Paging Procedure

Paging in AMF minimizes the load on the network and locates the UE with minimal area of paging and attempts.

The first response message from the UE terminates the paging.

Note

AMF considers only service request from UE as response to paging.

AMF initiates the paging procedure in the following three scenarios:

  • Clear Subscriber at AMF: AMF triggers paging, when a subscriber in CM-IDLE state gets cleared through AMF CLI. Paging is followed by the AMF-initiated UE deregistration procedure.

  • UDM-initiated UE Deregistration: AMF initiates paging when UDM sends Deregistration notification (UDM-initiated UE Deregistration) to AMF for a subscriber which is in CM-IDLE state. If UE doesn't respond, AMF Deregisters the UE and clears the UE in SMF and in PCF. If there’s no paging response, no response is sent back to UDM.

  • N1N2MsgTransfer received from SMF: AMF initiates paging when it receives N1N2MsgTransfer from SMF for a subscriber that is in CM-IDLE state. If UE doesn't respond, AMF sends N1N2MsgTxfrFailNotification back to SMF over the callback URI.

AMF performs the following procedures during paging:

  • Filling of the following IEs in NGAP messages other than mandatory IEs:

    • Paging Discontinuous Reception (DRX)

      It’s filled based on the requested DRX parameters received from the UE during registration.

    • Assistance Data for Paging

      • Assistance Data for Recommended Cells is derived from the Information on Recommended Cells and RAN Nodes for Paging IE. AMF receives this IE as part of the UE Context Release Complete message from gNB.

      • AMF fills the Paging Attempt Information IE based on the paging algorithm used for the respective paging stage. Whenever there’s a change in the paging stage, AMF marks Next Paging Area Scope as changed. AMF doesn't fill Next Paging Area Scope for the last attempt of the last paging stage.

    • Paging priority

      It’s applicable if paging trigger is from SMF and priority mapping is configured.

  • Handling of the new incoming N1N2Msg when paging is in progress for previous N1N2Msg:

    • Precedence Calculation: AMF calculates the precedence (lower values considered as higher precedence) as per the incoming parameters along with the paging map configuration. It computes this precedence for the incoming N1N2Msg and performs the following:

      • If new precedence value is lower than ongoing precedence, AMF selects the new paging profile as per the new precedence.

      • If new precedence value is same or greater than ongoing precedence, no new profile is selected and AMF continues with the ongoing paging profile.

    • Priority Comparision: Paging priority comparison is based on incoming ARP value and its mapped NGAP paging priority under AMF configuration. AMF compares the paging priority (lower values considered as higher priority) of ongoing paging with the new incoming paging, and performs the following:

      • AMF rejects the incoming N1N2 with the cause HIGHER_PRIORITY_REQUEST_ONGOING as part of N1N2MsgTransferError when:

        • The ongoing paging has high or same priority as incoming paging.

        • The incoming paging doesn’t have any paging priority mapped to it.

        In this scenario, AMF doesn't select any new paging profile. AMF fills N1N2MsgTxfrErrDetail as part of the N1N2MsgTransferError. AMF fills the retryAfter attribute, based on the ongoing and pending paging stages.

      • If incoming N1N2Msg has high priority, AMF triggers a new paging message with a new paging priority value. AMF triggers new paging as part of the ongoing paging attempt of the ongoing paging stage.

    Note
    • Paging profile selection and triggering of new paging with new priority are independent of each other.

    • AMF supports handling of new N1N2 messages when UE is in IDLE mode and paging is already triggered due to a previous N1N2 message. AMF doesn't support handling of simultaneously received multiple N1N2 messages when UE is in IDLE mode and paging is not yet triggered.

  • Sending a PAGING message to each gNB (belonging to the UE tracking areas) based on the actions configured under paging algorithm for respective paging stages

  • Paging in stages, where each stage defines the scope of paging. Each paging stage is associated with a paging algorithm that consists of paging action.

    AMF supports the following paging actions:

    • PAGING_LAST_GNB_LAST_TAI

    • PAGING_LAST_N_GNB_LAST_TAI

    • PAGING_ALL_GNB_LAST_TAI

    • PAGING_ALL_GNB_ALL_TAI

    • PAGING_ALL_GNB_REM_TAI_ALL

    • PAGING_ALL_GNB_REM_TAI_SEQ

    PAGING_LAST_GNB_LAST_TAI - As part of this action, AMF pages the last gNB in the last Tracking Area Identifier (TAI) from which UE contacted AMF.

    PAGING_LAST_N_GNB_LAST_TAI - As part of this action, AMF pages the last n gNB in the last TAI from which UE contacted AMF.

    PAGING_ALL_GNB_LAST_TAI - As part of this action, AMF pages all the gNBs in the last TAI from which UE contacted AMF.

    PAGING_ALL_GNB_ALL_TAI - As part of this action, AMF pages all the gNBs in all the TAIs as part of UE restricted area.

    PAGING_ALL_GNB_REM_TAI_ALL - As part of this action, AMF pages the remaining TAIs (Except the last known TAI) all together.

    Example: If UE's registration area contains TAI A, B, C, and the last known TAI is A, AMF pages gNBs in TAI B, C together at the same time.

    PAGING_ALL_GNB_REM_TAI_SEQ - As part of this action, AMF pages the remaining TAIs (Except the last known TAI) in a sequential manner. There’s no specific order in which AMF selects the TAI when paging is sequential.

    Example: If UE's registration area contains TAI A, B, C, and the last known TAI is A, AMF first pages gNBs in TAI B. When no response is received for paging after reaching the maximum attempts, AMF proceeds to page gNBs in TAI C.

    Note

    AMF doesn’t support PAGING_ALL_GNB_REM_TAI_ALL and PAGING_ALL_GNB_REM_TAI_SEQ as first stage of paging profile.

    The t3513 timeout value and number of retries at a given stage is configurable as part of paging-algo. The maximum number of gNB to page is also configurable and it is applicable only to paging action PAGING_LAST_N_GNB_LAST_TAI.

    AMF uses default paging-algo, when no paging map or profile or algo or matching triggers are configured for the incoming paging trigger.

    Default paging-algo has only one stage and has the following parameters:

    • action is all_gnb_last_tai, which is configured automatically.

    • max-paging-attempts and timeout value is fetched from the configured t3513 timer under Call Control Profile.

      The default value of the t3513 timer is set to five seconds and the default paging retry count is set to two.