The VLR shall handle the timers, queuing and retransmission for sending the SGsAP-PAGING-REQUEST message on the SGs interface in the same way that it handles the sending of a PAGING message on the A or Iu interface.

When a VLR has to page a UE, the VLR shall check whether the VLR has a SGs association for that UE. The VLR sends SGsAP-PAGING-REQUEST messages to the MME if the MME is in service and the state of the SGs association for the UE is in any of the following states:

• SGs-ASSOCIATED;
• LA-UPDATE-PRESENT or
• SGs-NULL and the "Confirmed by Radio Contact" restoration indicator is set to "false".

If the VLR detects that the MME serving the UE is no longer in service and the VLR supports MT CS services delivery via an alternative MME in the MME pool as defined in TS 23.007, the VLR shall send the SGs-PAGING-REQUEST message to one alternative MME in the same MME pool. The VLR shall set the CS restoration indicator in the Additional paging indicators information element. The sending of the SGsAP-PAGING-REQUEST message does not change the state of the SGs association with the MME. If the "Confirmed by Radio Contact" restoration indicator is set to "true", the VLR shall include the Location area identifier information element into the SGsAP-PAGING-REQUEST message, otherwise (i.e. after a VLR failure), the VLR shall not include the Location area identifier information element. When sending the SGsAP-PAGING-REQUEST message, the VLR shall start timer Ts5. If the state of the SGs association is SGs-NULL and the "Confirmed by Radio Contact" restoration indicator is set to "false", the VLR shall also perform a search procedure as specified in TS 23.018. In this message, the VLR includes the Service indicator information element which will be used to indicate the type of CS service. If the SGs paging request is sent as a result of reception of Provide Subscriber Information Request message, the VLR sets the Service indicator information element to either "SMS indicator" or "CS call indicator" as specified in subclause 7.2.3.5 of TS 23.018. For SMS, SMS indicator is used. For all the other CS services, CS call indicator is used. If the network supports CSFB priority call handling (see TS 23.272) and the call was received with an eMLPP priority level indication (see TS 24.008, TS 22.067 and TS 23.067), the VLR shall include the value of the received priority level indication in the eMLPP priority information element in the SGsAP-PAGING-REQUEST message. The eMLPP priority information element may be used to derive the appropriate priority of a SCTP association for the SGsAP-PAGING-REQUEST message. If the Calling Line Identification of the service (see TS 24.081) is available in the VLR, the VLR may include the CLI information element in the SGsAP-PAGING-REQUEST message. The conditions specified in TS 23.081 and TS 29.011 apply also here. If the paging is due to a NW-initiated Call Independent SS procedure as defined in TS 24.010, the VLR may include the SS code in the SGsAP-PAGING-REQUEST message as defined in TS 29.002. If the paging is due to a Mobile Terminated Location Request as defined in TS 24.030, the VLR shall include LCS indicator in the SGsAP-PAGING-REQUEST message. Additionally, the VLR may include LCS client identity as defined in TS 29.002 in the SGsAP-PAGING-REQUEST message. For Deployment Option 2 (see subclause 8.2.4a.1 of TS 23.272), if the paging was due to SMS and the SM Delivery Timer and SM Delivery Start Time IEs were received from the SMS-GMSC as defined in TS 29.002, the VLR may include these IEs in the SGsAP-PAGING-REQUEST message. For Deployment Option 2 (see subclause 8.2.4a.1 of TS 23.272), if the paging was due to SMS and the Maximum Retransmission Time IE was received from the SMS-GMSC as defined in TS 29.002, the VLR may include the received Maximum Retransmission Time IE in the SGsAP-PAGING-REQUEST message. While domain specific access control of the PS domain is ongoing, the VLR shall be configured to send paging messages on both the SGs and the A/Iu interface. The VLR may apply implementation specific rules for sending the paging on the A/Iu interface. Dependent on network configuration or operator policy, if the UE does not respond to a first paging on SGs interface or the VLR considers UE fallback was failed as described in subclause 5.15.1, and A/Iu paging has not been initiated already, the VLR shall page on the A/Iu interface.

