NG Application Protocol

Description

The NG Application Protocol (NG-AP) is a critical control plane protocol defined by 3GPP for the 5G System (5GS). It operates over the NG interface, specifically the N2 reference point, which connects the Next Generation Radio Access Network (NG-RAN) to the 5G Core Network (5GC). NG-AP is responsible for all signaling interactions between the gNB (or ng-eNB) and the Access and Mobility Management Function (AMF). It is a key enabler for the service-based architecture of 5G, facilitating the decoupling of the control and user planes.

NG-AP works by exchanging structured messages, known as Elementary Procedures (EPs), between the RAN and the core network. These procedures are categorized into Class 1 (requiring a response) and Class 2 (not requiring a response). Key procedures include the Initial UE Message, which establishes the first signaling connection for a user equipment (UE); the UE Context Management procedures for setup, modification, and release; and the Handover Preparation and Resource Allocation procedures for seamless mobility. The protocol also handles Paging, UE Capability Info Indication, and the setup of NG interfaces themselves.

Architecturally, NG-AP is carried over Stream Control Transmission Protocol (SCTP) to ensure reliable, message-oriented transport. It is a peer-to-peer protocol where the gNB and AMF are the logical endpoints. Its design supports network slicing by allowing the RAN to be aware of and route traffic to the appropriate network slice instance via the associated AMF. NG-AP's role is central to session management, mobility management, and the overall orchestration of UE connections within the 5G network, providing the foundational signaling layer that makes advanced 5G features possible.

Purpose & Motivation

NG-AP was created as part of the 5G architecture in 3GPP Release 15 to provide a new, flexible, and service-based signaling protocol between the RAN and the core network. It addresses the limitations of previous protocols like S1-AP (used in 4G LTE between eNB and MME), which were not designed for the cloud-native, service-based, and slice-aware architecture of 5G. The primary motivation was to support new 5G requirements, including enhanced mobile broadband (eMBB), ultra-reliable low-latency communications (URLLC), and massive machine-type communications (mMTC), which demand more agile and efficient control plane signaling.

The protocol solves the problem of rigid, point-to-point signaling by enabling a more decoupled and scalable interaction model. It allows a single gNB to connect to multiple AMFs, supporting advanced features like network slicing and AMF redundancy. Furthermore, NG-AP is designed to be forward-compatible, with a modular structure that can accommodate new procedures and parameters in future releases without breaking existing functionality. Its creation was motivated by the need for a protocol that could handle the increased complexity and diversity of 5G services, from high-speed data to critical IoT communications, while operating in a virtualized and software-defined network environment.

Key Features

• Manages UE context setup, modification, and release
• Handles NG interface management and configuration transfer
• Orchestrates handover preparation and execution procedures
• Supports paging initiation and delivery to the RAN
• Enables transport of NAS messages between UE and AMF
• Facilitates UE capability information transfer and RAN configuration updates

Evolution Across Releases

Defining Specifications