We use our mobile phones every day, and we can connect to the cellular network for phone calls, text messages and mobile data no matter where we go. While the network coverage and quality depend on which mobile operator we are using, the connectivity is available irrespective of our location and mobility. This is because our mobile network operators connect us to their cell towers that can be found throughout towns and cities for nationwide network coverage. So let’s find out what these cell towers are, where they can be found and how they connect us to the mobile network.
A mobile network consists of a large number of cells
A mobile cellular network consists of a large number of interconnected coverage zones called cells that are deployed throughout the geographical areas that a mobile network operator wants to cover. These cells are created by the radiations from the cell towers owned by the mobile operators.
Our cellular service providers employ various 2G, 3G, 4G and 5G technologies such as GSM, UMTS, CDMA2000, LTE, and NR built into the radio units within the cell towers. In technical terms, the cell towers are called radio base stations or base stations. The radio units in the base station emit the mobile signals (radio waves) at various frequencies that our mobile phones and other SIM-enabled devices can pick up to get us connected.
Cell towers are tall masts carrying radio antennas
Cell towers or radio base stations are the tall masts carrying cellular antennas that you can spot from a distance. A cellular tower can have many antennas installed on it, and the same tower may be used for 2G, 3G, 4G and 5G cells depending on the coverage of a given mobile operator.
The radio base station is installed at a mobile operator’s site, also known as a “cell site”. The electricity powers it, and this power determines how far the radio signal can travel. The higher the power, the longer the signal can travel. These signals are sent at specific frequencies to don’t interfere with signals coming from other base stations.
Only the mobile phone that the signal is intended for can decode that signal based on the digital techniques used by mobile operators. The frequency also determines how far the signal can travel; lower frequencies travel much further than higher frequencies. The picture below shows what a cell tower (base station) looks like.
Cell site means the location where a cell tower is installed
A cell site is a location or “site” where a mobile network operator installs a 2G, 3G, 4G or 5G radio base station (cell tower). Mobile operators own or rent many cell sites within a country to place their base stations in order to provide nationwide cellular coverage to their customers.
One cell site may contain the necessary radio equipment for multiple cells created by the same amount of radio units. The most prominent indication of a cell site is a cellular tower equipped with the antennas and necessary equipment to transmit radio signals to mobile phones and receive signals back from the mobile phones.
If you are on a phone call and start moving away from the cell range within a cell site serving you, your call can be handed over to the nearest available cell. That way, the mobile network makes sure that you stay connected when on the move so that your calls (or any other services) can continue without interruptions.
A cell is a network coverage zone created by radio waves
A cell is a network coverage area created by transmitting and receiving signals from the antennas of a radio base station. The cells are defined by the range (in kilometres) within which the base station can transmit and receive the mobile signals.
The cells are generally not omnidirectional but always in a specific targeted direction to avoid any potential interference with other nearby cells. However, for cell planning purposes and technical documentation, cells are represented as small hexagons, as shown below. The size of a cell is determined by its radiation power.
Generally, the larger cells are deployed on highways and rural areas, whereas smaller cells are deployed in busy towns and cities. Let’s now dive into the cell towers, but if you want to learn more about cells, you can check out my detailed post on the concept of cells in mobile communications and the various types, including macrocells, microcells and small cells.
Cell site consists of radio units and other radio equipment
The main installation at a cell site consists of a cabinet that contains radio units and other radio equipment connected through a backhaul to the radio network controller or mobile core network, depending on which network technology (3G, 4G, 5G etc.) is being used.
The radio units are assigned specific frequencies that mobile operators own as part of their frequency spectrum. The antennas for the radio units must be mounted at specific heights so that the resulting signals can reach the cellular devices with as few obstructions as possible.
These radio network entities are called base stations, and depending on the network technology, they can be called different things. In 2G GSM networks, the base station is called Base Transceiver Station. The base station is called Node B in UMTS networks, eNodeB in LTE networks, and gNodeB in 5G networks.
Ownership of cell sites and base stations
The cell sites and base stations are owned by mobile network operators such as Vodafone, T-Mobile, Rogers, AT&T, Verizon etc. The base stations represent the radio part of the mobile network, and one base station typically contains multiple cells which operate on specific radio frequencies.
The radio network is what connects a mobile phone to the mobile network. When you are using your mobile phone to make a call, it is the radio network that your phone communicates with first. The radio network then connects your phone to the other parts of the mobile network and/or any other external networks. Let’s have a look at the simplified network diagram below to visualise this concept.
As we may observe from the simplified diagram above, the mobile core network is a central part of the mobile network that allows mobile subscribers to use all the services they are entitled to. In addition, it provides vital functions such as subscriber profile information, subscriber location, authentication of services and the necessary switching functions for voice and data sessions. Both the radio and core networks have evolved considerably with the evolution of mobile networks from 2G to 3G, 3G to 4G, and then 4G to 5G.
Types of cellular networks – Analogue and Digital
There have been two main types of mobile networks: Analogue and digital. The analogue networks represented the first generation (1G); however, they are now obsolete. The 2G, 3G, 4G and 5G networks are digital cellular networks.
