What is Radio Access Network in telecom?

If you study mobile communications in an academic or professional capacity, you will surely come across terms like radio network, RNC, RAN, NodeB, eNodeB, base station, mast etc. Looking at mobile networks at a very high-level, we can conceptually break them down into at least four areas i.e. user device (e.g. smartphone), radio access network, core network, and other external networks (e.g. internet). While the radio access network may seem like only one of the network entities, it is arguably the most crucial one. In very simple terms, a radio access network is the entry point for your smartphone or other mobile devices to access the mobile network.

A radio access network, also known as radio network is one of the key entities within a mobile cellular network. It uses radio waves to enable transmission and reception of mobile signals between your device and the overall network. The radio network connects your device to other parts of the network including mobile core network and external networks such as PSTN or internet. Let’s have a look at a simplified diagram to visualise this concept.

The most visible part of the radio access network for the general public is the radio base station. These are the tall masts that we see quite frequently with cellular antennas and other communication links mounted on them. In 2G GSM and 3G UMTS networks, the radio networks can be seen as base stations installed nationwide that are connected to other network entities that control them. In 4G LTE as well as 5G, no separate controller entities are used because the base stations are capable of performing the controller functions on their own.

2G radio access networks

The BTS or Base Transceiver Stations can be seen as the high masts with cellular antennas mounted on them. BTS is responsible for managing all the radio communication between a mobile handset and the mobile network. The “coverage” or “signal” in a 2G GSM network is created by this key network entity.

BTS is controlled by another network entity called the Base Station Controller (BSC). Base Station Controller usually controls a number of Base Transceiver Stations (BTS). BSC has the intelligence to manage mobile radio resources, and it controls tasks such as handover and frequency allocation. The BSC is situated between Mobile Switching Centre (MSC) and BTS. If a BTS is facilitating a mobile call and the call quality starts to deteriorate due to decreasing signal strength, the BSC may intelligently assign the call to another BTS within its control with better signal strength. If the BSC cannot find a BTS with sufficient signal strength, then the MSC may assign the call to another BSC which in turn hands over the call to a BTS within its control to continue the call while ensuring appropriate service quality levels.

Both BTS and BSC are part of the Base Station Subsystem (BSS) in the GSM network, which then connects them to the mobile Core Network. It may be interesting to note that BSS is also short for another very important entity in mobile communications called Business Support Systems. More information on that in a separate article WHAT IS THE DIFFERENCE BETWEEN OSS AND BSS?

3G and 4G radio networks

Similar to BTS in GSM networks, Node B is the base station in UMTS (3G) networks. Node B communicates with the mobile handsets just like BTS does in a GSM network. Node B is controlled by another network entity known as Radio Network Controller (RNC). Both Node B and RNC are part of the UMTS Radio Access Network (UTRAN). As part of the UTRAN, RNC then connects the Node Bs to the mobile core network. Radio Network Controller has the responsibility to control a number of Node Bs. Radio resource management and mobility management are among the key tasks performed by an RNC. The RNC is situated between NodeB and the mobile core network. The RNC is connected to the circuit-switched MSC on the one hand and the packet-switched SGSN on the other hand. Have a look at the green boxes in the diagram below to visualise this concept.

The base station in LTE (4G) networks is called eNodeB which stands for evolved Node B and connects the mobile phones to the mobile network. In contrast to BTS and Node B, eNodeB has an additional role as the radio network controller in 4G LTE networks. In the GSM and UMTS networks, the radio control function is handled by separate controller entities BSC and the RNC respectively.

5G networks use a new technology called New Radio (NR) that employs specialised 5G base stations for the radio part. These NR compatible 5G base stations are called gNB. 5G also has other base stations that support LTE radio access.

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