Many of us take the subway every day, automatically switching to mobile data or searching for free Wi-Fi. It seems magical: you're diving deep underground, surrounded by concrete and steel, yet your connection to the outside world remains uninterrupted. However, behind this illusion of stability lies a complex engineering system that requires constant maintenance and specialized technical solutions.
Unlike terrestrial use, where the signal is transmitted from base stations located on the roofs of buildings, underground infrastructure has its limitations. Radio waves They don't penetrate well through thick soil or the massive structures of tunnels. This is why providers are forced to lay kilometers of fiber optic cable and install hundreds of access points directly in tunnels and at stations.
Connection speed and stability depend not only on the number of connected users but also on the physical properties of the signal propagation environment. In this article, we'll take a detailed look at how the subway network is organized, why your smartphone might lose connection while driving, and how city Wi-Fi differs from operator-provided traffic in the subway.
Principles of radio wave propagation in tunnels
The subway tunnel is ideal waveguide for radio signals. Due to the tunnel's cylindrical shape, radio waves of certain frequencies can propagate over long distances, repeatedly reflecting off walls, ceilings, and floors. This phenomenon allows for signal coverage over long distances without the need to install equipment every few meters.
However, not all frequencies perform equally well. The standard 2.4 GHz and 5 GHz frequencies used in consumer routers have different penetration capabilities and ranges. In metro environments, engineers must take this into account. signal attenuation and interference. If access points were too far apart, dead zones would form in the middle of the route.
⚠️ Please note: The waveguide effect only works with certain tunnel geometry. At stations with columns and complex architecture, the signal dissipates, requiring the installation of additional antennas.
Furthermore, train movement makes its own adjustments. The metal body of the train car shields the signal, creating a Faraday cage effect. Therefore, to ensure coverage inside the cars, antennas are often placed directly in the tunnel, hoping that some of the signal will penetrate through the windows, or special antennas are used. lying antennas, laid along the tracks.
Why is the connection better in old tunnels?
In old, shallow tunnels (like those on the Moscow Ring Line), the signal propagates more easily due to the thinner ground above and the proximity to the surface where the operators' base stations are located.
Underground access network architecture
The internet access system in the metro is not just a collection of routers scattered across platforms. It is complex. fiber optic network, which unites all access points into a single infrastructure. The cable is laid in special trays along the tracks and connects station equipment to the central communications hub.
At each station or at certain intervals in the tunnel, cabinets with active equipment are installed. From them, the signal is transmitted to Access Points, which can be directed in different directions to cover platforms and routes. MIMO technology is often used, allowing for the transmission of multiple data streams simultaneously.
- 📡 Access points are installed on the tunnel vaults or special masts on platforms.
- 🔌 Power supply Equipment is often backed up because power outages in the metro are critical for security systems.
- 🛡️ Moisture protection and dust is essential: the equipment in the tunnels has a high protection class of IP65 and above.
It's important to understand that the channel's bandwidth is limited by the width of the fiber-optic channel reaching the station. During peak hours, when thousands of passengers are simultaneously trying to download videos or update social media feeds, this can lead to congestion (channel congestion), which results in a drop in speed for all users.
| Parameter | Station (platform) | Tunnel | Train carriage |
|---|---|---|---|
| Type of coating | Open, many points | Waveguide, directional antennas | Shielded, signal through windows |
| Interference | High (many devices) | Average | Low (inside the carriage) |
| Speed | High (at low load) | Unstable | Depends on the position of the windows |
Handover Technology: Why the Connection Is Dropped
One of the main problems with using Wi-Fi in the metro is the connection dropping out while the train is moving. This is due to a process technically called Handover. When you move from one access point to another, your smartphone must break the connection with the first and quickly re-establish it with the second.
Under ideal conditions, this process takes a fraction of a second and is unnoticeable to the user. However, in metro conditions, where access points can be hundreds of meters apart and trains travel at high speeds, the switching algorithms don't always have time to work correctly. The device may cling to a fading signal until the very end, instead of switching to a stronger one.
The situation is complicated by the fact that different access points may operate on different channels or have different loads. Roaming protocols (802.11r/k/v) are designed to speed up this process, but not all smartphones and provider equipment support them. This is why you may experience sudden speed spikes or a complete loss of network service mid-hop.
