8 Handover Failure

23.2163GPPRelease 16Single Radio Voice Call Continuity (SRVCC)Stage 2TS

8.1 Failure in EUTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN (v)SRVCC

8.1.1 Failure before MSC Server initiates Session Transfer

If a failure is encountered before the MSC Server initiates Session Transfer, then the standardised handover failure procedures apply according to TS 23.401 [2]. No further action is required by the UE.

8.1.1a Failure after MSC Server initiates Session Transfer

8.1.1a.1 Failure before responding to PS to CS HO request

If the MSC Server receives a negative response from IMS during the Session Transfer procedure (e.g., due to invalid STN-SR, or temporary failure, etc) and the MSC Server has not yet responded back to MME/SGSN with a positive PS to CS Response message due to successful target CS radio resource reservation, then MSC Server shall reject this PS To CS Request with a Reject cause pointing to either permanent or temporary "Session Transfer leg establishment error" and MSC Server shall release the CS radio resource at the target RAT.

NOTE 1: An example of permanent error can be an invalid STN-SR (i.e., IMS returns with "404 user unknown").

When MME/SGSN receives the PS to CS Response with a reject cause related to "permanent Session Transfer leg establishment error", MME/SGSN may take that indication into account to prevent further SRVCC handover attempts.

NOTE 2: It is an implementation option on when the MME/SGSN may retry SRVCC after receiving a "permanent Session Transfer leg establishment error".

The following figure 8.1.1a-1 shows the overall procedure when the MSC Server aware of the Session Transfer failure prior to responding to PS to CS HO request.

Figure 8.1.1a-1: SRVCC Handover Rejection due to Session Transfer leg establishment error before responding to PS to CS HO request

8.1.1a.2 Failure after responding to PS to CS HO request

If the MSC Server receives a negative response from IMS during the initiates Session Transfer procedure (e.g. due to invalid STN-SR, or temporary failure, etc) and the MSC Server has already responded back to MME/SGSN with a positive PS to CS Response message due to successful target CS radio resource reservation and has not yet sent PS to CS Complete Notification, then MSC Server shall indicate to MME/SGSN with an error cause pointing to either permanent or temporary "Session Transfer leg establishment error" in PS to CS complete notification message, MME/SGSN shall perform the bearer release procedure as according to the normal SRVCC procedure as defined in clauses 6.2 and 6.3.

NOTE: MSC Server may wait for the status of Session Transfer procedure before sending the CS Complete Notification to MME/SGSN.

When MME/SGSN receives this PS to CS Complete notification message with an error cause pointing to "permanent Session Transfer leg establishment error", MME/SGSN may take that indication into account to prevent further SRVCC handover attempts.

The following figure 8.1.1a-2 shows the overall procedure when the MSC Server aware of the Session Transfer failure after the PS to CS HO request has been responded to MME/SGSN.

Figure 8.1.1a-1: SRVCC Handover Rejection due to Session Transfer leg establishment error after responding to PS to CS HO request

8.1.2 Failure after UE receives HO command

If the UE encounters a failure at the radio level after it receives the handover command and does not successfully transition to 3GPP UTRAN/GERAN RAT, the UE attempts to return to E-UTRAN/UTRAN by sending a re-INVITE to the SCC AS. The core network (MME, MSC Server) shall take no (v)SRVCC specific action in the event of not receiving the Handover Complete message from the UE.

8.1.3 Handover Cancellation

If the source E-UTRAN/UTRAN decides to terminate the handover procedure before its completion, the MME/SGSN shall return to its state before the handover procedure was triggered. The MME/SGSN attempts to trigger, at the MSC Server enhanced for SRVCC, handover cancellation procedures according to TS 23.009 [18].

The MME/SGSN shall also send a session reestablishment trigger notification to UE to start the recovery procedure if it receives notification from the MSC Server that the Session Transfer procedure is in progress.

The source E-UTRAN/UTRAN shall not attempt any new SRVCC procedures until the cancellation has successfully been completed and source E-UTRAN/UTRAN have received an acknowledgment of the previous HO cancellation. If MSC has already triggered session transfer, then the UE needs to be provided with a Session Reestablishment trigger before a new cancellation attempt.

Figure 8.1.3-1 shows the overall procedure for SRVCC handover cancellation.

For vSRVCC the MME and MSC also behave the same way as in the case of SRVCC handover cancellation.

Figure 8.1.3-1: SRVCC Handover Cancellation Procedure

1. Network has started the SRVCC procedure. SGSN/MME has sent the SRVCC PS to CS request to MSC Server.

2. MSC Server is performing the CS HO procedure with target network, and has also started the Session Transfer procedure with IMS with STN-SR, see TS 23.237 [14].

3. Source UTRAN/E-UTRAN decides to cancel the SRVCC HO Procedure by sending a Cancel message to SGSN/MME.

4. Source SGSN/MME indicates SRVCC PS to CS Cancel Notification to MSC Server to start the HO cancellation procedure as according to TS 23.009 [18].

