Entering the subway these days rarely involves automatically connecting your smartphone to the city's Wi-Fi network. Passengers take this for granted, but behind the stable video image or the fast loading of pages lies a complex engineering challenge.
Establishing communications in the confined space of a tunnel requires specific standards and equipment capable of operating at high train speeds. Unlike above-ground hotspots, the signal must penetrate through thick metal and concrete.
The system is built on hundreds of access points connected to a single network, allowing the user's device to switch between them without losing connection. This ensures uninterrupted communication throughout the entire route.
Network architecture and data transmission standards
The foundation of any modern metro system is a distributed architecture, with access points located on platforms, in walkways, and directly in tunnels. Data transmission is most often accomplished using the standard IEEE 802.11ac or newer Wi-Fi 6, operating in the 2.4 and 5 GHz frequency ranges. Choosing the 5 GHz band is critical, as it is less noisy and allows for high channel throughput.
Each access point is connected to switches, which are in turn linked via fiber optic lines to a central authorization server. This topology allows for centralized traffic management and load balancing across hundreds of devices. As a passenger moves through the station or boards a train, the process roaming, when the device seamlessly switches to the nearest antenna with the best signal.
To ensure stability, directional antennas are used in tunnels, forming a narrow signal beam along the tracks. This minimizes interference and allows data packets to be transmitted over long distances without loss of quality. It's important to note that the throughput of the entire system is limited not only by the channel width but also by the number of simultaneously connected users.
- 📡 Using the standard 802.11ac Wave 2 for maximum speed.
- 🔗 Backbone network based on fiber optics to minimize delays.
- 🔄 Seamless roaming between hundreds of access points.
- 🛡️ Isolation of client devices from each other for security.
Equipment in tunnels and stations
The placement of equipment in the metro is dictated by strict fire safety and electromagnetic compatibility requirements. All active network elements, such as access points and switches, are enclosed in sealed enclosures with a protection class of at least IP65This is necessary to protect against dust, moisture and vibration created by passing trains.
In tunnels, antennas are often hidden in special niches or mounted under the ceiling to prevent physical contact and vandalism. For communication between remote sections of the tunnel, radio relay lines or the installation of additional fiber optic lines. This allows for continuous coverage even in deep sections of the line.
High-density clusters of access points are installed at stations with the highest passenger traffic. They operate in reduced-power mode to cover a small area with a large number of users. This prevents overloading of a single point and distributes traffic evenly.
⚠️ Attention: Operation of equipment in tunnels requires special permits and is carried out only during technical windows (at night), when train traffic is stopped.
To power active equipment at remote points in a tunnel, technology is often used. PoE (Power over Ethernet)This allows data and electricity to be transmitted over a single cable, simplifying installation and reducing the risk of short circuits.
Authorization and billing system
Access to wireless networks in the metro is generally free, but requires mandatory user identification. This is a legal requirement aimed at ensuring digital security. Login occurs through an authorization portal, which opens automatically when attempting to access any resource.
The most common method is authorization by mobile phone number through SMS or a push notification from the mobile operator. The system sends a request to the operator's database, checks that the number matches the SIM card's IMSI, and then grants access. The entire process takes a few seconds.
Technologies are being introduced to simplify the procedure Pass-Tag or Wi-Fi Pass, allowing you to log in to the network without entering codes. In this case, the smartphone automatically transmits an encrypted token to the telecom operator, which verifies the subscriber's identity. Authorization via social media or email is also possible, although this method is less common.
| Login method | Speed | Security | Convenience |
|---|---|---|---|
| SMS code | Average | High | Low (needs to be entered) |
| PUSH notification | High | High | High |
| Social networks | High | Average | Average |
| Automatic (PassTag) | Instant | High | Maximum |
What happens to your login data?
When authorizing via SMS, the system transmits your number and temporary session ID to the mobile operator for verification. The operator confirms that the number is active and belongs to you, after which the metro gateway opens internet access. Your full browsing history is not transmitted to the mobile operator, but IP addresses are logged in accordance with legal requirements.
