 to a notification (paging: MT access); - sending an SMS (MO SMS and SMSoIP); - sending an MDT; - a request for a telephone call or videoconference from the mobile (MO MMTel Voice, MO MMTel Video). The base station can accept or refuse the request of the mobile (RRC SetupMessage or RRC RejectMessage) depending on the importance of the message and the congestion of the radio access or congestion of the core network. The mobile acknowledges the received message by responding with the RRC SetupComplete message. PUSCH RRC SetupRequest PDSCH RRC Setup PUSCH - RRC SetupRequestComplete Figure 3.8. Establishment procedure for the RRC connection 3.2.3. Register request Excepting emergency calls, the first procedure for the mobile is to register with the 5G core network. The registration procedure allows the mobile to use 5G network services. The registration procedure is divided into three steps: - mutual authentication: identification and authentication; - access to services; - mobile registration. NG-RAN and 5G-NR Once registered, the mobile will be able to send or receive its traffic through an E2E bearer. The initial bearer establishment is at the initiative of the mobile and its reestablishment can be implemented by the mobile or by the core network. 3.2.3.1. Mutual Identification and authentication During the RRC connection procedure (Figure 3.8), the mobile sends the RRC SetupRequest. This message is used to initiate the Register Procedure by sending the Register Request as a NAS Payload (signaling message). After receiving the RRC Setup message from the base station, the mobile sends the RRC SetupComplete request with the following parameters: - the message NAS: Registration Request; - its identifier: SUPI/SUCI (Subscription Permanent identifier/Subscription Concealed identifier) or 5G-GUTI (5G-Global Unique Temporary identifier); - the expected S-NSSAI (Single Network Slice Selection Assistance Information) network slices; - its capacity; - the list of PDU sessions to activate; - its previous location (TAI). Both