Since free Wi-Fi was provided to the passengers, the highest number of users per day has been increased to over 10 million. The Shanghai Metro, which has the world’s longest subway line with Wi-Fi, has become a hot topic among the public recently. The Wi-Fi covers not only the station concourses and platforms, but also the train compartments for the first time, which is a big challenge for the metro Wi-Fi technology.

An online article “Piston Effect and the Night Traveler” has caught many people’s attention earlier, which is a story about the key technologies of metro Wi-Fi and the innovative development experience of the wireless automotive system. The article tells a story about an engineer performing site investigation in the tunnel of Shanghai Metro and finally solving the poor Wi-Fi signal quality problem. As the top metro system of the world, is it really able to provide wireless network for passengers to watch HD videos in the train compartments with their mobile devices?

Field Test: Superior Performance without Lag or Disconnection
With the world’s longest total route length and system of the largest scale, the Shanghai Metro offers total route length of 617km and 366 stations to provide services for over 10 million passengers per day during peak traffic. The Shanghai Metro Wi-Fi network is therefore known as the world’s largest Metropolitan Area Network (MAN). It covers not only the station concourses and platforms, but also the train compartments for the first time. Shanghai has become the city with the world’s longest subway line offering free Wi-Fi. Among the 16 operating subway lines of Shanghai, Line 1 to 4, 6 to 9, 11 (Xujiahui Station to Disney), 12 and 13 have been covered with Wi-Fi as at June 2016. The Shanghai Metro Wi-Fi is jointly built by the Shanghai Metro and the service provider Galaxy Valley, and Ruijie Networks is the technology provider behind.

In the field test during the initial metro Wi-Fi deployment on Line 13, the average download speed was 1.5Mbps, upload speed was 1.46 Mbps and the average latency was around 6ms according to the Speedtest software. After login at the station concourse, there was no Internet disconnection during the whole subway journey. Web browsing was smooth and no lagging was encountered when playing videos.

Speed test data during video playback in the train compartment of Shanghai Metro

Since the first subway line of Shanghai Metro entered operation 22 years ago, full-scale construction has extended to the city center of Shanghai including Xuhui, Changning, Jing’an and Huangpu districts as well as 15 other administrative regions such as Baoshan, Minhang, Songjiang and the Pudong New Area. Shanghai Metro has become Shanghai’s main public transport at present. Full Wi-Fi coverage for all routes of Shanghai Metro is definitely a good news for the citizens of Shanghai. However, there were a lot of challenges when implementing wireless network for all routes of the metro.

Targeted Goal: Network Stability at 120km/h
The first challenge is providing full coverage. How to provide wireless network service covering the subway platforms, station concourses and moving trains is a huge challenge. After implementation, the Wi-Fi service can bring high convenience for passengers as they can use their computers and handheld devices to access the network. While providing continuous improvement on passengers’ travel experience, it also has to ensure the efficient operation of rail traffic.  

The second challenge is the high moving speed of the trains. Nowadays, urban rail transit has entered the high-speed era of 120km/h. The types and quantity of services provided by the rail transit system are rising continuously. The train wireless APs are required to perform frequent switching with different trackside APs. The handoff latency, packet loss rate and bandwidth stability will be the key factors of train-to-trackside transmission of the subway service system. Only by guaranteeing these key factors can the rail Wi-Fi system provide high-quality, high-reliability and uninterrupted real-time information exchange for the users in subway stations and on the trains.

Shanghai Metro AP deployment location

In order to achieve uninterrupted connection between the train and trackside APs, the roaming problem occurred when the train is moving at high speed must be solved. Currently, the roaming technologies most manufacturers adopted are for low-speed mobile environment with switching time from 500ms to 3s (including time for re-authentication and other security purposes). For high-speed (around 80km/h) operating environment, if 50m of signal interruption is observed for every 2s of roaming, this will not only affect passengers’ experience, but also the normal transmission of traffic information, causing control signal disorder.
To solve the above problems, a high-performance and high-reliability train network is required for the fast moving scenario. The existing 802.11g/n and LTE technology can no longer meet the requirements. New products and architecture of the 802.11ac technology must be employed. To this point, we have finally discovered the origin of the article “Piston Effect and the Night Traveler” as well as multiple inspiring new technologies.

Ruijie’s New Patent: Fast Roaming within Milliseconds
As a leading brand of data communication solutions in the industry, Ruijie first introduced the 802.11ac technology to meet the high-performance automotive bandwidth requirement and demonstrated an outstanding performance in the actual deployment of the Shanghai Metro Wi-Fi system. The automotive bandwidth was 200Mbps as targeted in the design plan but an actual result of over 600Mbps was observed and there was almost zero packet loss in the field test, which has fully met the high-stability, high-speed and uninterrupted real-time network service requirements of the subway information system. In the solution, Ruijie has developed a fast-roaming patented technology which can offer fast switching between train APs and trackside APs in milliseconds to ensure minimal packet loss during the switching process.  
Technically, the trackside APs operate on the same frequency and broadcast the same BSSID. All packet transmissions are centrally coordinated by the AC. For the train APs, the trackside APs of the entire network can be regarded as a “large virtual AP”.

Bridged network diagram between the train and trackside

First, when the train is moving, the train APs only communicate with the “large virtual AP”. The train APs automatically determine and select the trackside APs with the best signal quality for communication and they are unable to detect the roaming process. Secondly, the AP supports the operating mode of receiving dual signals as well as intelligent path update for information exchange so as to ensure the reliability of data transmission in the fast moving train (packet loss tends to zero). In addition, all devices comply to the required standards with operating temperature from -40℃ to 70℃, operating humidity from 5% to 95% and IP67 protection rating. The devices are also easy to deploy and easy to manage.

AP deployment in tunnels

10 Million Passengers: Superior Wi-Fi Experience with Full Coverage
The Shanghai Metro Passenger Wi-Fi Project in which Ruijie has involved offers coverage for a total of more than 600 trains as well as all rail lines with a total length of 617km (at the end of 2015). Taking the uplink and downlink intervals into account, the wireless coverage distance is over 1000km. The project offers dynamic bandwidth of over 600Mbps for the train and provides passengers with superior Internet services, ensuring the efficient operation of rail traffic while continuously improving passengers’ travel experience. Every weekday, there are millions of people traveling by rail transport in Shanghai. In the last century, foreign experts have asserted that the soft soil layer of Shanghai was not suitable for building subway. However, in only 22 years, the Shanghai Metro was built from scratch to the world’s largest subway at present. Behind this rapid development, the subway builders and operators have paid unimaginable efforts. In the future, Ruijie will continue to advance the Shanghai Wi-Fi network construction together with Shanghai Metro and Galaxy Valley to provide more value-added services for the passengers.