What is O-RAN?

Before we get to O-RAN¹, let’s lay down some basics.

Or, if you want to skip to the end, jump to But what is O-RAN, really?.

Basics of Mobile Telecommunications §

In simple terms, a typical mobile telecommunications architecture consists of:

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

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

An example RAN architecture §

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

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

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

For example, with a RAN architecture, a cell tower may have a one more more RRHs (Remote Radio Heads) placed at the top of the tower.

The UE communicates with the RRHs via RF, and the RRHs 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.

What is O-RAN? §

In contrast, O-RAN is the idea of making the RAN more composable and compatible between different providers, which will enable telecommunications companies to assemble a RAN from various component parts. This also permits splitting the architecture up in more ways, so that the different protocol layers can be impemented where needed.

An example O-RAN architecture §

  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 O-RAN 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).²

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.

But what is O-RAN? §

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 20.5³ fairly simple sentences. It briefly covers the evolution from D-RAN, to C-RAN, vRAN and finally O-RAN.

But what is O-RAN, really? §

• 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.⁴