In just a couple of years, a Moscow resident’s trip to the subway has ceased to be a daily routine. If earlier the only entertainment subway had to offer was reading books or newspapers and MP3-players, it’s about time that online shopping, watching TV shows, business correspondence, even dating on Tinder and quests were added to them.
And all that thanks to the advent of a free Wi-Fi network in the subway. About 80% of Moscow residents regularly connect to the MT_FREE network in the subway, without thinking about how it works and who was in charge of setting it up. It has been said that Wi-Fi in the subway appeared on its own, but that is not entirely true. The wireless network is MaximaTelecom’s progeny. For the company it was the first experience of building a high-speed Wi-Fi network with unique for global practice engineering and technical solutions. In this article we will talk about the configuration of the Moscow subway Wi-Fi network.
We actually have two networks:
Radio Network Inside the Subway Cars
You enter the subway car, you see a sticker with the network’s name or you automatically turn on the Wi-Fi on your phone. At the exact same time, the device is getting connected to the network using the SSID MT_FREE. It is organized with high-density access points, which are located in each subway car, they operate in two 2.4 GHz and 5 GHz bands and support the 802.11a /b/g/n standards. They are operated by the controller in the head car. The rolling stock contains two such subway cars, which means there are also two controllers. All the equipment in the rolling stock, including the one between the subways cars, is connected through cables — the twisted pair.
A Matter of Technique
In order to set up a Wi-Fi network and network infrastructure in the subway cars, we used Cisco equipment: most notably, the air-cap2602i access points, the air-ct2504 controllers, the 29xx series switch boards and the 8xx series routers. To improve the error tolerance, we installed two cable passages between the subway cars. If you delve deeply into the network architecture, Layer2 for user traffic is terminated on the router in the rolling stock, while NAT (Network Address Translation) is carried out on the edge network routers in the same way as it is organized for the majority of the wired access operators.
Train-Tunnel Radio Network
After passing through the internal train network, the data is transferred to the stationary network infrastructure using the train-tunnel radio channel. It is installed between the cell tower located in each head car and the base stations located along the path of the rolling stock in the tunnel, as well as in the open sections of the tracks. The placement of the base stations along the tracks is such that the train moves in a continuous radio field. This ensures that the breaks in communication are kept to a minimum. The base stations on the train are placed in the same way as the access point switch boards on each head car, with only one station being active when the train is on the move. The radio channel operates in the same frequency
A Matter of Technique
In order to set up a communication channel, Radwin 5000 series equipment has been used. It employs the Wi-Fi chips that comply with the 802.11n standard, in which case the data is transmitted via a proprietary TDM-based protocol (Time Division Multiplexing) that is generated by an additional microcircuit. Synchronization of the base stations is carried out in accordance with the protocol similar to PTP 1588v2.
The permitted frequency spectrum of 5,150 — 5,350 MHz is divided into five disjoint channels of 40 MHz each. On each line, all five channels are used, usually in the 1-3-5- 2-4 sequence, in an attempt to avoid the interference effects when operating the neighbouring devices in the same frequency range.
Each cell tower along the train’s route is connected to the switching nodes located in the subway’s service rooms using a dedicated fiber-optic network. Uninterrupted power supply of the base stations can also be achieved with the help of the equipment installed in these switching nodes The architecture of a fixed data transfer network is not that different from the typical architecture used by the cell phone operators. This is what we call a «double star» with geographical redundancy of communication channels and key equipment. The network has several communication channels with the backbone telecom operators, with a total bandwidth of more than 60 Gb/s.
A Matter of Technique
Network equipment at the access level (switch boards that can be connected directly to the base stations), aggregations, as well as the network’s core are represented by Cisco switch boards and routers. The base stations are connected to the switch boards through the WDM-technology for the sake of saving on fiber (that is, a single fiber at different wavelengths simultaneously receives and transmits data). Access switch boards have two uplinks with geo-redundancy (FOL cables are physically located in different tunnels) to aggregation switch boards of 1 Gb/s each. By the same token, they are connected over the geo-redundant communication lines to the core switch boards, but with 10 Gb/s interfaces.