1. Introduction

Fronthaul is the transport segment between the radio unit (RU, deployed as an active antenna unit, AAU) and the distributed unit (DU), and it is the only part of the 5G network where transport equipment carries radio-layer payload under radio-layer deadlines. A single S111 site with three AAUs presents 3 × 25 Gb/s of eCPRI interface capacity from the day the radio powers on, per operator planning estimates published in industry white papers, and every bit of it must arrive within a 100 µs one-way budget set by the hybrid automatic repeat request (HARQ) timing of the air interface. No other segment combines constant line-rate bandwidth, a microsecond-class latency bound, and a ±1.5 µs absolute time-alignment requirement in one link.

Those three constraints are why fronthaul gets its own engineering discipline rather than a corner of the backhaul design. This article covers the segment end to end: the architecture between AAU and DU pool, the requirement set that any candidate transport must satisfy, the bandwidth mathematics of CPRI and eCPRI, the four optical transport options and where each wins, packet fronthaul over time-sensitive Ethernet, synchronization delivery, and a product capability checklist usable in an RFP. The segment's position inside the full three-segment xHaul model is covered in the companion overview of 5G transport network architecture; this article goes deep on the RU–DU link alone.

2. Fronthaul Architecture

2.1 From C-RAN to the Disaggregated RU–DU Link

The fronthaul concept predates 5G. 4G centralized RAN (C-RAN) pulled baseband units out of cell sites into pooled hotel locations and connected them to remote radio units over CPRI, trading transport cost for pooling gain and simpler site engineering. 5G kept the pooling economics but moved the split point: the DU absorbs the real-time protocol layers (Radio Link Control, Medium Access Control, high physical layer) while the AAU keeps the low physical layer, beamforming, and the RF chain. The result is the O-RAN split 7.2x interface carried over eCPRI, replacing the raw I/Q sample stream of CPRI with frequency-domain data whose rate scales with traffic and spatial layers rather than antenna count.