The radio access network (RAN), also known as a radio network, is one of the most fundamental parts of the overall mobile cellular network. It creates a wireless connection between the SIM-enabled mobile cellular devices and the mobile network.
A Radio Access Network (RAN) is an essential part of the mobile network which uses radio (RF) waves to wirelessly connect cellular devices (e.g. phones) to the cell towers. The cell towers are linked to the mobile core network that connects them to external networks like PSTN, ISDN and the Internet.
In mobile communications, you often encounter terminologies like RNC, RAN, NodeB, eNodeB, gNodeB, radio base stations, etc. These terminologies belong to the radio access network (RAN), which we will cover in this post.
What does a Radio Access Network (RAN) do?
The radio access network (RAN) connects all cellular devices, including mobile phones, mobile broadband routers and other SIM-enabled devices, including tablets, smartwatches and IoT sensors and actuators, to the mobile network.
The most visible part of a radio network is the cell tower, technically known as a base station. The base station is represented by different terminologies in 2G, 3G, 4G and 5G networks. A mobile phone communicates with the base station via two radio links called uplink and downlink. The downlink is the transmission from the base station to the mobile phone, whereas the uplink is the transmission from the mobile phone to the base station.
In CDMA networks including IS-95 and CDMA2000, the downlink is called a forward link and the uplink is called the reverse link. Have a look at the diagram below that shows the basic concept of a radio access network.
While the basic concept of a radio access network is relatively straightforward, it is the intricacies of the underlying radio access technologies like FDMA, TDMA, narrowband CDMA, wideband CDMA, OFDMA, SC-FDMA that make the overall cellular experience so seamless.
A mobile network consists of a radio network, core network, user device, and external networks at a very high level. The radio base station is the most visible part of a radio access network for the general public. Base stations are generally called cell towers, and they are tall masts with cellular antennas and other communication links mounted on them.
At its simplest, the mobile core network is like a telephone exchange where all the switching of the calls and data sessions takes place. Finally, the mobile core network is connected to external networks like PSTN and ISDN for voice calls and the public internet and other data networks for data services.
The radio network is one of the most expensive investments for a mobile operator because base stations’ presence ensures cellular connectivity. If your mobile operator does not deploy the radio network properly, e.g. if they don’t have enough base stations or frequency spectrum, that can lead to service degradation due to lack of coverage and capacity.
The diagram below provides a simplified view of the network nodes that form part of the overall network architecture. Please note that the list below is simplified and does not include all the network nodes.
GRAN and GERAN– Radio Access Network for 2G GSM
The radio access network within the second-generation GSM (Global System for Mobile Communications) networks consists of a radio base station known as the Base Transceiver Station or BTS. The BTS is managed by another entity called the Base Station Controller or BSC.
The BTS or Base Transceiver Stations is responsible for managing all the radio communication between a mobile handset and the mobile network. This crucial network entity creates the “coverage” or “radio signal” in a 2G GSM network. BTS is controlled by another network entity called the Base Station Controller (BSC).
Base Station Controller usually controls several 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 important entity in mobile communications called Business Support Systems.
When GSM networks were initially introduced, they only had the circuit-switched capability that allowed them to perform voice calls the text messages (SMS) only. However, with the introduction of packet-switched capability through GPRS and EDGE, they were able to accommodate efficient mobile data services.
The original GSM radio network architecture that only facilitated circuit-switched voice calls and SMS is called GSM Radio Access Network (GRAN), whereas the evolved radio network architecture after the introduction of packet-switched EDGE is called GSM EDGE Radio Access Network (GERAN).
UTRAN – Radio Access Network for 3G UMTS
The network generation that follows GSM and EDGE is 3G UMTS, where UMTS stands for Universal Mobile Telecommunication System. In 3G UMTS networks, the base station is called Node B, which communicates with the mobile handsets just like BTS does in a GSM network. Node B is controlled by another network entity called the Radio Network Controller or RNC.
The Radio Access Network in 3G UMTS is called UMTS Terrestrial Radio Access Network (UTRAN), which consists of Node B and RNC. As part of UTRAN, RNC connects multiple Node Bs to the mobile core network. The RNC 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 for voice calls and text messaging (SMS) and to the packet-switched SGSN (Serving GPRS Support Node) for mobile data (internet) services. Have a look at the green boxes in the diagram below to visualise this concept.
E-UTRAN – Radio Access Network for 4G LTE
The fourth-generation LTE (Long Term Evolution) networks employ the OFDMA (Orthogonal Frequency Division Multiple Access) technology for radio access. The responsibilities for all radio communication in LTE sit with Evolved Node B or eNodeB, which is the base station in LTE networks. Unlike GSM and UMTS networks, where a combination of a base station and a controller is used for the radio access network functions, LTE uses eNodeB for all radio communication.
The LTE radio access network is called Evolved UMTS Terrestrial Radio Access Network or E-UTRAN. As part of E-UTRAN, eNodeB connects directly to the 4G core network components, MME (Mobility Management Entity) and Serving Gateway (S-GW).
The core network in 4G LTE is called Evolved Packet Core (EPC), to which MME and S-GW belong. The EPC is fully packet-switched, and it works alongside the IMS (IP Multimedia Subsystem) to enable IP based voice calls and SMS.
NG-RAN– Radio Access Network for 5G New Radio (NR)
The fifth generation of mobile networks is enabled by the New Radio technology, abbreviated as NR. In 5G NR, the radio access network is called Next Generation Radio Access Network (NG-RAN), and it consists of two kinds of base stations: gNodeB and ng-eNodeB (Next Generation Evolved Node B).
gNodeB is the radio base station that allows 5G cellular devices to connect to the 5G radio network, whereas ng-eNodeB is a special kind of 4G LTE base station that allows 4G LTE mobile devices to connect to the 4G radio network in 5G deployments where a 5G core network provides core capabilities for both LTE and NR radio networks. Like LTE, the radio access technology used by 5G NR is OFDMA (Orthogonal Frequency Division Multiple Access). 5G NR is also a packet-only network that enables all services, including voice calls and SMS over IP.
I have written a detailed post on the base stations used by 2G, 3G, 4G and 5G networks. If you want to understand how the responsibilities of gNodeB and ng-eNodeB differ in a 5G network deployment, you can read the last section of that post.
Here are some helpful downloads
Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:
Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.
Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.