Traffic congestion and balancing issues
The main drawback to high-quality Wi-Fi in the metro is the sheer number of simultaneous connections. During rush hour, a single station can have thousands of users, each consuming bandwidth. A typical home hotspot wouldn't handle even a tenth of that load, instantly crashing.
To solve this problem, an intelligent balancing system is used. It dynamically distributes clients among available access points, switching the device to a less loaded antenna, even if its signal is slightly weaker. Technology is also used. QoS (Quality of Service), which prioritizes traffic. For example, voice calls or video streaming can be prioritized over background app refreshes.
Telecom operators and Wi-Fi providers often set speed limits per user or restrict the use of resource-intensive protocols (such as torrents). This ensures the network remains accessible to all passengers.
- 📉 Dynamic signal power change for cell control.
- 🚫 Blocking P2P traffic to relieve bandwidth congestion.
- ⚖️ Uniform distribution of users across frequency ranges.
- 📶 Real-time load monitoring.
☑️ Why Wi-Fi might not work
Connection security and encryption
Using open Wi-Fi networks always carries potential risks. Although the subway network is protected by corporate-level security, data transmission between your device and a public hotspot could theoretically be intercepted by an attacker using specialized equipment.
Modern encryption standards WPA2/WPA3 protect the communication channel itself, but do not guarantee data security if the site does not use the protocol HTTPSTherefore, it is highly recommended not to conduct financial transactions or enter passwords for important services while on a public network without using additional security measures.
For maximum safety, experts recommend using VPN connectionIt will create an encrypted tunnel between your device and a remote server, rendering intercepted data useless to hackers. This is especially useful when working with corporate email or banking apps.
⚠️ Attention: Do not connect to networks with similar names (for example, "Metro_Free" instead of the official "Moscow_Free"), which are created by scammers to steal data.
Network administrators constantly monitor traffic for attacks such as Man-in-the-Middle and block suspicious activity. However, responsibility for the security of personal data also lies with the user.
Development Prospects: Wi-Fi 6 and 5G
Technology does not stand still, and the subways of large cities are gradually switching to the standard Wi-Fi 6 (802.11ax)This standard was developed specifically for high-density connection scenarios. It enables more efficient use of airtime, reduces latency, and increases overall network throughput by 3-4 times compared to previous generations.
One of the key features of Wi-Fi 6 is the technology OFDMA, which allows a channel to be divided into subchannels and data to be transmitted to multiple devices simultaneously rather than sequentially. This is ideal for crowded telephone stations where hundreds of phones are trying to send small data packets.
Cellular communications are also developing in parallel. The introduction of stations 5G In subway tunnels, it creates an alternative or complement to Wi-Fi. In the future, deep integration of these technologies is possible, with the smartphone automatically selecting the best available channel (Wi-Fi or 5G) without user intervention, ensuring a completely uninterrupted connection.
Does the metal body of the carriage affect the signal?
Yes, the metal body of the train car acts as a Faraday cage, shielding external radio waves. This is why access points are installed inside the cars or use special antennas that transmit the signal through windows and doors. In older train models without internal repeaters, the signal can be significantly weaker.
Why does the speed drop when the train is moving?
As the train moves, the device constantly searches for the best access point. During handovers, a brief connection loss or speed drop may occur. Furthermore, rapidly changing signal reception conditions in the tunnel require constant readjustment of the modulation parameters.
Is it possible to watch 4K videos via Metro Wi-Fi?
Technically, this is possible if the channel bandwidth in a given area allows it. However, due to the large number of users, the system may limit the video bitrate or prioritize other types of traffic, so stable 4K streaming is not guaranteed.
Do you need internet to access the network?
Mobile internet access is required for initial authorization (receiving SMS or PUSH notifications). If there's no mobile network coverage, you won't be able to access Wi-Fi. After successful authorization, the session is saved for a certain period of time, and re-logging in within a coverage area may not require a mobile network.