Published June 21, 2026 • 8 min read • Telecom Insights
The 5G Centralized Unit (CU) — or O-CU in O-RAN parlance — hosts the non-real-time processing layers of the 5G NR protocol stack: Radio Resource Control (RRC), Service Data Adaptation Protocol (SDAP), and Packet Data Convergence Protocol (PDCP). As the aggregation point for potentially hundreds of DUs, the CU is the locus of radio resource management, mobility control, and security — and it is the gNB's interface to the 5G Core Network (5GC) via the NG interface.
Key Takeaway: The CU can control up to 256 DUs across a metro-scale deployment from a single centralised data centre location, providing pooled processing, coordinated scheduling, and seamless inter-DU mobility — the foundational architecture for cloud-RAN.
RRC Layer: Connection Management and Mobility
The RRC (Radio Resource Control) layer manages the UE's connection states — RRC_IDLE (no connection, cell selection/reselection), RRC_INACTIVE (suspended connection with stored context, introduced in 5G for fast resume), and RRC_CONNECTED (active data transfer). The RRC handles: connection establishment and release, security activation (NAS and AS key derivation), measurement configuration and reporting (intra-frequency, inter-frequency, inter-RAT), handover (Xn-based between gNBs, NG-based via 5GC), and carrier aggregation / dual connectivity configuration. The RRC's ASN.1 message encoding — while more efficient than LTE's — remains a significant processing load at scale.
SDAP: QoS Flow to DRB Mapping
The SDAP (Service Data Adaptation Protocol) layer — new in 5G NR — maps QoS flows from the 5G Core to Data Radio Bearers (DRBs). Each QoS flow is identified by a QFI (QoS Flow Identifier) and characterised by a 5QI (5G QoS Identifier) that defines resource type (GBR, non-GBR, delay-critical GBR), priority level, packet delay budget, and packet error rate. The SDAP's reflective QoS feature enables the UE to derive uplink QoS rules from downlink packets, eliminating the need for explicit NAS signalling. The SDAP header carries the QFI and a reflective QoS activation indicator.
PDCP: Ciphering, Integrity, and Duplication
The PDCP (Packet Data Convergence Protocol) layer provides: ciphering (AES-128/256, SNOW 3G, or ZUC using keys derived from the K_gNB), integrity protection (mandatory for user plane in 5G — a departure from LTE where it was optional), header compression (ROHC — RObust Header Compression — with profiles for IP/TCP/UDP/RTP), duplication (sending the same PDCP PDU over two independent RLC entities for URLLC reliability), and reordering and duplicate detection. The PDCP's ciphering engine must sustain crypto throughput exceeding 20 Gbps per CU instance to support multi-gigabit cell aggregates.
CU-DU Split: F1 Interface
The F1 interface connects the CU to the DU, carrying both control-plane (F1-C) and user-plane (F1-U) traffic. F1-C uses SCTP transport for reliable F1AP (F1 Application Protocol) signalling — including UE context setup, bearer management, and RRC message transfer. F1-U uses GTP-U tunnels per DRB. The F1 interface is designed to tolerate up to 10–30 ms one-way latency, enabling CU-DU geographic separation across metro fibre networks. The E1 split further divides the CU into CU-CP (control plane) and CU-UP (user plane), connected via the E1 interface, enabling independent scaling of control and data processing resources.
Cloud-Native CU Deployment
The CU is uniquely suited for cloud-native deployment on containerised infrastructure (Kubernetes). Unlike the DU with its strict real-time constraints, the CU's ~10–100 ms processing deadlines align well with virtualised environments. Microservices architecture decomposes the CU into independently deployable services — RRC service, SDAP service, PDCP-CP, PDCP-UP — communicating via service mesh (Istio/Linkerd). Horizontal pod autoscaling (HPA) adjusts CU-UP instances based on throughput demand. Service-based interfaces align the CU with the 5GC's SBA architecture. Major vendors (Nokia, Ericsson, Samsung) now offer fully cloud-native CU solutions running on commercial-off-the-shelf (COTS) servers.
The 5G CU represents the convergence of cellular protocol engineering and cloud computing — a domain where telecom-grade reliability (five-nines) meets the agility and cost-efficiency of hyperscale cloud infrastructure.
