What Is the Radio Access Network (RAN)?

A radio access network (RAN) is a type of network infrastructure used commonly for mobile networks that consist of radio base stations with large antennas. A RAN wirelessly connects user equipment to a core network.

In a RAN, the radio unit (RU) processes digital radio signals and transmits, receives, and converts the signals for the RAN base station. When the RU receives signal information from the antennas, it communicates with the baseband unit (BBU) using the Common Public Radio Interface (CPRI). The BBU takes the signal information and processes it so it can be forwarded to the core network. Data returns to the user via the reverse process.

A traditional RAN architecture where the radio unit, also called remote radio head (RRH), receives information from user equipment (UE) and sends it to the BBU via the CPRI for processing and transmission to the core network. Source: “Flying to the Clouds: The Evolution of the 5G Radio Access Networks” via Springer

The amount of area a RAN node can cover varies depending on the capabilities of the antennas, RAN hardware, and software at the node.

RANs are a key aspect of mobile networks. Network operators have used RANs since the days of 1G and they are still used in current 5G networks.

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Network Infrastructure

The basic structure of a RAN base station consists of a BBU, radio unit RU or remote radio unit (RRU), antennas, and various software-based interfaces. In 5G RANs, including 5G cloud-based RANs, the BBU is broken up into the distributed unit (DU) and the central unit (CU).

Opting for a DU and CU architecture can reduce some of the cost of deployments, allow for flexibility in the design of RAN infrastructure, and can be used in a cloud-RAN infrastructure.

The DU runs the radio link control and medium access control (MAC) layers in addition to some of the physical layer at a base station. It in turn is controlled by the CU.

The CU runs the radio resource control protocol, which conducts many functions, including information broadcasting, establishing and releasing connections between the user equipment and the RAN, and controlling the quality of service.

The CU also works with the packet data convergence protocol, which compresses and decompresses IP data stream headers and transfers user data, among other technical functions.

The CU can remain at the base station or it can be placed at a more central aggregation site. A DU, on the other hand, is kept at a base station that is not at an aggregation or core network location.

SDN principles can be applied to the CU. The control plane and user plane functions of the CU can be separated from each other, just as the planes are in an SDN.

The different types of RANs reflect advancements in networking technology. These types of RANs include GSM RAN (GRAN), GSM EDGE RAN (GERAN), UMTS RAN (UTRAN), and Evolved UTRAN (E-UTRAN).

Global System for Mobiles (GSM) RAN

The Global System for Mobiles (GSM) RAN uses base stations and controllers to transmit and manage radio links for circuit-switched and packet-switched core networks. More advanced RAN standards have been adopted over time as network operators deployed newer mobile networks.

GSM EDGE RAN (GERAN)

The EDGE in GERAN does not refer to edge computing, but instead is an acronym for Enhanced Data Rates for GSM Evolution. The standards body behind most RAN standards, 3GPP, provides examples of services GERANs can offer in its technical report TR 45.912 version 16.

The services for GERAN include improved upload and download connections to the internet, improved Voice over IP (VoIP) and cellular conversations, constant connection to the most appropriate base station, and higher bit rates in general.

Universal Mobile Telecommunications System (UMTS) RAN

The base stations in a Universal Mobile Telecommunications System (UMTS) RAN are referred to as Node Bs and Radio Network Controllers (RNCs). RNCs are base stations that are between the Node B and the network core. The UTRAN standard was designed and primarily used for 3G mobile networks.

A Node B is different from a GSM base station because a Node B uses a different type of air transport technology called Wideband Code Division Multiple Access (WCDMA). A GSM base station uses basic CDMA. Radio frequencies received at a Node B are turned into a data stream, which is forwarded to the RNCs to be sent to the network core. For the reverse, the Node B turns the data stream into radio frequencies to be transported to the user equipment.

Evolved Universal Mobile Telecommunications System Terrestrial RAN (E-UTRAN)

An Evolved Universal Mobile Telecommunications System Terrestrial RAN (E-UTRAN) is the RAN designed for 4G LTE networks. The air transport technology is unrelated to WCDMA and is in fact incompatible with it. E-UTRAN uses Orthogonal Frequency Division Multiple Access (OFDMA) for downlink connections and Single Carrier-Frequency Division Multiple Access (SC-FDMA) for uplink.

Features of E-UTRAN include peak data rates of 100 Mb/s downlink and 50 Mb/s uplink, reduced latency, scalable bandwidth, and support for connecting devices that are moving slowly or up to 500 kilometers per hour.

Cloud RAN (C-RAN)

Cloud RAN (C-RAN) is another variant on RAN technology. There are three primary components: a centralized BBU pool, RRU networks, and transport network.

The BBU pool, located at a central point, operates similarly to a cloud. It provides the RRUs with resources they need based on network needs, just as a cloud provides computers with resources remotely. The RRU network connects wireless devices to the internet like RAN towers do in a traditional RAN. The transport network uses optical fiber, cellular communication, or millimeter wave (mmWave) radio frequencies as its connection types. It is the high-bandwidth connection layer between the BBU pool and the RRUs.

What’s on the Horizon

The future of RANs will likely be influenced largely by two standards: 5G and open source RAN. 5G RAN, as of this writing, is still nascent and in the process of being deployed by various telecoms globally. What sets it apart from previous RANs is how it brings together capabilities of different RAN technologies — such as antennas, radio equipment, and BBUs — to reduce the distance and hops a signal makes. A key part of 5G RAN is virtualization, so it makes sense that it will use virtual RAN (vRAN) technology.

Open RAN is a type of open standard for RAN interfaces. It isn’t currently used widely in existing RAN deployments, though it is receiving a lot of attention. Many organizations are working on open RAN standards. These organizations include the Telecom Infra Project, the O-RAN Alliance, the O-RAN Software Community, the Small Cell Forum, and the Open RAN Policy Coalition.

There aren’t many 5G RAN deployments from brownfield network operators that use open RAN technology; however, greenfield operators like Rakuten Mobile are exploring an open standards approach for their mobile network RANs.

What Is the Radio Access Network: Key Takeaways

A Radio Access Network (RAN) consists of a baseband unit, radio unit or remote radio unit, antennas, and software interfaces.
Data from a user reaches the network core after being received by a radio unit and transformed into a digital format by a baseband unit.
Different generations of mobile networks use different variations of RANs.
The future of RANs will be heavily influenced by 5G deployments and the push for open source RANs.

Updated February 2021 by Connor Craven