Deregistration is the 5GMM procedure used to remove the UE registration from the 5G system for one or more access types. It can be initiated by the UE or by the network. A common UE-initiated case is switch-off, where the UE informs the network that it is leaving 3GPP access and the network can release the corresponding UE context.
Signaling Sequence
The exact sequence depends on whether the UE or the network initiates the procedure and whether the UE is switching off. The following table shows a UE-originated switch-off case.
# |
Direction |
5GMM Message |
Comment |
1 |
UE -> AMF |
Deregistration request |
UE indicates deregistration type, switch-off indication, access type and UE identity. |
2 |
AMF |
Context release |
For switch-off, the UE does not wait for a NAS accept message before powering down. |
Message Structure
The deregistration request contains the deregistration type, ngKSI and 5GS mobile identity. The deregistration type tells whether the procedure is for normal deregistration or switch-off and whether it applies to 3GPP access, non-3GPP access, or both.
Example : Deregistration Request
The following example is based on an Amarisoft MME log showing a UE-originated switch-off deregistration. The whole decoded NAS message is shown below.
04:10:00.930 [NAS] UL 0065 5GMM: Deregistration request
Protocol discriminator = 0x7e (5GS Mobility Management)
Security header = 0x2 (Integrity protected and ciphered)
Auth code = 0xcb40f253
Sequence number = 0x07
Protocol discriminator = 0x7e (5GS Mobility Management)
Security header = 0x0 (Plain 5GS NAS message, not security protected)
Message type = 0x45 (Deregistration request)
Deregistration type = 0x9 (Switch off=1, Access type=3GPP access)
ngKSI:
TSC = 0
NAS key set identifier = 3
5GS mobile identity:
5G-GUTI
MCC = 001
MNC = 01
AMF Region ID = 128
AMF Set ID = 4
AMF Pointer = 1
5G-TMSI = 0x6e8f8c2c