gNB Central Unit User Plane

Description

The gNB-CU-User Plane (gNB-CU-UP) is a logical node in the 3GPP 5G NR architecture, introduced in Release 15 as part of the centralized unit (CU) and distributed unit (DU) functional split. It hosts the user plane portion of the Packet Data Convergence Protocol (PDCP) for an en-gNB and both the user plane PDCP and the Service Data Adaptation Protocol (SDAP) for a gNB. This separation of user plane functions from control plane functions allows for flexible deployment and efficient handling of user data traffic.

The gNB-CU-UP terminates two key interfaces: the E1 interface, which connects it to the gNB-CU-Control Plane (gNB-CU-CP), and the F1-U interface, which connects it to the gNB-DU. The E1 interface carries control information necessary for managing user plane resources, such as bearer setup and modification. The F1-U interface transports user data between the CU and DU, supporting functions like data forwarding, flow control, and quality of service (QoS) enforcement. These interfaces are defined in 3GPP TS 38.401 and TS 28.540.

In dual connectivity (DC) scenarios, the gNB-CU-UP assumes specific roles based on whether it operates as a master or secondary node. The MgNB-CU-UP designates the gNB-CU-UP of a gNB or en-gNB acting as the master node, while the SgNB-CU-UP designates the gNB-CU-UP of a node acting as the secondary node. This distinction is essential for managing user plane data across multiple radio links, ensuring efficient aggregation and splitting of data flows to enhance throughput and reliability.

The gNB-CU-UP is critical for the user plane functionality of the 5G radio access network. It handles PDCP functions such as header compression, ciphering, and integrity protection for user data, as well as SDAP functions that map QoS flows to data radio bearers. By centralizing these functions, the gNB-CU-UP enables efficient resource utilization and simplifies the integration with the 5G Core network, particularly the User Plane Function (UPF) via the N3 interface. This architecture supports advanced features like network slicing, where different slices can have tailored user plane policies, and ensures low-latency and high-throughput data transmission.

From a deployment perspective, the gNB-CU-UP can be virtualized and placed in edge data centers to reduce latency for latency-sensitive applications. It interacts with the gNB-CU-CP over the E1 interface to receive configuration and control information, and with the gNB-DU over the F1-U interface to transmit and receive user data. This separation of control and user plane functions allows operators to scale user plane resources independently, adapting to varying traffic demands and service requirements. The gNB-CU-UP is thus a cornerstone of the 5G RAN's ability to deliver diverse services with high efficiency.

Purpose & Motivation

The gNB-CU-UP was created to enable a flexible and scalable user plane architecture in 5G NR. In previous generations like LTE, user plane functions were tightly integrated with control plane functions in a single base station, limiting deployment flexibility and making it difficult to optimize for different traffic types. The CU/DU split in 5G allows for the separation of user plane functions, enabling independent scaling and placement of resources.

By centralizing user plane functions in the gNB-CU-UP, operators can deploy these functions in cloud environments, reducing costs and improving resource utilization. This is particularly beneficial for handling high-throughput applications like enhanced mobile broadband (eMBB) and for supporting network slicing, where each slice can have dedicated user plane resources. The gNB-CU-UP also facilitates dual connectivity, allowing devices to aggregate data from multiple cells, which improves throughput and reliability.

Historically, the limitations of monolithic base stations included inefficient use of hardware and difficulty in adapting to new services. The gNB-CU-UP addresses these by promoting virtualization and software-defined networking principles. It allows for dynamic scaling of user plane resources based on traffic demand and supports the integration of edge computing, where user plane functions can be placed close to the user to minimize latency. This architectural evolution is essential for meeting the diverse requirements of 5G use cases.