# O-RAN

## Summary


Traditionally, mobile network operators relied on network equipment vendors for RAN hardware. Vendors are few in number and their equipment is specialized and expensive. With 4G/5G, small cell deployments are more common, which is driving up deployment and maintenance costs.

The real problem is that operators are at the mercy of vendors. Vendors implement proprietary protocols and technologies resulting in vendor lock-in. O-RAN changes this by enabling multi-vendor deployments and open interoperable interfaces. O-RAN also enables virtualization and AI-based optimization. 

O-RAN is a set of standards that specifies open RAN interfaces, nodes, profiles and services. The operator-led O-RAN Alliance oversees the development of O-RAN. It also facilitates interoperability testing. Operators can therefore buy O-RAN compliant hardware from any vendor. To speed up adoption and innovation, O-RAN Alliance sponsors open source development of RAN software.

## Discussion

### Who started O-RAN and how are they organized?

The main organization behind O-RAN standards is the **O-RAN Alliance**. This was started in February 2018 by five mobile network operators AT&T, China Mobile, Deutsche Telekom, NTT DOCOMO and Orange. In fact, the Alliance was a merger of xRAN Forum (led by NTT DOCOMO) and C-RAN Alliance (led by China Mobile).  

By mid-2020, O-RAN Alliance expanded to 24 global operators and 148 contributors. Contributors include chipset manufacturers, network equipment manufacturers, radio element manufacturers, software vendors, service providers, and academic institutions. Despite this diversity, the Alliance is led by operators. Operators are the only members who get to vote at the AGM. 

O-RAN Alliance defines RAN standards and supports members towards interoperability testing. In cooperation with the Linux Foundation, it also sponsors the **O-RAN Software Community (SC)** for open source development for the RAN. Thus, O-RAN Alliance defines open architecture and specifications that O-RAN SC uses to develop open source software for industry deployment. 

Ultimately, O-RAN Alliance's mission is, 

> To reshape the RAN industry towards more intelligent, open, virtualised and fully interoperable mobile networks.


### Could you clarify the terms Open RAN, OpenRAN,  O-RAN, vRAN and C-RAN?

**Open RAN** is a generic term that refers to open RAN architectures including open interfaces, virtualization, and use of AI. 

**OpenRAN** is a project initiated by the Telecom Infra Project (TIP). It's an attempt to realize the Open RAN concept. Its work covers 2G/3G/4G/5G. As inputs, OpenRAN uses 3GPP and O-RAN specifications. 

**O-RAN** can refer to the O-RAN Alliance or standards created by the Alliance. It complements 3GPP specifications by defining interface profiles, new open interfaces and new nodes. O-RAN addresses 5G RAN interfaces including the X2 interface between 4G and 5G base stations. 

**vRAN (Virtualized RAN)** makes the RAN software-defined and programmable. Whereas Open RAN focuses on openness, vRAN is really about moving functionality from hardware to software. vRAN started in 4G with proprietary interfaces, later extended to 2G/3G (OpenRAN) and 5G (OpenRAN or O-RAN). 

**C-RAN (Cloud RAN)** is vRAN built on cloud native technologies, such as microservices, containers and CI/CD. Confusingly, C-RAN is also sometimes used to mean Centralized RAN where processing is centralized in CU or DU, away from RU.  


### When 3GPP has already standardized RAN, why do we need O-RAN?

NG-RAN nodes are not designed as monoliths. They are disaggregated such that multiple Radio Units (RUs) connect to a Distributed Unit (DU) and multiple DUs connect to a Centralized Unit (CU). The RU-DU link is called **fronthaul**. The DU-CU link is called **midhaul**. The CU-Core link is called **backhaul**. 

3GPP has standardized many NG-RAN interfaces: NG (NR-RAN and 5GC), Xn (gNB and gNB/ng-eNB), X2 (gNB and eNB), F1 (gNB-DU and gNB-CU), W1 (ng-eNB-DU and ng-eNB-CU), and E1 (gNB-CU-CP and gNB-CU-UP). However, fronthaul interfaces have not been standardized, thus forcing operators to buy RU and DU hardware from the same vendor. Moreover, X2 was seen as an optional interface. Vendors often added proprietary techniques to this interface. Therefore, when expanding to 5G NR, a 4G/LTE operator with eNB equipment is forced to buy gNB equipment from the same vendor. 

O-RAN standardizes the fronthaul. It opens up new interfaces including A1/O1/O2/E2. It specifies aspects of RAN virtualization. It also standardizes F1/W1/E1/X2/Xn interfaces already covered by 3GPP but the intention is to complement 3GPP standards. For example, O-RAN specifies profiles. 


### What's the O-RAN architecture?

O-RAN reference architecture disaggregates the RAN into RU, DU and CU. Moreover, user plane (UP) and control plane (CP) are disaggregated. RRC, SDAP and PDCP are in CU. Given a lower layer functional split, RLC, MAC and higher-PHY are in DU; lower-PHY and RF are in RU. 