The VLR stops the paging procedure towards the MME on expiry of timer Ts5 or on receipt of a SGsAP-SERVICE-REQUEST message from the MME. On receipt of an SCCP connection establishment containing the Initial L3 message from the UE via the A or Iu interface, the VLR shall stop the paging procedure. Upon receiving the SGsAP-SERVICE-REQUEST message with the UE EMM mode information element indicating "EMM-CONNECTED", if the Service indicator information element in the SGsAP-SERVICE-REQUEST message indicates "CS call indicator" and Call Forwarding on No Reply (CFNRy) is activated for the subscriber, the VLR shall start the CFNRy timer as specified in TS 23.082. If the paging response is received via the A or Iu interface from a location area which differs from the one stored in the VLR, the VLR shall move the SGs association to the SGs-NULL state after the UE has been authenticated successfully.

On receipt of an SGsAP-PAGING-REJECT message before the timer Ts5 expires, the VLR stops timer Ts5. If the SGs cause information element in the SGsAP-PAGING-REJECT message does not indicate "Mobile terminating CS fallback call rejected by the user", the SGs association is moved to the SGs-NULL state and within this state the SGs association is marked with the contents of the SGs cause information element. If the SGs cause information element in the SGsAP-PAGING-REJECT message indicates "IMSI detached for EPS services" the VLR shall send the paging message on the A/Iu interface. If the SGs cause information element indicates "Mobile terminating CS fallback call rejected by the user", the SGs association state shall not be changed. When the VLR receives the SGsAP-PAGING-REJECT message with the SGs cause information element indicating "Mobile terminating CS fallback call rejected by the user", the VLR shall trigger User Determined User Busy (UDUB) as specified in TS 24.082.

On receipt of an SGsAP-UE-UNREACHABLE message before the timer Ts5 expires, the VLR stops timer Ts5, and the paging procedure for that paging request towards the MME is stopped. The state of the SGs association at the VLR is unchanged. For Deployment Option 2 (see subclause 8.2.4a.1 of TS 23.272), if the paging was due to SMS and the VLR included the SM-Delivery-Timer and SM-Delivery-Start-Time IEs in the SGsAP-PAGING-REQUEST, and if the SM Buffer Request Indicator bit is set in the Additional UE Unreachable indicators IE in the SGsAP_UE_UNREACHABLE message, the VLR may buffer the Short Message until the time indicated in the SGsAP-PAGING-REQUEST or until receipt of any earlier SGsAP message for the UE. For Deployment Option 2 (see subclause 8.2.4a.1 of TS 23.272), if the Retransmission Time IE was included in the SGsAP-UE-UNREACHABLE message, the VLR may forward the received Requested Retransmission Time IE to the SMS-GMSC as defined in TS 29.002. In this case, the VLR shall not set the MNRF flag.

The MME accepts SGsAP-PAGING-REQUEST messages in any state of the SGs association. In the SGs-NULL state, the MME accepts SGsAP-PAGING-REQUEST messages only if:

• the "MME-Reset" restoration indicator is set to "true"; or
• the MME supports MT CS services delivery via an alternative MME in the MME pool as defined in TS 23.007 and the CS restoration indicator is set in the Additional paging indicators information element in the SGs-PAGING-REQUEST message.

When a MME receives a SGsAP-PAGING-REQUEST message from a VLR, the MME shall first check if the UE is known by the MME. The handling of the paging request depends on the state of the SGs association, the EMM context variables at the MME, and the Service indicator information element in the SGsAP-PAGING-REQUEST message. The MME shall process the paging request and subsequent SGs procedures related to this request preferentially compared to other normal procedures if the SGsAP-PAGING-REQUEST message includes the eMLPP priority information element and if the MME determines the CS call has high priority based on the received value in the eMLPP priority information element (see TS 23.272). If the Service indicator information element in the SGsAP-PAGING-REQUEST message indicates "CS call indicator", the MME shall handle the paging request as follows:

