[Editor's Note] Tongxinlianmeng, the author of this article, reprinted on his own 。 For the recent WiFi forced stop of the subway, Shenzhen Metro believes that this is due to mobile WiFi, but the author believes that this is mainly due to the poor subway technology. The use of 2.4GHz in the subway is just to save time. "Good news" two years ago
On March 16, 2010, a news from SASAC did not attract people's attention at that time. This should be the news that China Railway Signal Group Corporation (CRSC for short) is offering rewards and celebrating: "CRSC has won the bid for Shenzhen Metro Line 2". According to the news, this project is the most professional and comprehensive urban rail transit construction project independently contracted by CRSC, with the largest number of construction enterprises participating in the construction within the group, It is also the project with the highest amount of money, and many specialties have achieved "zero breakthrough" in the subway field. The system equipment comprehensive installation project of Shenzhen Metro Line 2 is divided into two parts: the first phase project and the east extension project. The project includes signal system (including computer interlocking), communication system (including communication, wireless, police communication and AFC) and other projects.
No one knows what kind of hidden danger will be buried for the public two years later when this listed company celebrates?
"Public panic" two years later
On September 5, 2012, the worst subway outage accident ever occurred in Shenzhen on the Longhua Line of the subway;
On November 1, 2012, the subway Shekou Line was emergency braked for many times;
On November 5, 2012, the subway Longgang Line was shut down in an emergency;
On November 7, 2012, the emergency braking of the central line of the metro ring was carried out.
So far, Shenzhen Metro Group said it was still under investigation
Cause of the incident, a word from an insider. Wi Fi becomes "unknown signal"
From 8:15 to 9:30 on November 1, several trains on Shekou Line of Shenzhen Metro experienced emergency braking for many times. The metro said that it was caused by the interference of "unknown electromagnetic signals" from outside on the signal system controlling train operation. On the 2nd, it was reported by the media that some insiders of the group had disclosed that the signal generated by the passenger's hand-held portable Wi Fi device was the reason for the emergency shutdown of the train. While Shenzhen Metro sent an official letter to Shenzhen Communications Administration in a hurry, hoping to coordinate with wireless operators to shield 3G signals when the facts were not clear and the investigation was not carried out. He accused WiFi of interfering with the normal operation of the subway.
In this regard, the Communications Administration made it clear that "the closure of 3G data service signals of mobile communications must be handled in strict accordance with relevant national regulations and procedures", and did not support the subway group's request to shield 3G signals all day long. He also believes that the reason for the Shekou Line accident is that the subway should not use the open 2.4G Hz channel, and refuses to question the products of the three major operators.
The Notice on Adjusting the 2.4GHz Frequency Band Transmission Power Limit and Relevant Issues issued by the Radio Administration of the Ministry of Information Industry clearly states that "2.4GHz frequency band is used as the common frequency band of various types of radio stations, such as wireless LAN, wireless access system, Bluetooth technology equipment, point-to-point or point-to-point multi-point spread spectrum communication system", "Interference between radio stations within this frequency band is not protected in principle and should be solved by themselves or through negotiation."
There has been a research report on the Internet for a long time, and the Preliminary Research Report on Train ground Wireless Transmission of Mobile Block Signal System also argued that "the use of 2.4GHz public frequency band can not completely avoid the possibility of CBTC interference, and only by configuring a dedicated frequency band can it be completely solved." The report said that France has taken this approach as a local policy, and in some areas, A dedicated working frequency band is specially configured for the wireless CBTC system in the 5.8GHz frequency band.
Since 2.4GHz is an open channel, designers should take this into consideration at the beginning of design. If such a time happens, the person in charge should not be the user using portable WiFi, but Shenzhen Metro.
Does Metro Group use 2.4GHz public band to save money or trouble?
At present, the CBTC system communication of Shekou Line, Huanzhong Line and Longgang Line of Shenzhen Metro where the accident occurred are all using the 2.4GHz public frequency band, and only Longhua Line uses the 5.8GHz paid frequency band. In this regard, communication experts said, "France, which was ridiculed by us, is using paid special frequency bands, but Shenzhen Metro has to cut down on major projects involving public safety, which is really unreasonable."
As for "saving money", the Metro said that actually the 5.8Ghz applied for payment does not need much money. Through Shenzhen Radio Administration, we also confirmed that the frequency band is a national resource. If the subway uses the paid frequency band, the annual cost will not exceed 10000 yuan. According to a person familiar with Shenzhen Metro, the choice of 2.4Ghz frequency band actually has something to do with the original bidding work: because of the popularity of 2.4Ghz, everyone thinks it is unnecessary to think about it again, which is simply to "save time".
Yang Chun, Deputy Director of the Business Department of Shenzhen Radio Office, also said that in fact, in the bidding work, he had suggested that Shenzhen Metro should not use the public frequency band because of the risk of signal interference. Obviously, many facts have proved that giving up 5.8GHz, which does not cost too much money, and using 2.4GHz "logically" is not to save money, but to save time.
In this regard, Chen Qiang, the person in charge of Shenzhen Rail Office, retorted that "as long as the CBTC system is used in some big cities, such as Beijing, Shanghai, Guangzhou, etc., the transmission frequency band is 2.4GHz." After several outage accidents, God knows why the subway still made such irresponsible remarks.
Is the Casco signal system defective?
At present, the three lines operated in Shenzhen, Shekou Line, Huanzhong Line and Longgang Line, all use the CBT C system, but the contractors of the signal system are inconsistent. Shekou Line and Huanzhong Line are Casco. It is reported that the Casco system was also used in the "7.23 Yong Wen Line EMU Accident", and the Casco system was also used in the rear end collision of Shanghai Metro Line 10.
Casco Signal Company introduced in an interview with the media that if the portable WiFi forced to stop the subway is to be completely solved, it needs to unify the national communication industry standard and give the subway signal system a special frequency band. At present, only protective measures of signal system can be strengthened. Since Casco knows this problem will occur, why use 2.4GHz?
The 2.4 band ISM public band is used in many places, and all mobile terminals have taken WiFi as the standard configuration. Obviously, Shenzhen Metro Group appeals to passengers not to use WiFi function and take passenger safety as a bargaining chip. This appeal is a hasty conclusion made when the facts are not clear. In particular, the signaling companies and the subway companies themselves have not yet made clear the tail of the accident in the past few years. They even accuse Shenzhen's wireless city construction is too fast, and face the separation of relationships with the counterparts of subway operators in various cities.
Obviously, the public can't help wondering if the WiFi phone in their hands will be able to disrupt the operation of the subway on any day. Will the public who take the subway have to be checked by the subway company in the future?
