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There are 2 types of deregistration requests:
1. UE to NW, if UE does not want to access the 5GS any longer.
2. NW to UE, to inform UE that it cannot access 5GS any longer.
3. NE to UE, to inform UE that the UE’s Registered PLMN is not allowed to operate at the UE location.
In this chapter we shall see “UE-initiated Deregistration” procedure.
1: Deregistration Request
-> The UE sends NAS message Deregistration Request (5G-GUTI, Deregistration type (e.g. Switch off), Access Type) to the AMF.
2: Nsmf_PDUSession_ReleaseSMContext Request
-> If the UE has no established PDU Session over the target access indicated in step 1, then steps 2 to 5 are not executed.
-> All PDU Sessions over the target access(es), which belong to the UE are released by the AMF sending Nsmf_PDUSession_ReleaseSMContext Request (SM Context ID) message to the SMF (or V-SMF) for each PDU Session.
3: SMF releases all resources
-> [Conditional] The SMF (or H-SMF) releases all resources e.g. the IP address / Prefix(es) that were allocated to the PDU Session and releases the corresponding User Plane resources:
3a. Conditional] The SMF (or H-SMF) sends N4 Session Release Request (N4 Session ID) message to the UPF(s) of the PDU Session.
3b. [Conditional] The UPF(s) acknowledges the N4 Session Release Request by the transmission of an N4 Session Release Response (N4 Session ID) message to the SMF.
4: Nsmf_PDUSession_ReleaseSMContext Response
-> [Conditional] The SMF (or V-SMF) responds with Nsmf_PDUSession_ReleaseSMContext Response message.
5: SM Policy Association Termination
-> [Conditional] If dynamic PCC applied to this session the SMF performs an SM Policy Association Termination procedure
6: AMF-initiated AM Policy Association Termination
-> [Conditional] If there is any association with the PCF for this UE and the UE is no more registered over any access, the AMF performs a AMF-initiated AM Policy Association Termination procedure
7: De-registration Accept
-> [Conditional] The AMF sends NAS message Deregistration Accept to UE depending on the Deregistration type i.e. if Deregistration type is switch-off, AMF does not send Deregistration Accept message.
8: Signaling Connection Release
-> [Conditional] AMF to AN: N2 UE Context Release Request (Cause)
Source: TS 23.502