Non-3GPP Connection

Description

The Non-3GPP Connection (N3C) is a protocol sublayer introduced in the 5G New Radio (NR) user plane protocol architecture, operating between the Service Data Adaptation Protocol (SDAP)/Packet Data Convergence Protocol (PDCP) layers and the underlying non-3GPP access technology's link layer. It functions as an adaptation layer, allowing the upper layers of the 5G radio protocol stack (specifically PDCP) to operate independently of the lower-layer characteristics of a non-3GPP radio link. The N3C entity resides in both the User Equipment (UE) and the network side (e.g., in a gNB-CU or a dedicated node supporting multi-RAT). Its primary role is to map PDCP Protocol Data Units (PDUs) onto the service data units of the non-3GPP link layer, handling aspects like segmentation, reassembly, and in-sequence delivery if the non-3GPP link does not natively provide these services.

Architecturally, N3C is part of the broader Multi-Radio Dual Connectivity (MR-DC) and Access Traffic Steering, Switching, and Splitting (ATSSS) framework. When a UE is configured with a non-3GPP connection as a secondary cell group or a path for traffic splitting, the gNB uses the N3C layer to manage the data flow over that link. The N3C layer may add its own header to the PDCP PDU, containing sequence numbers and length indicators necessary for the adaptation function. This enables the PDCP layer to maintain its core functions—such as ciphering, integrity protection, and duplicate detection—consistently, whether the underlying physical transport is 5G NR, LTE, or a non-3GPP technology. The specifications (e.g., TS 38.322, 38.323) define the precise procedures for N3C establishment, reconfiguration, and release, as well as its interactions with the RRC layer for control.

From an operational perspective, the N3C layer abstracts the vagaries of the non-3GPP link, presenting a more reliable and ordered data pipe to the PDCP layer. This is crucial for maintaining the end-to-end QoS and reliability expectations of 5G services when using heterogeneous access. For instance, if the non-3GPP link is a high-latency satellite connection, the N3C layer's buffering and sequencing mechanisms help mitigate the impact on the overall data flow. By standardizing this adaptation layer, 3GPP allows for the incorporation of a wide variety of non-3GPP radios into the 5G RAN framework in a clean, modular way, without requiring changes to the core PDCP protocol for each new access type.

Purpose & Motivation

The N3C was developed to address the growing need for deep, layer-2 integration of non-3GPP access technologies into the 5G RAN, going beyond the core network integration provided by N3AN. Previous approaches, like LTE-WLAN Aggregation (LWA), required specific adaptation and were limited in scope. The 5G vision of truly integrated multi-RAT operation, especially for ATSSS, demanded a more generic and flexible solution that could work with various non-3GPP links (e.g., Wi-Fi 6/7, satellite, private networks) as if they were native 3GPP radio links from the upper-layer perspective.

Its purpose is to solve the technical problem of protocol stack mismatch. Non-3GPP technologies have their own MAC and PHY layers with different characteristics (frame sizes, reliability mechanisms, absence of in-order delivery). N3C provides the necessary 'glue' to allow the 5G NR PDCP layer, which is designed for the 3GPP MAC/PHY, to function correctly over these disparate links. This enables advanced RAN features like packet duplication for ultra-reliability or intelligent traffic splitting across 3GPP and non-3GPP paths to be implemented seamlessly. The creation of N3C is motivated by the drive towards network convergence at the radio level, allowing operators to build more robust, high-capacity, and efficient radio networks by harnessing the best characteristics of all available wireless technologies in a tightly coordinated manner.