What is O-RAN?

The terms Open RAN and OpenRAN are often used interchangeably, but can have different meanings depending upon the context¹:

• Open RAN (with a space): Refers to the general concept of an open Radio Access Network (RAN) architecture, which can be disaggregated into component parts, with openly defined interfaces between them.
• OpenRAN (without a space): Refers to the specific implementation of an “Open RAN” as defined by the O-RAN Alliance.

To avoid ambiguity, this note will use the term O-RAN to refer to the specific implementation of OpenRAN by the O-RAN Alliance.

RAN architecture overview §

To help explain O-RAN, first here is an overview of RAN architecture.

Terminology §

A typical mobile telecommunications network consists of:

• User Equipment (commonly UE, e.g. a mobile phone), which communiates with a…
• Radio Access Network (commonly RAN, e.g. cell tower equipment²), which communicates with the…
• Core Network (sometimes CN or just core), which may communicate with other networks, such as…
• The Internet, or the PSTN.

An example 4G/5G RAN architecture §

  Protocol   Component   Portion
  --------   ---------   -------

                CN
       IP:       |       }: Backhaul
                BBU
               /   \     }
     CPRI:    /     \    }
             RU    RU    }: Fronthaul
       RF:   / \   / \   }
            UE UE UE UE

UE <--RF--> RU <--CPRI--> BBU <--IP--> CN

For example, with a 4G/5G RAN architecture:

• A cell tower may have a one more more RUs (Remote Units) placed at the top of the tower.
• The UE communicates with the RUs via RF, and the RUs communicate with a BBU placed at the base of the cell tower via CPRI.
• The BBU communicates via IP to the core.

Alternatively:

• The BBUs may be physically co-located in a central location (e.g. data center), in which case the architecture may be referred to as C-RAN (Centralised RAN).
  • In contrast, that architecture above may be more specifically described as a D-RAN (Distributed RAN), since the BBUs are distributed instead of centralised.
• The BBU may be virtualised (e.g. run in the cloud), known as a vBBU.

An example 5G RAN architecture §

In 5G, the BBU can be decomposed into a CU (Centralized Unit) and DU (Distributed Unit).

  Protocol   Component   Portion
  --------   ---------   -------

                CN
       IP:       |       }: Backhaul
                CU
       IP:       |       }: Midhaul
                DU
               /  \      }
eCPRI/ROE:    /    \     }
             RU    RU    }: Fronthaul
       RF:   / \   / \   }
            UE UE UE UE

UE <--RF--> RU <--eCPRI/RoE--> DU <--IP--> CU <--IP--> CN

For example, with an 5G architecture:

• A cell tower may have one or more RUs (Remote Units) placed at the top of the tower.
• The UE communicates with the RUs via RF, and the RUs communicate with a DU (Distributed Unit) via eCPRI (enhanced CPRI) or RoE (Radio over Ethernet).³
• The DU communicates with a CU (Centralized Unit) via IP.
• The CU communicates with the core via IP.

Alternatively:

• The DU and CU may be virtualized (e.g. run in the cloud), known as a vDU or vDU.
• The terms S-RU, S-DU and S-CU may be used in the context of small cells.

Remember, this RAN architecture is not necessarily O-RAN. It is 5G that permits decomposing the BBU into a CU and DU. A single vendor can still provide a CU/DU/RU which use proprietary interfaces.

What is O-RAN? §

Historically, a RAN has consisted of big, monolithic pieces, which may or may not be compatible between different providers.

O-RAN is an open standard by the O-RAN Alliance which defines interfaces between the different components. This makes it possible to assemble a RAN with a mix of CU/DU/RUs from different providers, so long as they implement the O-RAN standard.

O-RAN according to the web §

If you type “What is O-RAN?” into Google, many explanations begin with wonderful prose:

[…] Open RAN is the final piece of the unbundling puzzle that enables mobile network operators to use equipment from multiple vendors and still ensure interoperability."

Nokia

[O-RAN is] a shift in the way to designing and deploying radio access network infrastructure […]

Ericsson

Open Radio Access Networks (RAN) is a new approach to building the mobile networks […]

Vodafone

Some explanations are a bit more grounded:

An Open Radio Access Network (ORAN) is a nonproprietary version of [RAN] that allows interoperation between cellular network equipment provided by different vendors."

Cisco

[O-RAN] is a set of industry-wide standards designed to support equipment interoperability from multiple vendors.

Arm

[O-RAN] is a network infrastructure that enables greater choice and flexibility in telecoms supply chains."

Ofcom

If you make it to page 3 of Google, there is a fairly good article by Canonical Ubuntu, which explains O-RAN in just 20.5⁴ fairly simple sentences. It briefly covers the evolution from D-RAN, to C-RAN, vRAN and finally O-RAN.

Technical definition of O-RAN §

• O-RAN is a RAN architecture as specified by the O-RAN alliance.
  • Many companies are members of the O-RAN alliance.
  • The O-RAN specifications are freely available here on the O-RAN alliance website.
    • As a starting point, the overall architecture is documented in O-RAN.WG1.OAD-R003 “O-RAN Architecture Description”.
      • See section 5.1 for a high level architecture diagram.⁵