# O-RAN Architecture and Functional Split

:::info
**Resources:**
- https://docs.o-ran-sc.org/en/latest/architecture/architecture.html
- [5G O-RAN functional architecture and the protocol split](https://www.linkedin.com/posts/rahim-navaei_5g-4g-5gnr-activity-7023204657342636032-L0uY/?utm_source=share&utm_medium=member_ios)
- [William's notes](https://hackmd.io/gV7kLjbDSIu7wppoYV_N2A?view)
- [Ferlinda's notes](https://hackmd.io/otMWRbwcS-OwMBP5I2WGew?view)
- [O-RAN Architecture](https://www.docomo.ne.jp/english/binary/pdf/corporate/technology/rd/technical_journal/bn/vol21_1/vol21_1_006en.pdf)
:::

## O-RAN Architecture


The **radio side** includes:
- Near-RT RIC
- O-CU-CP
- O-CU-UP
- O-DU
- O-RU

The **management side** includes:
- Service management and Orchestration Framework that contains a Non-RT RIC function

**near-RT RIC**:
* < 1s
* a logical function that enables near-RT control and optimization of O-RAN elements and resources via fine-grained data collecton and actions over E2 interface
* provides Radio Resource Management (RRM) functions with embedded intelligence, QoS management, and seamless handover control

**non-RT RIC**:
* more than 1s
* a logical function that enables non-real-time control and optimization of RAN elements and resources, AI/ML workflow including model training and updates, and policy-based guidance of applications/features in near-RT RIC

**NMS**: Network Management System

**O-CU**: a logical node hosting RRC, SDAP and PDCP protocols

**O-CU-CP**: a logical node hosting the RRC and the control plane part of the PDCP protocol

**O-CU-UP**: a logical node hosting the user plane part of the PDCP protocol and the SDAP protocol

**O-DU**: a logical node hosting RLC/MAC/High-PHY layers based on a lower layer functional split.

**O-RU**: a logical node hosting Low-PHY layer and RF processing based on a lower layer functional split

> O-DU and O-RU functions consist of real-time Layer 2 functions and a function group performing baseband signal processing and radio signal processing. Open fronthaul interfaces are newly specified between O-DU and O-RU.

**O1**: Interface between management entities in Service Management and Orchestration Framework and O-RAN managed elements, for operation and management, by which FCAPS management, Software management, File management shall be achieved

**O1***: Interface between Service Management and Orchestration Framework and Infrastructure Management Framework supporting O-RAN virtual network functions

**xAPP**: Independent software plug-in to the Near-RT RIC platform to provide functional extensibility to the RAN by third parties

## Functional Splits

There are 8 splits in general:
* Option 1 (RRC/PCDP 1A-like split)
* Option 2 (PDCP/RLC Split 3C-like split)
* Option 3 (High RLC/Low RLC split, Intra RLC split)
* Option 4 (RLC-MAC split)
* Option 5 (Intra MAC split)
* Option 6 (MAC-PHY split)
* Option 7 (Intra PHY split)
* Option 8 (PHY-RF split)

### **Notes**
- To **pass the user data** received from the user plane function (UPF) within the 5G core from the gNB higher layer to the gNB RF, there are 8 splitting options (**Option 1 to Option 8**)
- 3GPP considers the protocol split of the higher layer (**Option 2**) a functional split between the gNB CU and gNB DU based upon the F1 reference point
- *F1: Reference point between RAN DU and RAN CU. It consists of F1‐C and F1‐U to exchange control and user plane traffic respectively. F1 is based on F1AP (F1 Application Part).*
- O-RAN Alliance adopted both higher layer protocol split (**Option 2**) and lower layer protocol split (**Option 7-2x**), so the functions of the gNB are divided into three functional entities (**O-CU, O-DU, and O-RU**). The **higher layer split** is based upon the **F1 reference point** and the **lower layer split** is based upon the **Fronthaul-LLS** (between the O-DU and O-RU)
- The eCPRI (Enhance Common Public Radio Interface) or RoE (Radio over Ethernet) are used as transport mechanism to encapsulate the Control Plane and User Plane between the O-DU and O-RU.
- T*he CPRI is an interface that sends data from remote radio units to base station units. Enhanced CPRI (eCPRI) is a way of splitting up the baseband functions to reduce traffic strain on the fiber.*
- These functional split options depend on network services, available transport links radio network deployment scenarios, constraints, etc. The comparison between these functional splitting options can be seen in [this document](https://www.hubersuhner.com/en/documents-repository/technologies/pdf/fiber-optics-documents/5g-fundamentals-functional-split-overview)

## Option for O-RAN

O-RAN’s functional split option begins at 7.2x, called Split Option 7-2x.

Split Option 7-2x is a specification for functional splitting between O-DU and O-RU adopted by O-RAN fronthaul specifications.