⚠️ Note: Some antivirus programs and smartphone power-saving settings may aggressively disable Wi-Fi when the signal is weak, considering the network unsafe or unnecessary, which prevents normal handover.
Differences between free Wi-Fi and mobile internet
Users often confuse free Wi-Fi provided by the metro or its partners with mobile internet (4G/LTE/5G) distributed through cellular operator base stations. These are two fundamentally different technologies with different architectures. Mobile internet in the metro operates through a system DAS (Distributed Antenna System).
In a DAS system, the signal from the operator's base station is transmitted via fiber optics to multiple remote antenna modules scattered throughout the tunnel. To your phone, it appears as one giant base station covering the entire tunnel. Therefore, switching between "cells" in the metro is often avoided while the train is moving, and the connection is more stable than Wi-Fi.
- 📶 Wi-Fi — is a local network that requires authorization (often via SMS) and has restrictions on traffic or session time.
- 📱 Mobile Internet — part of the operator's global network, operating via a SIM card, with voice priority and a stable IP address.
- 🔒 Security: Open Wi-Fi networks in the metro are less secure for transmitting confidential data than encrypted LTE channels.
In addition, the frequency resource of cellular operators is licensed and protected from interference, while the Wi-Fi range (especially 2.4 GHz) is oversaturated with signals from thousands of passenger devices, which creates additional radio noise.
Security issues and authorization
Using public Wi-Fi networks always carries risks. In the subway, where traffic passes through ISP gateways, the primary method of protecting user data is application-level encryption (HTTPS). However, the very fact of being on the same network with thousands of strangers requires caution.
The authorization process you see when logging in (entering your phone number, receiving an SMS) is necessary not only for legal user identification but also to create a unique session. This allows the provider to limit speed and connection time, and block suspicious activity.
It is worth remembering that in an open network, a type of attack is theoretically possible Man-in-the-MiddleIf you visit websites without a secure protocol, modern browsers mark such sites as "Not Secure" but don't block them completely.
⚠️ Warning: Never conduct financial transactions or enter passwords for important services while on an open Wi-Fi network without using a VPN, especially if the site does not use HTTPS.
☑️ Safety rules for public Wi-Fi
Development Prospects: 5G and New Standards
Technology does not stand still, and metro systems in major cities are gradually introducing solutions based on 5GFifth-generation communications promises not only high speed but also low latency, which is critical for video calls and online gaming. However, implementing 5G in the subway requires the installation of new frequency equipment, as the higher 5G frequencies have poorer penetration through obstacles.
Technology is also developing Wi-Fi 6 (802.11ax), which better handles a large number of connected devices. In a crowded train car, where every passenger is holding a smartphone, Wi-Fi 6 allows for more efficient distribution of data transmission time slots, reducing competition for airtime.
In the future, metro navigation systems are expected to integrate with Wi-Fi networks. Your smartphone will be able to pinpoint your location with meter-level accuracy using access point signals, allowing you to create door-to-door routes even underground, where GPS doesn't work.
Infrastructure development is proceeding in parallel with rolling stock upgrades. New trains are already being designed with improved screening and the ability to install signal repeaters inside the car, which will solve the problem of "blind spots" inside the cars.
Why is 5G in the metro so difficult?
High 5G frequencies (mmWave) have a very short range and poor obstacle avoidance. Covering a 5G tunnel would require antennas every 10-20 meters, which is both economically and technically challenging.
Why is there no GPS reception in the metro, but the internet works?
GPS satellites are in space, and their signal is too weak to penetrate the thick soil and concrete above the metro station. The internet works because the signal source (access points and base stations) is located inside the tunnel, right next to you.
Does the number of passengers affect Wi-Fi speed?
Yes, directly. A communication channel has limited bandwidth. If 500 people start watching a 4K video at the same time, the channel will simply become overwhelmed, and everyone's speed will drop. This is called channel congestion.
Is it possible to improve Wi-Fi reception in the metro on a phone?
Absolutely not, since the phone's antenna is fixed. However, you can try disabling "smart network switching" in the settings to prevent the phone from frantically searching for the best network. Manually switching between 2.4 and 5 GHz frequencies also helps, if the phone allows it.
Is it safe to pay by card on the metro via Wi-Fi?
Payment systems (NFC, Apple Pay, Google Pay) use secure communication channels between banks and payment systems, often via mobile internet or an encrypted token. Directly using open Wi-Fi to enter card details in a browser is less secure.