ORAN 5G SA Registration Call Flow

|This story is basically how I would explain it to anyone.|

We usually take the PCAP(Packet capture) from the CU worker node where the Control plane is running.

When RAN is being deployed, we usually follow the sequence where we deploy CUCP 1st, which will be talking to AMF for the NGC Client.

Then we deploy the CUUP which will talk to UPF via the NGU interface and CUCP via the E1 interface.

Then DU is deployed which talks to the CUCP 1st to have F1 setup via F1c Interface, in F1 setup DU initiates the F1 request and sends it to CUCP where CUCP will match all the config and send it to AMF via NGC interface, which will be ng setup initiation if the config sent from gnb to core matches, Core acknowledges the ngsetup request and send a successful response to CUCPand then CUCP sends the successful message to the F1 setup to DU and cell comes up afterwards

Then once the cell is up, we do the UE attach, where UE sends the RRC setup request towards DU and DU sends it to CUCP. After that, CUCP acknowledges the RRC setup, UE sends message RRC a complete message, in the RRC setup complete message UE sends the Reg request which goes towards core via F1C and NGC interfaces from DU to CUCP to Core.

Then CUCP sends the initial UE message with all the details like MCC MNC, SUPI, GUTI, ST and SST etc details.

Core responds to that by matching the details, if the details match then, there will be a security mode command which will check the ciphering algorithm used by gnb and will match with the cores algorithm

Once that matches and the security mode gets completed next there will be UE capability inquiry where it will be checked that what kind of bands, tech, CA modes, band combo and BW are supported

Once that completes, the Reg request completes and UE s reg request gets accepted if all is good from the Core end.

Then UE sends the Reg complete message.

Then UE sends the PDU session establishment request towards the core, and it matches the APN details, also bearer context setup happens before that between CUCP and CUUP over the E1 interface

Once the PDU session request goes to core it responds successfully if APN details are correct, and UE gets the IP.