OCUDU RAN Disaggregated FAPI Split

Mon, 01 Jan 0001 00:00:00 +0000

Split the gNB into two processes - odu_low (L1/PHY) and odu_high (L2/MAC) - and connect them through an xFAPI translator-bridge over an xSM (DPDK shared-memory) transport. The split lets L1 and L2 run in different DPDK domains on the same host, or on different servers connected by a dedicated DPDK-Ethernet link.

FAPI split architecture. odu_low creates the shared xSM memzone with two slot pairs; xFAPI attaches to both, and odu_high attaches to pair 1. The two-host xFAPI split places L1 and L2 on different machines connected by a DPDK-Ethernet link between two xFAPI instances.

Highlights

Process-level FAPI split - odu_high (L2/MAC, F1, scheduler) and odu_low (L1/PHY, OFH, radio) run as independent binaries; FAPI P5/P7 messages are serialised over an xSM ring buffer between them.
xFAPI translator-bridge - a separate process owns the FAPI translation and message dispatch, decoupling L1 and L2 DPDK domains and enabling cross-host deployments without code changes on either side.
Two supported topologies - single-host (all three processes on one server, one shared xsm_bridge memzone) and two-host xFAPI split (xFAPI-L1 and xFAPI-L2 connected over DPDK-Ethernet, each host has its own local memzone).
xSM shared-memory transport - DPDK-backed SPSC ring with hugepage memzones, pair-indexed slot allocation, and a precompiled libxsm.so exposing a small C API.
Configuration-driven activation - fapi_split_l1 (odu_low) and fapi_split_l2 (odu_high) YAML blocks control device names, pair indices, DPDK proc-type, and file-prefix. The same binary serves both monolithic and split deployments.
FAPI stats recorder - optional in-memory ring buffer captures every FAPI message (P5 + P7) with direction and PDU summary, dumped as JSON at shutdown for offline analysis.

1. Prerequisites

1.1 Hardware

x86-64 host with DPDK-capable NIC for the OFH front-haul (single-host topology) or two such hosts connected by a dedicated 25G link (two-host topology).
For the two-host topology, an additional DPDK-Ethernet-capable NIC (Intel XXV710 / E810 or equivalent) bound to vfio-pci on each host.
≥ 2 GB of 1 GiB hugepages on each host (xSM memzones use 1 GiB pages by default).
The DPDK-Ethernet link NIC (two-host only) and the OFH NIC must be on the same NUMA node as the upper-PHY / cell_rt worker cores.

1.2 Software

Component	Minimum version	Notes
DPDK	22.11	Tested with 25.11. Both xFAPI and OCUDU must build against the same DPDK ABI.
Linux kernel	5.15+	IOMMU enabled (`intel_iommu=on iommu=pt`).
GCC / Clang	C++17 capable	Standard Ubuntu 22.04 toolchain.
CMake	3.18+
xFAPI source tree	`coranlabs/xfapi`, branch `main`	Provides `libxsm.so` and `xsm/xsm.h` that OCUDU links against.
OCUDU build flags	`ENABLE_DPDK=True`, `ENABLE_XSM_FAPI_SPLIT=ON` (default ON)	See Section 5.

1.3 Kernel and hugepage setup

# Kernel boot parameters (then update-grub + reboot):
intel_iommu=on iommu=pt default_hugepagesz=1G hugepagesz=1G hugepages=4

# Mount hugetlbfs if not done by distro:
sudo mkdir -p /mnt/huge
sudo mount -t hugetlbfs -o pagesize=1G none /mnt/huge

# Confirm 1 GiB pages are available:
grep Huge /proc/meminfo

# Load VFIO modules (needed for any DPDK-bound NIC):
sudo modprobe vfio-pci