1. If the UE is known:
   • if the UE is considered to be IMSI attached for EPS services and "SMS only", the MME shall return an SGsAP-PAGING-REJECT message to the VLR indicating in the SGs cause information element "Mobile terminating CS fallback call rejected by the user";
   • if the UE is considered to be IMSI attached for EPS and non-EPS services (i.e. the SGs association is not in the state SGs-NULL), the MME shall page the UE based on the location information stored in the MME, i.e. in all tracking areas of the stored list. If the SGsAP-PAGING-REQUEST message does not include the Location area identifier information element, the MME may set the "VLR-Reliable" MM context variable to "false". If ISR is activated, and in addition the SGsAP-PAGING-REQUEST message does not include the Location area identifier or the "VLR-Reliable" MM context variable in the MME is set to "false", then the MME shall send Detach Notification (Local Detach) message to the associated SGSN;
   • if the MME detects that the UE currently has an ongoing IMS emergency session as specified in subclause 4.6a of TS 23.272, the MME shall return an SGsAP-PAGING-REJECT message to the VLR indicating in the SGs cause information element "Mobile terminating CS fallback call rejected by the user";
   • if the MME detects that the UE currently has an EPS bearer context with QCI=1 and the MME determines that SRVCC (as specified in TS 23.216) is not supported in the network or it determines that the UE does not support SRVCC, MME shall return SGsAP-PAGING-REJECT message to that VLR with SGs cause information element "Mobile terminating CS fallback call rejected by the user";
   • if the UE is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services (i.e. the state of the SGs association is SGs-NULL):
     • if the UE is in the EMM-DEREGISTERED state (see TS 24.301) and if the MME supports MT CS services delivery via an alternative MME in the MME pool and the CS restoration indicator is set in the SGsAP-PAGING-REQUEST message, the MME shall send the paging request with the location information provided by the VLR, regardless of the value of the "MME-Reset" indicator. If no such location information is provided, the MME may either page the UE in all the tracking areas served by that MME or in the tracking areas served by the MME and by the VLR, or reject the paging request per operator policy;
     • otherwise, the MME shall return an SGsAP-PAGING-REJECT message to that VLR indicating in the SGs cause information element the detach circumstance ("IMSI detached for EPS services", "IMSI detached for non-EPS services" or "IMSI implicitly detached for non-EPS services");
   • if the UE is marked as unreachable, indicated by Paging Proceed Flag set to "false", and the ISR is not activated, the MME shall return an SGsAP-UE-UNREACHABLE message to that VLR indicating in the SGs cause information element "UE unreachable". The state of the SGs association does not change at the MME; or
   • if the UE is marked as unreachable, indicated by Paging Proceed Flag set to "false", and the ISR is activated, the MME shall not return an SGsAP-UE-UNREACHABLE message to that VLR. If the SGsAP-PAGING-REQUEST message does not include the Location area identifier or the "VLR-Reliable" MM context variable in the MME is set to "false", then the MME shall send Detach Notification (Local Detach) message to the associated SGSN; Otherwise the MME shall forward the paging request to the associated SGSN, as described in TS 29.274.
2. If the UE is not known and the "MME-Reset" restoration indicator at the MME is set to "false" and if:
   • the MME does not support MT CS services delivery via an alternative MME in the MME pool; or
   • the MME supports MT CS services delivery via an alternative MME in the MME pool and the CS restoration indicator is not set in the SGsAP-PAGING-REQUEST message;
   the MME shall handle the paging request as follows:
   • the MME shall return an SGsAP-PAGING-REJECT message to that VLR indicating in the SGs cause information element "IMSI unknown".
3. If the UE is not known and if:
   • the "MME-Reset" restoration indicator at the MME is set to "true"; or
   • the MME supports MT CS services delivery via an alternative MME in the MME pool and the CS restoration indicator is set in the SGsAP-PAGING-REQUEST message;
   the MME shall handle the paging request as follows:
   • if the MME only supports "SMS only", the MME shall return an SGsAP-PAGING-REJECT message to the VLR indicating in the SGs cause information element "Mobile terminating CS fallback call rejected by the user";
   • if the SGsAP-PAGING-REQUEST message includes the Location area identifier information element, the MME shall page the UE in all the tracking areas served by the MME that can be mapped to the location area indicated in the Location area identifier information element; or
   • if the SGsAP-PAGING-REQUEST message does not include the Location area identifier information element, the MME may page in all the tracking areas served by the MME, or the tracking areas served by the MME and by the VLR or reject the paging request per operator policy.

