Mobile device registration in 5G Network

Procedure for a mobile device (UE) registration to 5G System (RAN+Core).

The control plane (C-plane) is responsible for initial UE registration procedures.

Before understanding the procedure for UE registration with 5GS, it is important to understand the C-plane protocols between UE, gNB, and 5G Core (5GC). (all three entities combine to make a 5G System or 5GS).

• Between the UE and gNB is the NR air interface. The control plane protocol used here is the RRC protocol.
• Between gNB and AMF within the 5GC is the N2 which uses NG-AP, i.e. Next Generation Application Protocol.
• The interface between UE and AMF is the N1 interface which shares messages over the Non-Access Stratum (NAS messages).

N1 Interface

N1 interface is used for multiple C-plane functionalities, including mobility management, session management (TS 23.501, TS 24.501) etc. N1 interface is used for NAS message transmission between UE and AMF, transparent to gNB (i.e. gNB doesn't decode them and passes them untouched).

NAS Messaging procedure

As mentioned, the NAS messages (on the N1 interface) are transparent at the gNB level, i.e. gNB does not decode the NAS messages and relays them as-it-is to AMF or UE (depending on the direction. The gNB acts only as the relay for NAS transmission (see Section…