Initial Registration Request Call Flow
This section describes the Initial Registration Request call flow.

Step |
Description |
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1 |
An UE which wants to register itself with the 5G core sends a Registration-Request N1 message towards AMF with the following contents:
If the subscriber is unknown, AMF allocates AMF-NGAP-id to the NGAP connection and subscriber data-store. The AMF-NGAP-id to AMF-Service is stored in etcd so that subsequent messages over the NGAP connection reach same AMF-Service. gNB selects an AMF and forwards the registration-request message to AMF. |
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2 |
If the identity received from the UE was either a SUCI, SUPI, or GUTI allocated by this AMF, the AMF authenticates the UE as presented in the authentication procedure. |
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3 |
If the AMF is configured to do EIR checks during registration, the AMF retrieves the PEI from the UE during security mode command procedure. It then checks the status of the equipment during registration procedure. |
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4 |
Depending on the status of the equipment from EIR, the AMF either rejects the registration or proceeds with the call. Actions to be taken when the status is grey listed is configurable on the call control policy currently active for the UE. |
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5 |
The AMF selects an UDM based on the PLMN information through NRF query or via static configuration and registers the UE with the UDM using Nudm_UECM_Registration. |
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6 |
The UDM stores the AMF identity and responds to the AMF request. |
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7 |
The AMF requests from the UDM the Access and Mobility Subscription, and SMF Selection Subscription Data using Nudm_SDM_Get and using multiple data set names. If integrity check passes and UDM subscription data already exit in UE context, AMF skips Steps 7 - 10. |
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8 |
The UDM responds to the request from the AMF. The AMF stores the subscription information. |
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9 |
The AMF subscribes to be notified using Nudm_SDM_Subscribe when the data requested is modified. |
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10 |
The UDM registers the AMF and responds to the AMF. |
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11 |
The AMF selects PCF based on PLMN-info and slice-info and performs a policy association establishment. PCF sends policy data to AMF with restrictions and other policies to be applied for the UE.
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12 |
The PCF responds to the AMF request along with AM-Policy configurations for the subscriber. |
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13 |
The AMF sends a Registration Accept message to the UE indicating that the Registration Request has been accepted. Registration Accept contains (registration area, mobility restrictions, PDU session status, allowed NSSAI, configured NSSAI for the serving PLMN, periodic registration update timer, emergency service support indicator, accepted DRX parameters). |
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14 |
If the AMF sends an Initial Context Setup Request, the gNB responds with an Initial Context Setup Response. This message could come after the message in Step 12. |
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15 |
The UE sends a Registration Complete message to the AMF to acknowledge that a new 5G-GUTI was assigned if a new 5G GUTI was included in the Registration Accept message. |
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16 |
If the UE did not include a follow-on indication in the request, the AMF releases the UE gNB context by sending a UE Context Release Command to the gNB |
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17 |
The gNB responds with a UE Context Release Complete message to the AMF. |