Description
The Next Generation Application Protocol (NGAP) is a critical signaling protocol defined by 3GPP for the interface between the 5G Core Network (5GC) and the Next Generation Radio Access Network (NG-RAN), specifically the gNB or ng-eNB. It operates over the SCTP transport layer on the NG interface (N2 for control plane). NGAP is responsible for all control plane signaling between these two domains, enabling the core network to manage the RAN and vice versa. Its primary functions include the establishment, modification, and release of the UE context in the RAN, which is fundamental for any data or signaling session. NGAP also handles the transfer of Non-Access Stratum (NAS) signaling messages between the UE and the AMF, meaning the RAN transparently relays these messages without interpreting them. Furthermore, it manages UE mobility procedures, such as handovers within the 5G system and from legacy systems, and it supports Paging initiation from the core network. The protocol is designed to be modular and forward-compatible, with a procedure-based model where each elementary procedure (EP) is defined as either Class 1 (requiring a response) or Class 2 (without a response). Key NGAP messages include INITIAL UE MESSAGE, DOWNLINK NAS TRANSPORT, UPLINK NAS TRANSPORT, HANDOVER REQUIRED, and PAGING. Its architecture separates the concerns of user plane management (handled by other protocols on the N3 interface) and control plane signaling, providing a clean, efficient, and scalable mechanism for network control. NGAP is a cornerstone of the 5G Service-Based Architecture (SBA), allowing for flexible network function interaction and enabling advanced features like network slicing by carrying slice information between the RAN and the core.
Purpose & Motivation
NGAP was created to provide a unified, efficient, and future-proof signaling protocol for the 5G system, replacing and enhancing the functionality of protocols like S1-AP from the 4G EPS. The evolution to 5G introduced a new Service-Based Core architecture (5GC) and new RAN requirements, such as support for network slicing, ultra-reliable low-latency communication (URLLC), and massive IoT. The older S1-AP protocol was not designed to natively carry the information elements required for these new services and architectural paradigms. NGAP solves this by being designed from the ground up for 5G, with explicit support for Network Slice Selection Assistance Information (NSSAI), multiple PDU sessions per UE, and differentiated QoS flows. It addresses the limitations of previous approaches by offering greater flexibility, improved efficiency for control signaling, and inherent support for the decoupling of the control and user planes, which is a key tenet of 5G architecture. Its creation was motivated by the need for a protocol that could scale to support a vastly increased number of connected devices, diverse service requirements, and cloud-native network functions, thereby enabling the full vision of 5G.
Protocol Stack
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (40 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, NGAP was introduced to manage the logical per-UE association between a 5G-AN node and an AMF, including procedures for the AMF UE NGAP ID and the binding of this association to a specific transport layer. It defined key capabilities such as the AMF's ability to command the release or update of the NGAP UE-TNLA-binding for a UE in CM-CONNECTED state while maintaining user-plane connectivity, and introduced mechanisms for overload control using NGAP OVERLOAD START messages. Furthermore, it specified the handling of the Initial Context Setup procedure and the inclusion of user location information within NGAP messages.
In Release 16, key enhancements to NGAP included the formalization of the NGAP UE-TNLA-binding update procedure, allowing the AMF to command the 5G-AN to change or release the binding of a UE's signalling association to a specific transport layer connection. Furthermore, the protocol was extended to support providing recommended cells for paging, where the AMF can include the last used cell ID in the **Assistance Data for Recommended Cells IE** within a Paging message to improve efficiency. Other updates comprised various clarifications and corrections, such as the usage of the New AMF UE NGAP ID in the UE CONTEXT MODIFICATION REQUEST message and support for the UE Radio Capability for Paging.
- NGAP UE-TNLA-binding update TS 23.501CR2632
- Provide recommended cells for paging information in NGAP UE CONTEXT RESUME REQUEST message TS 23.501CR2664
- NGAP Rapporteur corrections TS 38.413CR0320
- NGAP Rapporteur corrections TS 38.413CR0365
- NGAP tabular corrections and asn.1 review TS 38.413CR0416
- Rapporteur cleanup of NGAP TS 38.413CR0417
+ 3 more changes
In Release 17, NGAP was enhanced to support new functionalities including NR positioning, Redundant PDU Sessions for URLLC, and Multicast/Broadcast Services (MBS), while also introducing corrections for UE Power Saving, MUSIM, and Inter-System Load Balancing. The updates refined procedures for managing the NGAP UE-TNLA-binding and introduced mechanisms for AMF overload control, including the use of Weight Factors and OVERLOAD START/STOP messages. Furthermore, corrections addressed specific information elements and parameters, such as the Global eNB ID and Excess Packet Delay handling.
- Introduction of NR positioning enhancements to NGAP TS 38.413CR0754
- 5G URLLC Redundant PDU Session correction for NGAP parameters TS 23.501CR3035
- NGAP ASN.1 review for MBS TS 38.413CR0764
- UE Power Saving Correction in NGAP TS 38.413CR0798
- MBS NGAP Corrections TS 38.413CR0801
- NGAP Inter-System Load Balancing corrections for procedure and IEs TS 38.413CR0807
+ 10 more changes
In Release 18, NGAP enhancements included updates to the DOWNLINK NAS TRANSPORT message for transferring mobile IAB authorization state and corrections to support QoE mobility during inter-MN handover. The release also introduced specific impacts for Mobility Restrictions with NR NTN and provided corrections for ECN marking behavior. Furthermore, it defined mechanisms for an AMF to command the release or rebinding of the NGAP UE-TNLA-binding for a UE in CM-CONNECTED state.
- NGAP DL Data Notification Message update TS 23.501CR4960
- Transfer of mobile IAB authorization state in NGAP DOWNLINK NAS TRANSPORT TS 38.413CR1071
- Correction on the behavior description of ECN Marking or Congestion Information Reporting over NGAP TS 38.413CR1099
- Correction on NGAP to support the QoE mobility during NG-based inter-MN HO TS 38.413CR1171
- Mobility Restrictions with NR NTN - NGAP Impacts TS 38.413CR1109
- NGAP rapporteur corrections TS 38.413CR1076
+ 1 more changes
In Release 19, the NGAP protocol was updated with new procedure texts for XR uplink rate control and included corrections for the Notification Cause IE, A-IoT related procedures, and the description of NGAP Services. The release also refined the mechanisms for managing the NGAP UE-TNLA-binding, including the use of an indicator during the NGAP Setup procedure to control its release or re-binding. Furthermore, enhancements were made to overload control procedures, such as the ability for an AMF to include S-NSSAI(s) in an NGAP OVERLOAD START message to restrict NAS signalling for specific network slices.
Explore further
Broader topics and technologies where NGAP plays a role.
Defining Specifications
3GPP specifications that define or reference NGAP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 29.214 vj20 | Policy and Charging Control over Rx | Rel-19 |
| TS 29.561 vj30 | 5G Interworking with External Data Networks | Rel-19 |
| TS 33.836 vg10 | Security Study for Advanced V2X Services | Rel-16 |
| TR 33.847 vh10 | 5G Proximity Services Security Study | Rel-17 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
| TS 38.414 vj00 | NG Interface User Plane Protocol | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
