1.

The UE sends a Registration request message including the following parameters: { AN

parameters, Registration Request (Registration type, SUCI or 5G-GUTI or PEI, last visited TAI
(if available), Security parameters, Requested NSSAI, UE Radio Capability Update, UE MM
Core Network Capability, PDU Session status, List Of PDU Sessions To Be Activated, Follow-on
request, MICO mode preference, Requested DRX parameters, [LADN DNN(s) or Indicator Of
Requesting LADN Information], [NAS message container]) and UE Policy Container (the list of
PSIs, indication of UE support for ANDSP and the operating system identifier))

 AMF selection: If a 5G-S-TMSI or GUAMI is not included or the 5G-S-TMSI or GUAMI

does not indicate a valid AMF the RAN, based on RAT and Requested NSSAI, if
available, selects an AMF

2. The AMF receives a NAS message Registration Request over N2

 During an initial Registration, the AMF will compare the PLMN of the presented SUCI
against internal configuration reflecting a list of SUPI Prefixes (=PLMNs) that the AMF
is to regard as supporting 5G SA operation. The AMF will respond to the UE with the
5GMM cause code #27 ‘N1 not allowed’, where there is no match and as a result the
UE will deregister from the NG-RAN, attempt to reselect/attach to another BT RAT
with its N1 capability flag withdrawn. In subsequent Registrations, the UE will send
the 5GGUTI which includes the PLMN (see Section 3.6.2) .
 AUSF selection: The AMF may decide to initiate UE authentication by invoking an
AUSF. In that case, the AMF selects an AUSF based on SUPI or SUCI. Note should the
AMF not be able to select an AUSF, it will provide the UE with the 5GMM cause code
#27 ‘N1 not allowed’ (see above)
 Note should the AMF successfully select an AUSF, but either the Subscriber using with
the UE has not been provisioned with a 5G SA UDM profile or the SUCI concealment
key on the SIM doesn’t match that of the network, them the AMF will map the
response code received from the AUSF to the following 5GMM cause code to the UE: -
#27 ‘N1 not allowed’ (to ensure the UE moves to another RAT while withdrawing it’s
N1 capabilityflag).

8. The AMF receives an Nausf_UEAuthentication Authenticate response from the AUSF, which
indicates that the UE is authenticated by the 5GS.

The AMF also obtains the following information from the response:

 The SUPI of the UE

 The security key that enables the AMF to provide integrity and encryption protection
to the subsequent NAS messages to the UE

9. The AMF sends a NAS message Security Mode Command to the UE to establish the 5G
security context for the UE. The message is integrity-protected.

10. The AMF receives a NAS message Security Mode Complete from the UE, which indicates
that the 5G security context for the UE is established in the UE. The message is ciphered and
integrity-protected and includes a complete Registration Request message.
11. Upon successful security procedures and if the AMF has changed, the new AMF sends a
Namf_Communication_RegistrationCompleteNotify to notify the old AMF that the registration
of the new UE in the new AMF is completed. If the authentication/security procedure fails,
then the Registration shall be rejected, and the new AMF invokes the
Namf_Communication_RegistrationCompleteNotify service operation with a reject indication
reason code towards the old AMF. The old AMF continues as if the UE context transfer service
operation was never received.

12. If the PEI was not provided by the UE nor retrieved from the old AMF, an Identity Request
procedure is initiated by the AMF sending an Identity Request message to the UE to retrieve
the PEI (encrypted).

13. The UE replies with an Identity Response

14. The new AMF initiates ME identity check by invoking the

N5geir_EquipmentIdentityCheck_Get service operation

15. The 5G-EIR replies with IMEI check result

 UDM selection. The new AMF, based on the SUPI, selects a UDM. The UDM may select
a UDR instance

16. The AMF sends an Nudm_SubscriberDataManagement Get request to the UDM to

retrieve the Slice Selection Subscription data of the UE.

17. The AMF receives an Nudm_SubscriberDataManagement Get response that contains the
Slice Selection Subscription data of the UE from the UDM.

18. If the AMF has changed since the last Registration procedure, or if the UE provides a
SUPI which doesn’t refer to a valid context in the AMF, the new AMF sends an
Nudm_UEContextManagement Registration request to the UDM to register as the
serving AMF for the UE. The request also enables the AMF to subscribe to be notified
when the UE has moved to another AMF.

