Network Initiated Service Request

During network initiated service request, SMF handles the temporary reject for N1N2 response message from AMF as mentioned in 3GPP TS 23.502, section 4.2.3.3.

Temporary Reject during Network Triggered Service Request - 1
Temporary Reject during Network Triggered Service Request - 1

Step

Description

1

On getting a trigger for service request in UP IDLE session state, SMF initiates a N1N2 message towards the AMF as part of Idle mode exit procedure.

2

If UE registration procedure with new AMF is in progress then AMF responds with temporary reject for N1N2 message with response code 409 and cause as:

TEMPORARY_REJECT_REGISTRATION_ONGOING

OR

TEMPORARY_REJECT_HANDOVER_ONGOING SMF

3

On receiving the response, SMF starts a guard timer of 2 seconds that is configured locally.

4

While guard timer is running, SMF expects either a SM Context Update with AMF ID change or SM Context Update for handover.

5

On receiving SM Context Update with AMF ID change, SMF:

  1. Stops the guard timer.

  2. Removes the reference to the discovery information for old AMF.

  3. Stores the new UE location information, PLMN information and AMF information.

  4. Send SM Context Update response success without content.

  5. Reinitiates N1N2 message transfer to the new AMF. This involves NF discovery and subsequent transmission to the new AMF.

6

On receiving SM Context Update for N2 handover, SMF:

  1. Starts the Handover procedure.

  2. Suspends the Idle mode exit procedure and stops the guard timer.

  3. As part of the Handover procedure completion, old AMF details are removed and new AMF information is stored.

  4. Idle mode exit procedure resumes after Handover procedure is complete.

  5. Reinitiates N1N2 message transfer, if required, to the new AMF. This involves NF discovery and subsequent transmission to new AMF.

Temporary Reject during Network Triggered Service Request - 2
Temporary Reject during Network Triggered Service Request - 2

Step

Description

1

On getting a trigger for service request in UP IDLE session state, SMF initiates a N1N2 message towards the AMF as part of Idle mode exit procedure.

2

If UE registration procedure with new AMF is in progress then AMF responds with temporary reject for N1N2 message with response code 409 and cause as:

TEMPORARY_REJECT_REGISTRATION_ONGOING

OR

TEMPORARY_REJECT_HANDOVER_ONGOING SMF

3

On receiving the response, SMF starts a guard timer of 2 seconds that is configured locally.

4

Once guard timer expires, SMF:

  1. Sets the UE CM status as NotReachable.

  2. Deactivates the UP session state.