NGAP Introduction

Note

Author: Wei, Chang
Date: 2025/10/15

NGAP (Next-Generation Application Protocol) is the control-plane protocol on the N2 interface between NG-RAN (gNB) and the 5GC (AMF). It defines modular Elementary Procedures (EPs) that carry Information Elements (IEs) to establish identities and capabilities, transport NAS end-to-end, manage UE context and PDU session resources, and support mobility (handover, path switch). Messages are ASN.1 APER-encoded and run over SCTP/IP, relying on shared identifiers (RAN-UE-NGAP-ID, AMF-UE-NGAP-ID, PDU Session ID) and transport anchors (TEIDs/IPs) to maintain state across procedures.

Main functions

Paging: Notify the RAN to locate the UE.
UE Context Management: Create, modify, and release the UE context in both the AMF and the gNB.
PDU Session Management: Manage the UE’s PDU sessions, e.g., establishing and releasing the resources required for connectivity to the UPF.
Mobility Management: Handle handover procedures as the UE moves between different gNBs.
NAS Message Transport: Transparently transfer NAS messages between the UE and the AMF.
NG Interface Management: Manage the NGAP connection between the RAN and the AMF.

Message model

Every NGAP PDU (Protocol Data Unit) is exactly one of:
- initiatingMessage (request/trigger)
- successfulOutcome (positive response)
- unsuccessfulOutcome (negative response)

Messages are made of IEs (Information Elements). Each IE has id / criticality / presence.

Presence

Mandatory: This IE must be present in the message. If it’s missing, it’s a protocol error and the whole message/procedure is typically discarded or fails.
Optional: This IE may be present or absent; omission is not an error.

Assigned Criticality

This tells the receiver how to handle the IE if there’s an issue.
- reject: If this IE is problematic, reject the entire message/procedure; treat it as a failure.
- ignore: If this IE is problematic, ignore this IE and continue processing the rest of the message.

Elementary Procedures (EPs)

There are two types of elementary procedure classified into Class 1 and Class 2.
- Class 1: Elementary Procedures with response (success and/or failure)

Class 1 EPs
- Class 2: Elementary Procedures without response.

Class 2 EPs

Interdependencies

Sequential: Initiating message → response (success/failure); failures abort or trigger error handling.
Contextual: Later procedures depend on earlier context (e.g., UE context before resource setup).
Shared IEs: UE IDs, security info, session details reused within/across procedures.

Key Procedure Summaries & IEs (Presence / Criticality)

(1) NG Setup Request (gNB→AMF)

Purpose: Introduce gNB; establish NG parameters.

Key IEs

IE	Presence	Criticality
Global RAN Node ID	M	reject
RAN node name	O	ignore
Supported TA List	M	reject
default paging DRX	M	ignore

Effect: Registers gNB identity; constrains UE placement (TA/PLMN match).

(2) NG Setup Response (AMF→gNB)

Purpose: Confirm setup; provide AMF identity/capacity.

Key IEs

IE	Presence	Criticality
AMF Name	M	reject
Served GUAMI List	M	reject
PLMN Support List	M	reject
Relative AMF Capacity	M	ignore

Effect: gNB caches AMF identifiers; slice/PLMN alignment influences routing.

(3) Initial UE Message (gNB→AMF)

Purpose: First UE-associated NGAP; carry initial NAS.

Key IEs

IE	Presence	Criticality
RAN-UE-NGAP-ID	M	reject
NAS-PDU	M	reject
User Location Information	M	reject
RRC Establishment Cause	M	ignore
5G-S-TMSI	O	reject
UE Context Request	O	ignore

Processing notes (38.413-8.6.1.2):
- gNB allocates unique RAN-UE-NGAP-ID and includes it.
- NAS-PDU is forwarded without interpretation.
- Selected PLMN is indicated by PLMN Identity in the TAI (network sharing).
- If UE Context Request present, AMF triggers Initial Context Setup.
- Additional optional IEs (AMF Set ID, CE-mode-B, LTE-M indication, EDT Session, NPN info) are handled per TS 23.501/23.502.

(4) Downlink NAS Transport (AMF→gNB)

Purpose: Deliver NAS to UE (e.g., Authentication Request, SMC, Registration Accept, PDU Session Accept).

Key IEs

IE	Presence	Criticality
AMF-UE-NGAP-ID	M	reject
RAN-UE-NGAP-ID	M	reject
NAS-PDU	M	reject

(5) Uplink NAS Transport (gNB→AMF)

Purpose: Forward UE NAS to AMF when a UE-associated logical NG-connection exists.

Key IEs

IE	Presence	Criticality
AMF-UE-NGAP-ID	M	reject
RAN-UE-NGAP-ID	M	reject
NAS-PDU	M	reject
User Location Information	M	ignore

Note: NAS is transferred without interpretation in NG-RAN.

(6) Initial Context Setup Request (AMF → gNB)

Purpose: Establish the UE’s initial control-plane and user-plane context at the gNB after authentication/authorization.

Key IEs

IE	Presence	Criticality
AMF-UE-NGAP-ID	M	reject
RAN-UE-NGAP-ID	M	reject
GUAMI	M	reject
Allowed NSSAI	M	reject
UE Security Capabilities	M	reject
Security Key	M	reject
NAS-PDU	O	ignore
Redirection Voice Fallback	O	ignore

Effect
- gNB creates the UE context and configures required resources based on the above security/slice parameters; prepares for subsequent session setup.
- Common side effect: triggers the gNB’s RRC Security Mode Command.

(7) Initial Context Setup Response (gNB → AMF)

Purpose: Indicates successful completion of the initial context setup initiated by the request; confirms radio/security configuration is in place at the gNB.

Key IEs

IE	Presence	Criticality
AMF-UE-NGAP-ID	M	ignore
RAN-UE-NGAP-ID	M	ignore

Effect: AMF acknowledges the UE context is active at the gNB; the UE can proceed with further signaling and data transfer (including subsequent NAS and PDU Session resource procedures).