The AMF provides the “Homogenous Support of IMS Voice over PS Sessions”
indication (see clause 5.16.3.3 of TS 23.501 [2]) to the UDM. The “Homogenous
Support of IMS Voice over PS Sessions” indication shall not be included unless the
AMF has completed its evaluation of the support of “IMS Voice over PS Session” as
specified in clause 5.16.3.2 of TS 23.501 [2].

19. The AMF receives an Nudm_UEContextManagement Registration response from the

UDM, which indicates that the AMF is registered as the serving AMF in the UDM.

20. If the AMF does not have subscription data for the UE, The AMF sends an

Nudm_SubscriberDataManagement Get request to the UDM to retrieve the Access

and Mobility Subscription data of the UE (including the Charging Characteristic

‘udm3gppChargingCharacteristics‘ for the DNN).

21. The AMF receives an Nudm_SubscriberDataManagement Get response that contains

the Access and Mobility Subscription data of the UE from the UDM.
22. The AMF sends an Nudm_SubscriberDataManagement Get request to the UDM to

retrieve the SMF Selection Subscription data of the UE.

23. The AMF receives an Nudm_SubscriberDataManagement Get response that contains

the SMF Selection Subscription data of the UE from the UDM.

24. After a successful response is received, the AMF subscribes to be notified when the

data requested is modified, using a Nudm_SDM_Subscribe (the UDM may subscribe to

UDR by Nudr_DM_Subscribe)

25. The AMF receives an Nudm_SubscriberDataManagement Subscribe response from the

UDM. The response indicates that the subscription to the Access and Mobility

Subscription data change of the UE is created for the AMF in the UDM.

26. The AMF sends an Nudm_SubscriberDataManagement Subscribe request to the UDM.

The purpose is to subscribe to be notified if the SMF Selection Subscription data of the

UE is modified in the UDM

27. The AMF receives an Nudm_SubscriberDataManagement Subscribe response from the

UDM. The response indicates that the subscription to the SMF Selection Subscription

data change of the UE is created for the AMF in the UDM.

28. The UDM initiate a Nudm_UECM_DeregistrationNotification to the old AMF. The old

AMF may have set a timer upon the reception of the UE Context transfer Response

message and removes the UE context when the timer expires. If the timer is not

running, the UE context is deleted upon the reception of this message.

29. The Old AMF unsubscribes with the UDM for subscription data using

Nudm_SDM_unsubscribe.

30. The AMF sends a Registration Accept message included in the Downlink NAS Transport

message to the UE, providing the UE with the registration area a 5G-GUTI and the IMS

Voice over PS session supported Indication (used by a Voice centric UE to determine if

it is to subsequently activate a PDU Session on the IMS DNN and Register with the IMS,

see Section 4.5.1).

31. The AMF receives a NAS message Registration Complete from the UE, which indicates

that the Initial Registration procedure is completed

[14b] The AMF retrieves the SMF Selection Subscription data and Access and
Mobility Subscription data and UE context using Nudm_SDM_Get, and the
UDM retrieves that information from the UDR via Nudr_DM_Query.

[14c] After receiving a successful response, AMF calls Nudm_SDM_Subscribe

to subscribe to the notification when the requested data has been modified,
and UDM can subscribe to the UDR via Nudr_DM_Subscribe.

[14d] [Conditional] After the UDM stores the Access type information, it
causes the UDM to initiate a Nudm_UECM_DeregistrationNotification to the
Old AMF so that the Old AMF deletes the corresponding UE context.
Roles of PCC Rules and Session Rules 💡

In 5G networks, managing service quality for diverse applications is critical.

PCC(Policy and Charging Control) rules play a central role in regulating traffic
flows, enforcing quality standards, and adapting network resources according
to real-time conditions.

Session Rules serve as the operational framework for implementing PCC

decisions at the user session level. These rules are responsible for managing
session establishment, maintenance, and termination while ensuring proper
resource allocation and QoS enforcement.

Process of Mapping PCC Rules into Session Rules:

▪️PCF transmits PCC rules to SMF.
▪️SMF processes PCC rules.
▪️Data path establishment.
▪️QoS configuration.
▪️Flow control setup.
▪️Charging rule generation.
▪️Session rule generation.

For the complete introduction please refer to the blog

👉🏻https://lnkd.in/g7eyU2vA