If the Service indicator information element in the SGsAP-PAGING-REQUEST message indicates "SMS indicator", the MME shall handle the paging request as follows:

1. If the UE is known:
   • if the UE is considered to be IMSI attached for EPS and non-EPS services or IMSI attached for EPS services and "SMS only", the MME shall page the UE based on the location information stored in the MME. If the SGsAP-PAGING-REQUEST message does not include the Location area identifier information element, the MME may set the "VLR-Reliable" MM context variable to "false". If the UE is using eDRX, the MME may, subject to local operator policy, return an SGsAP-UE-UNREACHABLE message to the VLR indicating in the SGs cause information element "UE temporarily unreachable" while continuing to page the UE. If ISR is activated, and in addition the SGsAP-PAGING-REQUEST message does not include the Location area identifier or the "VLR-Reliable" MM context variable in the MME is set to "false", then the MME shall send Detach Notification (Local Detach) message to the associated SGSN; or
   • for Deployment Option 2 (see subclause 8.2.4a.1 of TS 23.272), if the UE is using eDRX, the SGsAP-PAGING-REQUEST message includes the SM Delivery Timer and SM Delivery Start Time IEs, and the UE is expected to be reachable before the time indicated by the SM-Delivery-Timer and SM-Delivery-Start-Time IEs, the MME may return an SGsAP-UE-UNREACHABLE message to the VLR indicating in the SGs cause information element "UE temporarily unreachable" and setting the SM Buffer Request Indicator bit in the Additional UE Unreachable indicators information element. The state of the SGs association does not change at the MME. The MME should then page the UE when the UE is expected to be reachable.
   • for Deployment Option 2 (see subclause 8.2.4a.1 of TS 23.272), if the UE is using eDRX and the SGsAP-PAGING-REQUEST message includes the Maximum Retransmission Time IEs, the MME may return an SGsAP-UE-UNREACHABLE message to that VLR indicating in the SGs cause information element "UE temporarily unreachable" and including the Requested Retransmission Time IE. In this case, the MME shall set the NEAF. The state of the SGs association does not change at the MME.
   • if the UE is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services, or as unreachable, the MME shall proceed as specified for the case when the service indicator indicates "CS call indicator".
2. If the UE is not known and the "MME-Reset" restoration indicator at the MME is set to "false" and if:
   • the MME does not support MT CS services delivery via an alternative MME in the MME pool; or
   • the MME supports MT CS services delivery via an alternative MME in the MME pool and the CS restoration indicator is not set in the SGsAP-PAGING-REQUEST message;
   the MME shall handle the paging request as follows:
   • the MME shall return an SGsAP-PAGING-REJECT message to that VLR indicating in the SGs cause information element "IMSI unknown".
3. If the UE is not known and if:
   • the "MME-Reset" restoration indicator at the MME is set to "true"; or
   • the MME supports MT CS services delivery via an alternative MME in the MME pool and the CS restoration indicator is set in the SGsAP-PAGING-REQUEST message;
   the MME shall handle the paging request as follows:
   • if the SGsAP-PAGING-REQUEST message includes the Location area identifier information element, the MME shall page the UE in all the tracking areas served by the MME that can be mapped to the location area indicated in the Location area identifier information element; or
   • if the SGsAP-PAGING-REQUEST message does not include the Location area identifier information element, the MME may page in all the tracking areas served by the MME, or the tracking areas served by the MME and by the VLR or reject the paging request per operator policy.