5. MSC Server acks the PS to CS Cancel Notification with an indication that Session Transfer procedure is in progress.

5b. The MSC Server releases any resources established in CS and IMS as a result of the SRVCC preparation.

6. Due to the Session Transfer procedure in progress indication, the source SGSN/MME sends a Session Reestablishment trigger notification to UE to start the session re-establishment procedure

NOTE 1: The Session Reestablishment trigger notification will need to provide information to the UE to start the re-establishment procedure on the bearer used for IMS signalling.

6b. The source SGSN/MME confirms the Cancel message provided in step 3.

7. UE starts the re-establishment procedure, by attempting to return to E-UTRAN/UTRAN by sending a re-INVITE towards IMS for the related session. If the session is no longer active, then this session transfer request shall be rejected by the IMS.

NOTE 2: All originally established bearers in E-UTRAN still exist despite this process and assumed to be re-used after the HO cancelation.

8.1.4 Handover Failure due to alerting/pre-alerting state

If either an UE, MSC server, or IMS (e.g. SCC AS, EATF, or ATCF) does not support the required alerting or pre-alerting SRVCC capabilities as defined in TS 23.237 [14] then handover failure scenario as defined in clause 8.1.1a is followed with the following additional clarifications:

– At the session transfer request from MSC server with E-STN-SR or STN-SR, IMS returns a temporary failure indication to this request.

– MSC Server, if supported, shall be configured to not return a PS-CS Response to source MME/SGSN until it receives either a positive or negative indication with temporary failure from IMS, or due to other normal error handling (e.g., timeout).

Source E-UTRAN/UTRAN may retry SRVCC or other handover mechanism when receiving handover preparation failure from source MME/SGSN.

8.2 Failure in CS to PS SRVCC

8.2.1 Failure before UE initiates Session Transfer detected by MSC Server

Figure 8.2.1-1 provides the procedures of how the MSC server handles errors when HO Required is received.

Figure 8.2.1-1: Error handling during HO

1. The RNC/BSC sends a HO required to the MSC Server including an indication this HO is for CS to PS SRVCC.

2. The MSC Server checks if CS to PS HO is possible, e.g., that the UE is still registered in IMS. In this flow, the MSC Server detects that CS to PS SRVCC cannot be performed.

3. MSC Server cancels the HO with a specific cause that the RNC/BSC will not perform further HO attempts for CS to PS SRVCC.

If the HO cancellation is done prior that the Session Transfer Preparation request, but after the Session Transfer Notification, the MSC Server shall send a notification session transfer cancellation to IMS to indicate that the transfer has been cancelled to allow IMS to de-allocate any resources it may have reserved.

8.2.2 Failure before UE initiates Session Transfer detected by MME

Figure 8.2.2-1 provides the procedures of how the MSC server handles errors when HO Required is received.

Figure 8.2.2-1: Error handling during HO

1. The RNC/BSC sends a HO required to the MSC Server including an indication this HO is for CS to PS. If the MSC Server is the target MSC, it forwards the HO required to the anchor MSC Server.

2. The MSC Server sends a SRVCC CS to PS Request to the Target MME. If required, the IMSI is provided for identifying the UE, and the P-TMSI and RAI is provided to MME if available.

3. The MME has no UE context and tries to find the source SGSN/MME. For some reason, the UE context cannot be found. The MME cancels the HO.

4. The MME sends SRVCC CS to PS Response, with cause indicating failure, to the MSC Server.

5. MSC Server cancels the HO with a specific cause that the RNC/BSC will not perform further HO attempts for CS to PS SRVCC.

The cancellation is done prior that the Session Transfer Preparation request, but after the Session Transfer Notification, the MSC Server shall send a notification session transfer cancellation to IMS to indicate that the transfer has been cancelled to allow IMS to de-allocate any resources it may have reserved.

8.2.3 CS to PS Handover Cancellation

If the source RAN decides to terminate the handover procedure before its completion, the MSC Server shall start the Cancellation procedure if it has initiated the CS to PS HO command towards the MME. Figure 8.2.3-1 shows the overall procedure for SRVCC CS to PS handover cancellation.

Figure 8.2.3-1: SRVCC CS to PS Handover Cancellation Procedure

1-2. Network has started the SRVCC CS to PS procedure. MSC Server has sent the SRVCC CS to PS request to MME. MME is performing the SRVCC CS to PS HO procedure with target network.

3. Source RAN decides to cancel the SRVCC CS to PS HO Procedure by sending a Cancel message to MSC Server.

4. Source MSC Server indicates SRVCC CS to PS Cancel Notification to MME to start the HO cancellation procedure.

5. MME acks the CS to PS Cancel Notification.

6. The MSC Server acknowledges the HO Cancelation from BSC/RNC and resumes the CS session.

If the HO cancellation is done prior that the Session Transfer Preparation request, but after the Session Transfer Notification, the MSC Server shall send a Session Transfer Notification cancellation to IMS to indicate that the transfer has been cancelled to allow IMS to de-allocate any resources it may have reserved.