What is Fronthaul HLS?
Fronthaul HLS (Higher Layer Split) defines the open interface between a gNB-CU (Centralised Unit) and a gNB-DU (Distributed Unit) in the 5G RAN architecture. This split — designated as Option 2 by 3GPP — separates the RAN protocol stack at the PDCP/RLC boundary, allowing CUs and DUs to be supplied by different vendors and deployed at different physical locations. HLS is a core enabler of Open RAN disaggregation and is standardised by both 3GPP (F1 interface) and the O-RAN Alliance.
How Does Fronthaul HLS Work?
In the HLS architecture, the CU handles higher-layer functions: RRC (Radio Resource Control) for connection management and PDCP (Packet Data Convergence Protocol) for ciphering, integrity protection, and header compression. The DU handles lower-layer functions: RLC (Radio Link Control), MAC (Medium Access Control), and the upper part of PHY (scheduling, HARQ). The F1 interface connects CU to DU, with F1-C carrying control plane traffic and F1-U carrying user plane data. Because the HLS split point is above the time-critical scheduling functions, the fronthaul bandwidth requirements are proportional to user data throughput rather than to the full radio bandwidth — making it much less demanding than lower-layer splits like CPRI.
Use Cases
Open RAN multi-vendor deployments (CU from one vendor, DU from another), CU pooling for resource efficiency across multiple cell sites, CU placement at regional edge for coordinated mobility and dual connectivity, and network slicing with dedicated CUs per slice.
3GPP / Standards Reference
3GPP TS 38.401 (NG-RAN architecture — CU/DU split), 3GPP TS 38.470/473 (F1 interface specifications), O-RAN Alliance WG1 architecture specification
Related Terms
C-RAN | O-RU / O-DU / O-CU | Fronthaul | Midhaul | Open RAN
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
