43.129 [44].

6. Immediately after receiving the PS Handover Command message, the MS sends GA-RRC RELOCATION COMPLETE message to the GANC.

7. Immediately upon receiving the GA-RRC RELOCATION COMPLETE message from the MS, the GANC sends the Relocation Detect message to the SGSN.

8. The GANC sends the Relocation Complete message to the SGSN, indicating the completion of the PS handover. After the reception of the Relocation Complete message the SGSN shall be prepared to receive data from the target GANC.

9. If the SGSN has established Direct Tunnel, the SGSN sends an Update PDP Context Request message (RNC Address, TEID, QoS Negotiated, DTI) to the GGSN concerned. The SGSN provides to the GGSN the GANC address for the User Plane and TEID for downlink data and shall include the DTI to instruct the GGSN to apply Direct Tunnel specific error handling procedure as defined in 3GPP TS 23.060 [19]. The GGSN updates the PDP context fields and returns an Update PDP Context Response message (TEID). From now on the GGSN sends new incoming downlink IP packets to the target GANC instead of the SGSN.

10. The SGSN shall initiate PFC Management procedures towards the source GERAN cell in order to trigger the release of resources in the source cell.

11. The MS and SGSN perform the Routing Area Update procedure as described in 3GPP TS 43.129 [44].

9.23.2 PS Handover from UTRAN to GAN

9.23.2.1 Preparation Phase

The description of the UTRAN to GAN PS handover procedure assumes the following:

– the Iur interface between the UTRAN RNC and GANC is not supported. Therefore, only the Combined Hard Handover and SRNS Relocation is applicable for GAN – UTRAN PS handover. Consequently, only the "MS Involved" Relocation Type is supported;

– the MS has one or more active PDP Contexts with active RABs in the UTRAN;

– the MS has successfully registered with a GANC, allowing the MS to obtain GAN system information;

– the GANC has directed the MS to operate in GAN Iu mode; and

– the UTRAN provides information on neighbouring cells such that one of the cells in the neighbour list matches the cell associated with the GANC, as provided in the AS-related component of the system information obtained from the GANC. See Annex B.2.1 for more information.

Figure 69: UTRAN to GAN PS Handover Preparation Phase

1. Based on measurement results and knowledge of the RAN topology, the source SRNC decides to initiate a combined hard handover and SRNS relocation.

2. The source SRNC sends a Relocation Required message (Relocation Type, Cause, Source ID, Target ID, Source RNC To Target RNC Transparent Container) to the SGSN.

3. The SGSN determines the target cell is the GANC, based on the contents of Relocation Required. It then sends the Relocation Request message (Permanent NAS UE Identity, Cause, CN Domain Indicator, Source RNC To Target RNC Transparent Container, RAB To Be Setup) to the GANC.

If the IMSI is not present in the Permanent NAS UE Identity IE in the Relocation Request message, the GANC shall send the Relocation Failure message to the SGSN including Cause value "Relocation Failure In Target CN/RNC Or Target System" and abort the relocation procedure.

4. One or more GAN PTCs are established between the GANC and MS as specified in steps 3-4 in sub-clause 9.16.2.2. Upon the GA-RRC PTC establishment, the PS domain GA-RRC sublayer entity in the MS enters the PTC-ACTIVE substate and starts the PTC Timer for the PTC.

5. The GANC sends the Relocation Request Acknowledge message (Target RNC To Source RNC Transparent Container, RABs Setup, RABs Failed To Setup) to the SGSN.

9.23.2.2 Execution Phase

Figure 70: UTRAN to GAN PS Handover Execution Phase

1. Upon receiving the positive acknowledgement from the GANC to serve the MS, the SGSN initiates the Execution Phase by sending the Relocation Command to the source SRNC.

2. a) The RNC may start forwarding GTP PDUs to the GANC while still transmitting them in the downlink to the MS. This forwarding is routed via the Iu-PS interface. The GANC may buffer, start a blind transmission of downlink user data towards the MS over the allocated PTC(s), or discard these forwarded GTP PDUs, depending on the QoS profile, network conditions, and whether it supports data forwarding.

b) The RNC instructs the MS to initiate the switch to GAN via the Physical Channel Reconfiguration message.

c) The RNC sends the Forward SRNS Context message to the GANC via the SGSN. In this message, the next-expected sequence number of uplink and downlink GTP-U packets are indicated to the GANC by the RNC.

3. Immediately after receiving the Physical Channel Reconfiguration message, the MS sends GA-RRC RELOCATION COMPLETE message to the GANC. Upon receiving this message and the Forward SRNS Context message, the GANC becomes the Serving RNC.

4. Immediately upon receiving the GA-RRC HANDOVER COMPLETE message from the MS, the GANC sends the Relocation Detect message to the SGSN.

5. The GANC sends the Relocation Complete message to the SGSN.

6. The MS, GANC and CN exchange user data via the established PTC.

7. The SGSN releases the Iu-PS connection with the old RNC.

8. If the Routing Area of the GANC cell (as indicated by the GANC to the MS in GAN registration) is different from that under the old RNC, then the MS performs the Routing Area Update procedure.

9.23.3 PS Handover from GAN to GERAN

The description of the GAN to GERAN PS handover procedure assumes the following:

– the PS handover is intra-SGSN (i.e., assumes that the SGSN is capable of both 2G and 3G operation and interworking).

