Shanghai has become a hot topic recently because it provides a free Wi-Fi network in the longest Wi-Fi metro line in the world and provides the Internet access service for passengers (up to 10 million users one day at peak). The Wi-Fi network covers station halls and platforms and, for the first time, covers compartments, which is a great challenge for the metro Wi-Fi technology.

The report The "Wind" and "Night Traveler" has attracted many people to the network. It told the innovation history about the key metro Wi-Fi technology — the train-to-ground wireless system, field surveys on the tunnels of Shanghai Metro, and resolving the poor quality of Wi-Fi signals. In the metro that ranks No.1 in the world, can users really access the wireless network on their phones to watch HD movies at any time inside compartments?

Field Test: Videos Are Smooth

Shanghai Metro ranks No.1 globally in total operating length and scale. The metro lines total 617 km and cover 366 stations. In rush hour, the passenger flow can exceed ten million of person times a day. The Shanghai Metro Wi-Fi network is thus known as the world's largest MAN. It covers station halls, platforms, and compartments for the first time. Shanghai has become the city with the longest Wi-Fi metro lines in the world. As of June 2016, Shanghai operated 16 metro lines. Lines 1, 2, 3, 4, 6, 7, 8, 9, 12, 13 and 11 (Xujiahui station to Disney station) are covered by the Wi-Fi network. The Shanghai Metro Wi-Fi network was jointly built by Shanghai Metro and the operator Galaxy Valley. One of the technology providers behind is Ruijie Networks.

Shanghai Metro Map

In the test for Line 13, where the metro Wi-Fi network was provisioned first, Speedtest software showed an average download rate of 1.5 Mbps, average upload rate of 1.46 Mbps, and average delay of about 6 ms. After test personnel logged in from a station hall, the STA did not disconnect from the network throughout line 13. Web pages could be displayed very smoothly and videos did not freeze.

For the 22 years since the first Shanghai Metro line was opened, Shanghai Metro has progressively covered Shanghai urban centers such as Xuhui, Changning, Jing'an, Huangpu, and 15 administrative regions such as Baoshan, Minhang, Songjiang, and Pudong New Area. It has become the main transportation vehicle in Shanghai. The provisioning of Wi-Fi networks in all lines of Shanghai Metro definitely benefits Shanghai people. However, there are great difficulties in the process of advancing wireless network access throughout all lines. 

Building Goal: 120 km/h Speed without Network Interruption

The first difficulty is full coverage. The range of the wireless access service needs to be extended to platforms, station halls, and the trains of the metro rail transit, which is highly complex. After implementation, passengers can use computers and handheld devices to access the network at any time. The efficient operation of the rail transit needs to be ensured and the passenger experience needs to be improved constantly.

The second difficulty is high speed. Nowadays, the urban rail transit has moved to the 120km/hour high-speed era. Services carried in the rail transit system are growing in category and quantity. Vehicle-mounted wireless APs need to be handed over frequently between different trackside APs. Handover delay, packet loss rate, and train-to-ground bandwidth stability are key factors affecting the metro service system in train-to-ground link transmission. Only after these factors are guaranteed can the civil Wi-Fi system achieve high-quality, reliable, and uninterrupted real-time services for users in metro stations and trains.

AP Deployment Positions in Shanghai Metro

To implement the train-to-ground service without interruption, it is necessary to solve the roaming and handover problem during high-speed movement of trains. At present, the roaming technology adopted by most manufacturers is based on low-speed mobile environments. The handover time ranges from 500 ms to 3s each time, including the time required for re-authentication and other security overheads. For high-speed (about 80 km/hour) operating environments, if roaming succeeds within an average of 2 seconds, signal interruption will occur at a distance of 50 m, which will degrades passenger experience and affects the transmission of transport information, resulting in control signal disorder.

To achieve the construction goals, it is necessary to build a high-performance, highly reliable network for trains in high-speed movement scenarios. The previous 802.11g, 802.11n, and LTE technologies no longer meet requirements, and 802.11ac-compliant new products and new architecture are needed. Now, we have found the origin of the report The "Wind" and "Night Traveler" and multiple impressive new technologies.

Ruijie New Patent: Fast Roaming Handover within Several Milliseconds

As a leading brand in data communication solutions, Ruijie first adopted the 802.11 technology to meet high-performance train-to-ground bandwidth requirements. The technology functions well in Shanghai Metro scenarios. The target design of the train-to-ground bandwidth is 200 Mbps, the actual test bandwidth is more than 600 Mbps, the packet loss rate is nearly 0, which fully meets the stable, high-speed, and uninterrupted real-time network service requirements of the metro information system. For this solution, Ruijie developed its patented fast-roaming handover technology, which implements fast train-to-ground handovers within a few milliseconds and ensures almost zero data packet loss during the handovers.

Specifically, trackside APs work at the same frequency and have the same BSSID. All packet processing and interaction are unified and coordinated by the AC. For vehicle-mounted APs, trackside APs function as one large virtual AP.

Train-to-ground Bridging Network Topology

Vehicle-mounted APs can communicate only with the large virtual AP. They automatically make judgments and select trackside APs with superior link quality for communication. The vehicle-mounted APs do not perceive roaming. APs support the single-transmitting dual-receiving mode and intelligently update information interaction paths, thereby ensuring the reliability of data transmission (almost zero packet loss) when trains run at high speeds. All devices comply with standards. They can work in environments of -40°C to +70°C at 5% to 95% humidity, and offer IP67 protection. They can also easily be deployed and managed.

AP Deployment Inside Tunnels

Tens of Millions of Passengers: Satisfactory Full Wi-Fi Coverage Experience

In the Shanghai Metro Internet access project, Ruijie helped achieve full coverage for more than 600 trains and lines covering 617 km (at the end of 2015). If both uplink interval and downlink interval are included, the coverage distance of the train-to-ground wireless system exceeds 1,000 km. The project achieves dynamic train-to-ground bandwidth of above 600 Mbps, providing robust video, Internet access, and other service experience for passengers, while ensuring efficient rail transit operation.

Tens of millions of passengers use the metro system on any given work day. It has taken 22 years for Shanghai to go from no metro to full metro. In the last century, experts considered the ground to be too soft for a metro system. Now, it ranks No.1 in metro lines and scale. Behind the rapid development, the metro builders and operators have made incredible efforts. In the future, Ruijie Networks will continue to work with Shanghai Metro and Galaxy Valley to promote the Shanghai Wi-Fi network and provide more value-added services for passengers.