Look! There are "white mushrooms" on the ceiling of the subway station...

Have you ever noticed white objects that look like mushrooms on the ceilings of public places such as subway stations and shopping malls? What are they and what are their functions?

"White mushrooms" on the subway station ceiling

Source: Photographed by editor

01. What is the white mushroom-shaped device?

These devices are usually wireless access points (WAPs), which allow wireless devices (such as mobile phones, laptops, tablets, etc.) to connect to wired networks via wireless signals. We can think of it as a wireless signal "transmission tower" that can "broadcast" the Internet signal so that all wireless devices around can receive it and connect to the Internet.

Mushroom-shaped ceiling antenna Image source: ycantenna.com

To understand its function, we might as well first understand how the mobile phones we use every day achieve the Internet function?

02. How do mobile phones access the Internet?

When it comes to mobile Internet access, the two most common ways are cellular data networks and wireless local area networks. Cellular data networks are wireless communication networks that allow mobile devices to communicate with the Internet via wireless signals. The base station coverage area of ​​a cellular network is theoretically arranged in a hexagonal shape, similar to the hexagonal structure of a beehive, hence the name "honeycomb".

The 4G and 5G we are familiar with are a type of cellular mobile network. The mobile phone has a built-in cellular modem, which will actively search for the nearest cellular base station (Cell Tower), and then establish a connection with the cellular network of the network operator through the SIM card (the "identity certificate" for the mobile phone to access the cellular network). When we browse the web, download files or upload photos on our mobile phone, the mobile phone will package the data and send it to the base station through the cellular network. The base station is like a "transit station" that transfers data to the operator's core network (Core Network), and the core network is like a "super express hub" that quickly and accurately delivers data to every corner of the Internet, allowing us to easily complete various operations such as data access, downloading and uploading.

Image source: Photo Network

Another way to access the Internet with a mobile phone is a wireless local area network, which is commonly known as Wi-Fi. The built-in Wi-Fi module in the mobile phone will scan the surrounding area for Wi-Fi signals like a radar. When you choose to connect to a Wi-Fi network, you usually need to enter the network password. After the connection is successful, the wireless access point (such as a router) will start an "identity distributor" - DHCP (Dynamic Host Configuration Protocol), and issue a unique "ID card" to your mobile phone - a local IP address. With this "ID card", you can ensure that the communication in the network can be sent to your mobile phone correctly. When the mobile phone is successfully connected to the wireless access point, you can access the Internet through it. The wireless access point will send the mobile phone's data packets through a wired network (such as fiber optic, DSL, etc.) to the Internet Service Provider (ISP), and the ISP will then send the data packets to every corner of the Internet. At the same time, it will also send the data packets received from the Internet back to the mobile phone. All data enters and exits through this wireless access point, just like a "super portal" between the mobile phone and the Internet.

Image source: Photo Network

03. Why does the mobile network get stuck when taking the elevator?

Everyone has more or less experienced the moment when the mobile phone signal "drops", such as the sudden disconnection when taking the elevator, or the signal being strong or weak on the subway or high-speed rail. What affects the mobile phone signal? The first factor is the distance from the wireless access point or base station . Mobile phone signals are essentially electromagnetic waves. When electromagnetic waves propagate in the air, just like running a marathon, the farther you run, the more tired you are, and the signal strength will gradually decay as the distance increases. We have an intuitive feeling about this. For example, when we are at home, the closer the mobile phone is to the router, the better the signal, and the farther it is, the weaker it is.

Image source: sina.com

The second factor is the shielding effect . The most typical example is that there is no signal in the elevator. This is because the elevator car is usually made of metal materials, which will reflect and absorb radio waves, resulting in a significant reduction in signal strength. Similarly, the walls and floors of high-rise buildings are mostly reinforced concrete, which will also "set up barriers" for signal propagation, making it difficult for the signal to penetrate. The third factor is base station switching , which often occurs in fast-moving scenarios such as taking the subway or high-speed rail. The network signal will be strong and weak at times. During the fast-moving process, the mobile phone is like an athlete participating in a relay race, running from the coverage of one base station to another. The mobile phone will automatically switch base stations so that we are always connected to the strongest and most stable signal source. Although the entire switching process is very fast and smooth, if the movement speed is too fast (such as high-speed rail) and the switching frequency is too high, due to the millisecond-level interruption when the base station is switched, it is easy to cause signal dropout or delay.

Image source: Photo Network

That’s why there are always many mushroom-shaped wireless access points on the ceilings of public places. These wireless access points and base stations are part of the modern urban communication infrastructure, ensuring that we can still maintain efficient and convenient communication and Internet access experience in various complex environments. With the continuous advancement of technology and the promotion and application of new technologies such as 5G networks, future network connections will be faster and more stable, bringing us a richer digital life experience. Next time you go out, you might as well count how many "white mushrooms" you can encounter on the road~

Image source: Game "Super Mario"

Author: Yin Guangshui, Master of Communication Systems, EPFL

Planning & Editing: Rain

Acknowledgements: Xiaoping Liu, a lecturer at the School of Data Science and Engineering, East China Normal University, provided scientific guidance for this article