The first generation of mobile networks (1G) in the early 1980s used analogue technologies. These mobile networks were deployed in different parts of the world using various analogue standards. The most popular standards were Advanced Mobile Phone System (AMPS), Nordic Mobile Telephone (NMT), Radio Telephone Network C (C-Netz) and Total Access Communications System (TACS).
However, analogue communication systems are usually more sensitive to noise in the air interface, which deteriorates the clarity of the speech. They also lack encryption capabilities, making them more vulnerable to security threats.
Digital networks followed analogue networks in the early 1990s, which marked the beginning of the type of mobile networks we see today. Digital systems are much more resilient to noise and offer much higher security levels than their analogue counterpart. The second generation of mobile networks employed various digital standards.
The most popular standards are Global System for Mobile Communications (GSM), Digital Advanced Mobile Phone System (D-AMPS), and Interim Standard 95 (IS-95, proprietary name: cdmaOne). The third, fourth and fifth generations of the mobile networks (3G, 4G and 5G) are also digital.
Types of cell towers in 2G, 3G, 4G and 5G networks
While all generations of mobile networks have cell towers or base stations, there are different terminologies to refer to a base station for each of the generations. Let’s find out what base stations are called in 2G GSM (Global System for Mobile Communications), 3G UMTS (Universal Mobile Telecommunication System), 4G LTE (Long Term Evolution) and 5G NR (New Radio) networks.
The cell towers or base stations are called Base Transceiver Station or BTS in 2G GSM networks, Node B in 3G UMTS networks, eNodeB in 4G LTE networks and gNodeB or ng-eNodeB in 5G NR networks.
Cell towers or base stations in 2G GSM networks
In the second generation of mobile networks powered by GSM technology, the base stations are called Base Transceiver Stations or BTS for short. The word transceiver refers to the fact that the base station is doing a dual job of being a transmitter and a receiver, hence the term trans + ceiver.
In GSM networks, mobile phones are referred to as Mobile Stations or MS or short. The same terminologies continue to be used for the base stations in all the GSM networks enhancements, including GPRS (General Packet Radio Service) and EDGE (Enhanced Data for Global Evolution). Now you might think that the GSM networks are so old (originally launched in 1992), so when might you ever come across any of these BTS cell towers.
If your existing mobile operator is not using CDMA technologies (CDMA2000 and cdmaOne), they are highly likely to use GSM for the 2G services. It means that whenever you see an “E” or “G” symbol on your mobile phone screen, you are likely to be served by one of these BTS cell towers. If you are curious about these symbols on your phone, check out my dedicated post on the E symbol so you know what download speeds to expect.
Cell towers or base stations in 3G UMTS networks
In the third generation of mobile networks powered by UMTS technology, the base stations are called Node B. So Node B is just an equivalent of BTS but in 3G networks. The mobile phones in 3G networks are referred to as User Equipment or UE.
Node B can work with BTS to give customers a seamless user experience when accessing cellular services. This happens, for example, when you initiate a phone call in a rural area with only GSM coverage and during the call move into an area with UMTS coverage. As you move from the GSM coverage to UMTS coverage, the call is handed over from BTS to Node B.
If you ever notice the H or H+ symbol on your mobile phone, that is when you are served by a Node B. H or H+ symbols refer to High-Speed Packet Access (HSPA) which is an enhancement to the 3G UMTS network. I have a dedicated post on HSPA if you want to know what to expect from this enhancement.
Cell towers or base stations in 4G LTE networks
In the fourth generation of mobile networks that are enabled by the LTE (Long Term Evolution) technology, the base stations are called Evolved Node B or eNodeB or eNB. When you see a 4G, 4G+, LTE or LTE+ symbol on your mobile phone screen, you are being served by an eNodeB.
Unlike GSM and UMTS networks, in 4G LTE, the base station eNodeB has more intelligence and responsibilities as part of the mobile radio network. In 4G networks, there are fewer nodes in the radio network than in 2G and 3G networks and, therefore, many mobility and control responsibilities. Since 4G LTE networks are backwards compatible, like 3G UMTS, eNodeB can work with Node B (3G) and BTS (2G) for calls or data sessions handovers.
Cell towers or base stations in 5G NR networks
The fifth generation of mobile networks is enabled by a technology called New Radio or NR. However, unlike the earlier technologies, 5G has the flexibility to be deployed in multiple ways to improve the data rates for customers. Multiple network architecture scenarios use different combinations of 5G radio, 5G core, 4G radio and 4G core networks.
There are two types of base stations in 5G NR networks: gNodeB (gNB) and Next-Generation Evolved Node B (ng-eNB). gNodeB is a base station that connects 5G phones when 5G radio and 5G core networks are used. ng-eNB is the base station that connects 4G phones when 4G radio and 5G core are used.
I have written a dedicated post on Node B, eNodeB, gNodeB and ng-eNB that dives into the technical details so you can find out, for example, what scenarios require gNodeB and ng-eNB? There is also a dedicated post on 5G New Radio (NR) that explains what 5G technology is and how it works.
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.