gNB Central Unit

Description

The gNB Central Unit (gNB-CU) is a fundamental component of the 5G New Radio (NR) architecture, introduced in 3GPP Release 15. It is a logical node that hosts the Radio Resource Control (RRC), Service Data Adaptation Protocol (SDAP), and Packet Data Convergence Protocol (PDCP) layers for the gNB, or the RRC and PDCP layers for the en-gNB (the LTE node in EN-DC). The gNB-CU controls the operation of one or more gNB Distributed Units (gNB-DUs) through the F1 interface, which is a standardized interface defined in 3GPP TS 38.401. This split architecture allows for flexible deployment, centralizing control and higher-layer processing while distributing lower-layer functions to the edge.

The primary function of the gNB-CU is to manage the radio resource control and mobility management for UEs. It handles the RRC protocol, which is responsible for connection establishment, radio bearer configuration, handover decisions, and paging. The SDAP layer, also hosted in the gNB-CU, is responsible for mapping QoS flows to data radio bearers, a key feature of 5G QoS management. The PDCP layer performs header compression, ciphering, and integrity protection, and manages the reordering and retransmission of packets. By centralizing these functions, the gNB-CU enables coordinated control over multiple gNB-DUs, which is essential for features like dual connectivity, carrier aggregation, and centralized scheduling.

The gNB-CU connects to the gNB-DUs via the F1 interface, which consists of the F1-C (control plane) and F1-U (user plane) parts. The F1-C uses SCTP over IP for signaling, while the F1-U uses GTP-U for user data transport. The gNB-CU also connects to the 5G Core (5GC) through the NG interface, specifically the N2 interface for control plane (to the AMF) and the N3 interface for user plane (to the UPF). This architecture allows the gNB-CU to act as a concentrator for multiple gNB-DUs, reducing the number of core network interfaces and simplifying network management.

The split between gNB-CU and gNB-DU is not rigid; 3GPP defines different functional splits (options 1 through 8) to allow vendors to choose where to place the boundary between central and distributed units. The most common split is option 2, where the PDCP layer is in the gNB-CU and the RLC, MAC, and PHY layers are in the gNB-DU. This split is particularly suitable for centralized unit deployment in cloud environments, while the gNB-DUs are deployed at cell sites. The gNB-CU can be further split into a Control Plane (gNB-CU-CP) and a User Plane (gNB-CU-UP) in a later release, allowing for separate scaling and deployment of control and user plane functions.

In terms of deployment, the gNB-CU can be implemented as a virtualized network function (VNF) running on commercial off-the-shelf (COTS) servers in a data center, while the gNB-DUs are typically located at the cell site. This enables benefits such as centralized resource management, reduced latency for control plane functions, and easier upgrades. The gNB-CU also plays a crucial role in network slicing, as it can be instantiated per slice to provide dedicated resources and isolation. Overall, the gNB-CU is a key enabler of the flexible, scalable, and efficient 5G RAN architecture.

Purpose & Motivation

The gNB Central Unit (gNB-CU) was created to address the need for a more flexible and scalable radio access network architecture in 5G. Traditional RAN architectures, such as those in 4G LTE, had monolithic base stations (eNBs) that combined all protocol layers in a single physical entity. This made it difficult to centralize control functions, coordinate across multiple cells, and adapt to varying deployment scenarios. The gNB-CU/gNB-DU split, introduced in Release 15, allows network operators to separate the control and higher-layer user plane functions from the lower-layer, real-time functions.

This separation solves several problems. First, it enables centralized coordination of radio resources across multiple gNB-DUs, which is essential for features like Coordinated Multi-Point (CoMP) and dual connectivity. Second, it allows for more efficient use of network resources by pooling control plane processing in a central location, reducing the cost and complexity of cell sites. Third, it supports the deployment of cloud RAN (C-RAN) and virtualized RAN (vRAN), where the gNB-CU can run as a virtual machine in a data center, providing scalability and flexibility.

The gNB-CU also facilitates the introduction of new services and network slicing. By centralizing the SDAP and PDCP layers, it can enforce QoS policies and map flows to appropriate bearers, ensuring that different slices receive the required treatment. The split architecture also simplifies the upgrade path for new features, as the gNB-CU can be updated independently of the gNB-DUs. Overall, the gNB-CU is a cornerstone of the 5G RAN, enabling the performance, efficiency, and flexibility required for diverse 5G use cases.