A key functional module is the RAN Intelligent Controller (RIC). This is decomposed into two parts based on real time (RT) capabilities: 

    + **RIC non-RT**: Execution exceeding 1s. Part of the Orchestration/NMS layer. Functions include service and policy management, RAN analytics and model training for RIC near-RT. Core algorithms are developed by operators.
    + **RIC near-RT**: Execution within 1s. Next generation RRM that uses AI/ML models trained and passed on from RIC non-RT. Functions include load balancing, RB management, interference detection/mitigation, QoS management, connectivity management and seamless handover control. A robust, secure and flexible platform for third-party applications.RIC near-RT uses the **Radio-Network Information Base (RNIB)**, which gets near-RT state of the radio network via E2 and commands from RIC non-RT via A1. RIC near-RT and Multi-RAT CU platform are often virtualized so that capacity can be distributed across multiple network elements. 


### Which are the interfaces relevant to O-RAN?

The NG interface connects NG-RAN to 5G Core. It's completely specified by 3GPP. Otherwise, O-RAN interfaces include: 

    + **Open Fronthaul**: DU-RU lower layer functional split. Consists of Control, User and Synchronization (CUS) plane and Management plane. Although Common Public Radio Interface (CPRI) is a standard commonly used at fronthaul, vendors often modified it in proprietary ways. O-RAN standardizes this interface.
    + **A1**: Carries network or UE-level information from eNB/gNB to RIC non-RT to optimize network and ensure SLAs.
    + **E2**: Interfaces RIC near-RT with CU/DU. Carries measurements from CU/DU and configuration commands to CU/DU.
    + **F1/W1/E1/X2/Xn**: Existing 3GPP interfaces but enhanced by O-RAN for multi-vendor interoperation. Interfaces to Multi-RAT CU platform. F1 is the CU-DU higher layer functional split.
    + **O1**: An Operations and Maintenance (OAM) interface. Functions include management of provisioning, fault supervision, performance assurance, tracing, files, heartbeat, and Physical Network Function (PNF) software.
    + **O2**: Service provider functionality resides in Service Management and Orchestration (SMO) while cloud provider functionality resides in O-Cloud. This interface connects the two. It's decomposed into cloud infrastructure management and deployments on the infrastructure.

### What are O-RAN Workgroups and Focus Groups?

The work of O-RAN is organized into workgroups. These include **WG1**: Use Cases and Overall Architecture, **WG2**: The Non-real-time RAN Intelligent Controller and A1 Interface, **WG3**: The Near-real-time RIC and E2 Interface, **WG4**: The Open Fronthaul Interfaces, **WG5**: The Open F1/W1/E1/X2/Xn Interface, **WG6**: The Cloudification and Orchestration, **WG7**: The White-box Hardware Workgroup, **WG8**: Stack Reference Design, and **WG9**: Open X-haul Transport. 

In addition, there are a few focus groups: 

    + **Standard Development Focus Group (SDFG)**: Strategizes standardization effort. Coordinates and liaises with other standard organizations.
    + **Test & Integration Focus Group (TIGF)**: Defines test and integration specifications across workgroups.
    + **Open Source Focus Group (OSFG)**: Successfully established O-RAN SC. Otherwise, it's currently dormant.

### In O-RAN, what is meant by white-box hardware?

White-box implies general purpose vendor-neutral hardware, often called **Commercial Off-the-Shelf (COTS) hardware**. White-box hardware is common in IT. With O-RAN, it's coming to telecom. 

White-box hardware is expected to bring down deployment costs. It enables decoupling of hardware and software. It opens up the market to small players. Hardware vendors can make equipment conforming to O-RAN white-box specifications without worrying about the complexity of software. Operators and software vendors can focus on software with the foreknowledge of the hardware. 

O-RAN specifies "high performance, spectral and energy efficient white-box base station hardware". It publishes reference hardware designs and software architecture for both O-DU and O-CU, where the prefix "O-" signifies O-RAN specification. 

O-RAN has defined hardware requirements for different deployment scenarios (indoor pico, outdoor pico/micro/macro, IAB) and different architectures (split vs integrated). Performance requirements are specified in terms of peak data rate, peak spectral efficiency, bandwidth, latency, and mobility. Among the different split architectures, WG7 has considered options 6 (all PHY in O-RU), 7-2 (lower PHY in O-RU, higher PHY is O-DU) and 8 (all PHY in O-DU). 


### Could you share details on O-RAN slicing?

Network slicing is defined primarily by 3GPP but there are aspects studied by ONAP, ETSI and GSMA. O-RAN includes a slicing framework and architecture to realize 3GPP's network slicing. In particular, there's impact on RIC, O-CU, O-DU, and A1/E2/O1/O2 interfaces. 

A network slice has two parts, RAN slice and CN slice. O-RAN slicing is concerned with only RAN slices. Non-RT RIC should be aware of RAN slices, slice configuration and performance metrics. These become inputs to AI/ML models for slice assurance and optimization that's done over O1 interface. 