If the Service indicator information element in the SGsAP-PAGING-REQUEST message indicates "CS call indicator" and the MME accepts the paging request, the MME shall proceed as follows:

• If the SGsAP-PAGING-REQUEST message includes the Location area identifier and the "VLR-Reliable" MM context variable in the MME is set to "true":
  • if the UE is considered to be IMSI attached for EPS and non-EPS services, the MME pages the UE with CN domain indicator set to "CS".
    if
    • the SGsAP-PAGING-REQUEST message includes the IMSI without TMSI and the location area stored in the MME is the same as the one received in the SGsAP-PAGING-REQUEST message and S-TMSI can be retrieved, and the operator's policy allows the use of S-TMSI as paging address; or
    • the SGsAP-PAGING-REQUEST message includes the TMSI and the location area stored in the MME is the same as the one received in the SGsAP-PAGING-REQUEST message;
    then the MME uses the S-TMSI as paging address at the radio interface; else the MME uses the IMSI; and
  • if the UE is not known, or is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services, the MME pages the UE with IMSI and CN domain indicator set to "PS".
• If the SGsAP-PAGING-REQUEST message does not include the Location area identifier, or the "VLR-Reliable" MM context variable in the MME is set to "false":
  • if the UE is considered to be IMSI attached for EPS and non-EPS services, the MME uses IMSI as a paging address at the radio interface and CN domain indicator set to "CS". When the MME receives the paging response from the UE, the MME requests the UE to re-attach for non-EPS services as described in TS 24.301; and
  • if the UE is not known, or is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services, the MME pages the UE with IMSI with CN domain indicator set to "PS" to force the UE to re-attach to EPS and non-EPS services as described in TS 24.301.

If the Service indicator information element in the SGsAP-PAGING-REQUEST message indicates "SMS indicator" and the MME accepts the paging request, the MME shall proceed as follows:

• If the SGsAP-PAGING-REQUEST message includes the Location area identifier and the "VLR-Reliable" MM context variable in the MME is set to "true":
  • if the UE is considered to be IMSI attached for EPS and non-EPS services or IMSI attached for EPS services and "SMS only", the MME shall use the S-TMSI as paging address at the radio interface with CN domain indicator set to "PS". The MME takes action as described in subclause 5.6.2.4 of TS 24.301; and
  • if the UE is not known, or is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services, the MME pages the UE with IMSI as paging address at the radio interface with CN domain indicator set to "PS". The MME takes action as described in subclause 5.6.2.4 of TS 24.301.
• If the SGsAP-PAGING-REQUEST message does not include the Location area identifier information element or the "VLR-Reliable" MM context variable is set to "false":
  • if the UE is considered to be IMSI attached for EPS and non-EPS services or IMSI attached for EPS services and for "SMS only", the MME uses S-TMSI as paging address at the radio interface with CN domain indicator set to "PS". When the MME receives the paging response from the UE, the MME requests the UE to re-attach for non-EPS services as specified in TS 24.301; and
  • if the UE is not known, or is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services, the MME pages the UE with IMSI with CN domain indicator set to "PS" to force the UE to re-attach to EPS and non-EPS services as described in TS 24.301.

Independent of the Service indicator information element, if the MME has activated Idle mode Signalling Reduction for the UE, the MME shall forward the paging request to the associated SGSN, as described in TS 29.274. Independent of the Service indicator information element, the MME shall not retransmit the paging message to the UE. Additionally, if the MME has activated Idle mode Signalling Reduction for the UE, the MME shall not retransmit the paging request to the associated SGSN.

If the Service indicator information element in the SGsAP-PAGING-REQUEST message indicates "CS call indicator" and the MME accepts the paging request, the MME shall proceed as follows:

• The MME requests the UE to re-attach for non-EPS services as specified in TS 24.301 if the SGsAP-PAGING-REQUEST message does not include the Location area identifier, or the "VLR-Reliable" MM context variable in the MME is set to "false"; and
• otherwise, the MME sends the CS SERVICE NOTIFICATION message to the UE through the NAS signalling connection, including the CS service related parameters (CLI, SS code, LCS indicator and LCS client identity), received from the VLR.

If the Service indicator information element in the SGsAP-PAGING-REQUEST message indicates "SMS indicator" and the MME accepts the paging request, the MME shall proceed as follows:

• If the Location area identifier information element is not included in the SGsAP-PAGING-REQUEST message or the "VLR-Reliable" MM context variable is set to "false", the MME requests the UE to re-attach for non-EPS services as specified in TS 24.301; and
• otherwise, the MME need not take any action towards the UE.