EDUCATION

How communication and Wi-Fi works in the subway

If ten years ago, free Wi-Fi in public places was unthinkable, but now it is becoming a common practice. Public Wi-Fi networks gradually appeared in cafes and shopping centers. But subway rides were still torture for hundreds of thousands of people who spend a lot of time on the road. There was a time when cellular communication was difficult to catch there, to say nothing of 3G and 4G. But progress cannot be stopped, and gradually their own Wi-Fi networks began to appear in the metro of various cities of the world, which today have become familiar to millions of people. But how does all this complex structure work? Let’s look at a domestic example built in Moscow.

A little history of MT_Free
Wi-fi in the Moscow Metro is the largest wireless Internet network in Europe. The project started back in 2014 and has been constantly modified and improved over a long period of time. What is the key feature of MT_Free, which distinguishes this network from foreign analogues? Most importantly, it is the first network of its kind, which for a long time had no analogues in the world. In other countries, there have been attempts to make access points in the subway, but only at stations. In Moscow, Internet access is possible right during a train ride.

How it works?
The signal is transmitted between the base stations installed in the tunnel and the train cars. The main difficulty that the creators had to face is that trains move inside curved concrete tunnels, which creates serious interference with wireless data transmission.

Dipole receiver antennas and controllers are installed on the head cars, smoothly switching between base stations in the tunnel. The controllers are interconnected by a radiating cable, which is pulled along the train. Wi-Fi routers located in each car are connected to this cable . In old cars they are installed at the end of the car, and in new cars they are located in the center, under the ceiling panel. With the highest pace of metro work, taking a train off work for the sake of repairing the Internet is an unaffordable luxury. Therefore, for reliability, a spare cable route is installed along the length of the train, which, if necessary, is used for quick repairs.

Creating a network inside the car is not enough, since it does not have access to the “outside world” and remains isolated deep underground. In order for the Internet to appear, it is necessary to connect Wi-Fi inside the train with access points located outside. For a fixed network, a conventional twisted pair could be used. But in the case of the metro, this method is inappropriate, because the trains are constantly in motion. This problem is solved with the help of many base stations located at a distance of half a kilometer from each other (base station – a set of access points and antennas). The receivers on the head cars smoothly switch between base stations, which allows you not to lose connection to the network while the train is moving.

The train is equipped with two independent receivers that are constantly connected to the stations located in the tunnel. These receivers transmit the signal to the routers located in each car. Routers just distribute the MT_Free network, to which metro passengers connect. Schematically, it looks like this:

There are server racks near the stations, from which a fiber optic cable runs to the base stations. In parallel with the fiber optic cable, a power wire runs to the stations. Power supply in the subway is another problem for engineers. The voltage in the network “jumps” a lot and, due to the size of the subway, cannot be stable. Therefore, specifically for the installation of equipment, power supplies have been developed that can operate under conditions of voltage drops in the range from 30 to 150 V. and are immune to dust and dirt in tunnels. The installation of these power supplies greatly facilitated the task of setting up the equipment.

System Security
To prevent the failed equipment from causing a fire, each element of the network is equipped with capsules with a fire-fighting mixture, which, in the event of a fire, immediately extinguish the fire.

It was also necessary to create a fully modular system. Cars on trains are constantly changing, and it is not possible to run hundreds of meters of cable each time. Therefore, it was necessary to develop a scheme so that when replacing the composition, it would not be necessary to spend a lot of time on tuning. Engineers succeeded, and now, when assembling a train, the Internet automatically configures and network all access points installed in the cars. You just need to connect the necessary wires and you’re done. The receivers on the train are independent of each other, so the Internet will function even if one of the two head cars is disconnected from the train. But this raises another problem: how to distribute the signal from two receivers between routers?

The Internet goes quite a long way to the final consumer. First, a set of server racks are installed, which are the source of the Internet.

Please note that it is already difficult to install even one such rack at a depth of 20-30 meters (the average depth of stations). Server racks are supplied with energy from special power sources. After that, the base stations installed in the tunnels are connected to the server racks. Base stations operate as a replacement for conventional twisted-pair cable. The receivers on the head cars “pick up” the signal from the base stations and distribute the Internet inside the car. The throughput of one base station can reach 60 Gbps or more. To access the Internet, networks operating at a frequency of 2.4 and 5 GHz are available. Connection speeds on each train are limited to 100 Mbps, with a total throughput of 900 Gbps.

Foreign analogues of Wi-Fi in the subway
Moscow MT_FREE is unique in its kind, but, nevertheless, not the only version of Wi-Fi networks in the metro. Attempts to connect the Internet to the “underground” were in other countries.

In 2017, 286 stations were equipped with wireless Internet in New York . But, unlike Moscow, it is impossible to use it during a trip, because access points are installed only on platforms. This is the main disadvantage of New York subway Wi-Fi networks: you can connect only while waiting for the train. Also, the installation of a wireless network at the stations leads to an increase in the workload of platforms by people who want to use free Wi-Fi, which is not good for a metropolis where there are already an extremely large number of people. For this reason, in Moscow, access points are installed only on trains.

Wi-Fi was installed in London back in 2012 during the preparation of the city for the Summer Olympics. But, just like in New York, only at the stations. Unlike Moscow, where to access you need to connect to the network and watch ads, in London the Internet is paid.

Oddly enough, many Londoners were against this initiative, as they believed that the Internet would cause an increase in theft of gadgets and would have a bad effect on the health of residents. Therefore, providers found themselves in a sad situation and were forced to make paid access.

Japan has always been a technologically advanced country. The subway is no exception. The subway in Tokyo is really impressive, because it can be used to get to any place in the city. The intricacy of the scheme scares many tourists every time, but this is not about that. The Internet is present only at metro stations in the form of ordinary wi-fi routers installed underground. You can connect to the network by simple manipulations, after which access to the Internet will be provided for 180 minutes.

In the capital of Korea , there is also Internet in the subway. The Seoul subway is one of the few where the Internet is available directly in the tunnels. But, unlike the Moscow network, the connection was made using a radiating cable without the use of base stations. There is also a serious cost – to use the network, you must have a SIM card from a local operator.

The successful experience of implementing wireless technologies in Moscow motivated MaximaTelecom to take on the Wi-Fi project in the northern capital of Russia. So far, the work of the Internet in the St. Petersburg metro is similar to the New York and London models. In St. Petersburg, access is only available at stations, but perhaps the system will be improved to match MT_FREE, which has raised the bar for underground internet.

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