9.23.3.1 Preparation Phase

Figure 71: GAN to GERAN PS Handover Preparation Phase

1. The MS is in active packet flow exchange with active PDP Context(s) and PTC(s) in the GAN.

2. The GANC may send a GA-RRC UPLINK QUALITY INDICATION if there is a problem with the uplink quality for the ongoing session. Uplink Quality Indication is information sent by the GANC to the MS indicating the crossing of a uplink quality threshold in the uplink direction. Whenever the MS receives an indication of bad quality, it should start the PS handover procedure, as described in the next step. Alternatively, MS can use its local measurements to decide to initiate the PS handover procedure.

3. The MS decides to initiate a PS handover from GAN to GERAN by sending GA-RRC RELOCATION INFORMATION message to the GANC. The GA-RRC RELOCATION INFORMATION message indicates a list of target GERAN A/Gb mode cells, identified by CGI, in order of preference for PS handover, and includes the received signal strength for each identified GERAN A/Gb mode cell.

4. The GANC selects a target GERAN cell based on the contents of the GA-RRC RELOCATION INFORMATION message. It sends the Relocation Required message (Relocation Type, Cause, Source ID, Target ID, Source BSS To Target BSS Transparent Container) to the SGSN. The GANC sets Relocation Type to "UE Involved in relocation of SRNS". Target ID contains the identity of the target GERAN cell.

5-7. The SGSN and Target BSS complete the UTRAN to GERAN PS handover preparation as described in 3GPP TS 43.129 [44].

9.23.3.2 Execution Phase

Figure 72: GAN to GERAN PS Handover Execution Phase

1. The SGSN begins the Execution Phase by issuing a Relocation Command message (Target BSS to Source BSS Transparent Container (PS Handover Command with RN part and CN part), RABs to be Released List, RABs Subject to Data Forwarding List) to the GANC.

2. a) The GANC may begin data forwarding for the RABs subject to data forwarding according to the procedure defined in 3GPP TS 43.129 [44].

b) The GANC sends GA-RRC RELOCATION COMMAND to the MS. This message contains the information from the Relocation Command received in step 1.

c) The GANC sends the Forward SRNS Context message to the SGSN.

3. The MS executes the GERAN A/Gb PS handover access procedures as described in 3GPP TS 43.129 [44].

4. After successfully accessing the GERAN cell, the MS, target BSS, SGSN and GGSN complete the GERAN PS handover procedures as described in 3GPP TS 43.129 [44].

5. The SGSN releases the Iu-PS connection by sending the Iu Release Command message to the GANC, to which GANC responds with the Iu Release Complete message.

6. If the Routing Area of the UTRAN cell is different from that of the GAN cell, then the MS performs the Routing Area Update procedure.

9.23.4 PS handover from GAN to UTRAN

9.23.4.1 Preparation Phase

Figure 73: GAN to UTRAN PS Handover Preparation Phase

1. The MS is in active packet flow exchange with active PDP Context(s) and PTC(s) in the GAN.

2. The GANC may send a GA-RRC UPLINK QUALITY INDICATION if there is a problem with the uplink quality for the ongoing session. Uplink Quality Indication is information sent by the GANC to the MS indicating the crossing of a uplink quality threshold in the uplink direction. Whenever the MS receives an indication of bad quality, it should start the relocation procedure, as described in the next step. Alternatively, MS can use its local measurements to decide to initiate the handover procedure.

3. The MS decides to initiate a PS handover from GAN to UTRAN by sending GA-RRC RELOCATION INFORMATION message to the GANC.

4. The GANC selects a target RNC based on the contents of the GA-RRC RELOCATION INFORMATION message. It sends Relocation Required message to the SGSN containing the selected RNC information.

5. The SGSN sends a Relocation Request message to the target RNC.

6. The RNC performs the necessary allocation of radio and Iu transport resources.

7. The RNC returns Relocation Request Acknowledge message to the SGSN. This message contains a transparent container that contains channelization information needed by MS to access UTRAN.

9.23.4.2 Execution Phase

Figure 74: GAN to UTRAN PS Handover Execution Phase

1. The SGSN begins the Execution Phase by issuing the Relocation Command message to the GANC. The message contains the channel access information in the target UTRAN cell.

2. a) The GANC sends GA-RRC RELOCATION COMMAND to the MS. This message contains the information from the Relocation Command received in Step 1 earlier.

b) The GANC also sends Forward SRNS Context message to the target RNC via the SGSN.

3. The SGSN relays the Forward SRNS Context message to the target RNC.

4. Upon receiving the GA-RRC RELOCATION COMMAND, the MS immediately suspends uplink GTP PDU transfer. It immediately begins accessing the UTRAN using the indicated channelization parameters in the message. The MS’s access attempt is detected by the Node B and RNC, and is reported to the SGSN via the Relocation Detect message.

5. The MS completes the lower layer setup and configuration, and sends the RRC Physical Channel Reconfiguration Complete to the target RNC. This triggers the target RNC to send the Relocation Complete message to SGSN. At this stage, the target RNC assumes the role of SRNC for the MS.

6. The packet data flow is now active via the UTRAN.

7. The SGSN releases the Iu-PS connection by sending the Iu Release Command message to the GANC, to which GANC responds with Iu Release Complete message.

8. If the Routing Area of the UTRAN cell is different from that of the GAN cell, then the MS performs the Routing Area Update procedure.

Annex A (normative):
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