Dynamic slice optimization is done by Near-RT RIC to prevent SLA violations. Near-RT RIC collects slice performance metrics over E2 interface, applies its algorithms and optimizes slices over E2. 

3GPP defines Network Slice Management Function (NSMF) and Network Slice Subnet Management Function (NSSMF). For these functions, O-RAN provides multiple deployment options. They can be within or outside the SMO. 


### What are some challenges when it comes to adoption of O-RAN or Open RAN in general?

Even if operators introduce O-RAN compliant hardware for 4G/5G, their earlier investments in legacy equipment prevent them from reaping the benefits of a single virtualized and flexible RAN architecture. Lack of fibre or cost-efficient transport can be bottleneck for deployment of open fronthaul and midhaul. 

Some network vendors such as Ericsson and Huawei are holding out against O-RAN. Huawei hasn't joined the O-RAN Alliance. It believes that its integrated offering is more performant that can't be matched by disaggregated white-box hardware. However, Huawei supports the ONAP initiative for better interoperability. 

Detractors of O-RAN believe that disaggregated hardware can compromise overall network security. 

It's been said that the "main issue with disaggregation is aggregation". While disaggregated multi-vendor hardware is attractive, it's not trivial to integrate and achieve an end-to-end interoperable solution. Operators have the challenge of selecting the right hardware from multiple vendors and also build in-house integration expertise. On the other hand, there's a new opportunity for system integrators. 


### What are some useful resources to know more about O-RAN?

O-RAN specifications can be downloaded for free but requires acceptance of O-RAN ALLIANCE Adopter License Agreement. 

To follow the status of O-RAN open source implementation, visit the homepage of O-RAN SC. The site includes documentation of the latest release.

## Milestones

2016

The **Telecom Infra Project (TIP)** is founded to address the lack of innovation, high costs and a closed ecosystem ruling the telecom world. It's starts the **OpenRAN** initiative. OpenRAN is meant to spur innovation, enable supplier diversity and reduce costs. First OpenRAN trials start in 2017, in India and Latin America, particularly in rural areas where user base is low and Average Revenue Per User (ARPU) is also low. 

Feb  
2018

Operators AT&T, China Mobile, Deutsche Telekom, NTT DOCOMO and Orange come together and start the **O-RAN Alliance**. O-RAN Alliance creates standards, unlike TIP that doesn't create standards but promotes OpenRAN. 

2019

In an online article, Tamaskar of Radisys writes that 2019 may be the year for Open RAN. She notes the work of both O-RAN Alliance and Telecom Infra Project (TIP). We may see more collaboration between these two groups. Just as Self-Organizing Network (SON) brought automation to RAN management, planning and optimization, O-RAN will use AI to automate operations and reduce costs. 

Feb  
2019

Rakuten announces the world's first virtualized, cloud-native greenfield 4G network. However, it's not based on O-RAN standards. It's based on Nokia's X2 interface, that Nokia opened up for integration with another vendor. 

Jun  
2019

O-RAN WG5 publishes **O-RAN NR profiles for EN-DC (E-UTRA New Radio Dual Connectivity)** for both C-plane and U-plane. Initial 5G deployments are expected to be in *Non-Standalone (NSA)* mode, in which a UE will connect to LTE/eNB as master node and via Dual Connectivity connect to 5G/gNB as slave node. This is referred to as *EN-DC* or *option 3*. This deployment option was first standardized by 3GPP in Release 15 (early drop) in December 2017. 

Sep  
2019

NTT DOCOMO claims to be the world's first to deploy multi-vendor RAN conforming to O-RAN specifications. Its new pre-commercial 5G service is deployed with its existing 4G network. This is what we call 5G NSA network that makes use of X2 interface between 4G and 5G base stations. 

Nov  
2019

The O-RAN Software Community releases the first version (Amber Release) of its **open source software**. Since O-RAN specifications continue to be developed, this must be considered as pre-specification software with limited capabilities. Subsequent releases include Bronze Release (June 2020) and Cherry Release (December 2020). 

Feb  
2020

Mavenir founds the **Open RAN Policy Coalition (ORPC)**. This is a coalition to promote policies leading to open interoperable RAN solutions. It's not a standardization body. In May, Nokia joins ORPC. In January 2021, ORPC joins a coalition of coalitions that includes BSA, GSMA, CableLabs, and TIP. 

May  
2020

O-RAN Alliance and GSMA announce partnership to accelerate the adoption of Open RAN and thereby benefit from new virtualization architectures. This follows an earlier announcement in February when O-RAN Alliance partnered with TIP. 

Jul  
2020

Nokia and Samsung announce **OpenRAN products for 5G**. Meanwhile, Ericsson and Huawei make no similar announcements, though Ericsson is a contributor in O-RAN Alliance. 

Dec  
2020

The O-RAN Software Community makes the **Cherry Release** of O-RAN open source software. It's aligned with latest O-RAN specifications and brings the implementation closer to commercial deployments.