NGAP – Next Generation Application Protocol
(NGAP) stands for Next Generation Application Protocol. It is a protocol that enables the communication and coordination between the core network and the radio access network (RAN) in 5G networks. It is used for various purposes, such as mobility management, session management, connection management, and security.
The NGAP signaling flow is the sequence of messages exchanged between the core network entities, such as the Access and Mobility Management Function (AMF) and the User Plane Function (UPF), and the RAN entities, such as the Next Generation Node B (gNB), using the NGAP protocol. The NGAP signaling flow can be divided into two main categories: non-UE-associated signaling and UE-associated signaling. Non-UE-associated signaling refers to the messages that are not related to a specific user equipment (UE), such as configuration updates, error indications, and load balancing. UE-associated signaling refers to the messages that are related to a specific UE, such as registration, handover, and PDU session establishment.
A typical example of a NGAP signaling flow is the initial registration procedure, which is performed when a UE wants to connect to the 5G network for the first time. The following steps describe the main messages involved in this procedure:
• The UE sends a random access preamble to the gNB to initiate the connection.
• The gNB responds with a random access response, which contains the timing advance and the uplink grant for the UE.
• The UE sends a RRC connection request, which contains the UE identity and the establishment cause.
• The gNB responds with a RRC connection setup, which contains the RRC configuration for the UE.
• The UE sends a RRC connection setup complete, which contains the NAS registration request message.
• The gNB forwards the NAS registration request to the AMF using the NGAP initial UE message.
• The AMF performs authentication and security procedures with the UE and sends a NAS registration accept message to the gNB using the NGAP initial context setup request.
• The gNB sends a RRC reconfiguration message to the UE, which contains the NAS registration accept message and the radio bearer configuration for the UE.
• The UE sends a RRC reconfiguration complete message to the gNB, which indicates that the UE has applied the radio bearer configuration.
• The gNB sends a NGAP initial context setup response to the AMF, which indicates that the gNB has applied the NAS context for the UE.
• The AMF sends a NGAP PDU session resource setup request to the gNB, which contains the PDU session information and the UPF address for the UE.
• The gNB sends a PDU session establishment request message to the UPF, which contains the UE identity and the QoS parameters for the PDU session.
• The UPF responds with a PDU session establishment accept message to the gNB, which contains the tunnel endpoint identifier (TEID) and the IP address for the PDU session.
• The gNB sends a NGAP PDU session resource setup response to the AMF, which contains the TEID and the IP address for the PDU session.
• The UE and the UPF can now exchange user data packets over the PDU session.