Walk through how a 5G UE sets up a connection from registration to a working data path, and what NGAP's role is.

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NGAP signals between the gNB and the AMF to set the UE up. First the gNB registers itself with the core (NGSetupRequest); then the UE registers (InitialUEMessage carrying a NAS Registration Request); IDs and security are established; finally a PDU session is set up, which builds the GTP-U user-plane tunnel so user data can flow.

* Register the tower, register the phone, then build the data pipe. *
The setup sequence (control plane first, then user plane):

gNB announces itself: before any UE attaches, the base station sends an NGSetupRequest to the core, advertising its identity and capabilities — supported PLMNs (operator networks), Tracking Areas, and network slices.
UE registration begins: the UE's first NAS message (e.g. Registration Request) is carried up to the gNB over the radio protocols, and the gNB forwards it to the AMF inside an InitialUEMessage.
Identifiers are assigned: the gNB allocates a RAN-UE-NGAP-ID, then the AMF adds an AMF-UE-NGAP-ID. This pair uniquely identifies the UE on the NG interface and is reused for the whole connection.
Security & context: messages like DownlinkNASTransport and InitialContextSetupRequest set up security mechanisms, UE context, and initial QoS parameters.
PDU session / data path: the AMF/SMF send the gNB a PDUSessionResourceSetupRequest carrying the user-plane parameters — PDU-Session-ID, QoS info (QFIs), and tunnel endpoints (TEIDs + IP addresses). The gNB replies with the matching downlink-tunnel info, and the user-plane data path over GTP-U is then fully established.

NGAP's central role: the NG Application Protocol runs between the gNB and the 5G core over the open NG interface (split into NG-C for signalling and NG-U for user data). It carries the NAS messages from the RAN into the core and orchestrates both the signalling connection and the data-path setup. Throughout, a consistent set of identifiers is reused — UE-IDs, PDU-Session-IDs, QoS flows (QFIs), and TEIDs — to keep everything tied to the right UE and session.
Tip: Think "register the tower, register the phone, then build the pipe." Control-plane signalling (NGAP/NAS) comes first to authenticate and set up context; only then does the GTP-U user-plane tunnel get built so packets can actually reach the internet.