Why is there no signal on the mobile phone during the college entrance examination?

The college entrance examination will change the destiny of most people. The state also guarantees the fairness and justice of the college entrance examination from regulations and action monitoring. Cheating in the college entrance examination may face criminal offences, and during the examination process, the examination room is also ready to use various high-tech to avoid examination fraud. With the development of science and technology, various cheating methods are also advancing with the times. Wireless transmission, Bluetooth headsets... and even the latest popular drones can be tools for cheating. According to Hubei Xianning Public Security News, at 8 o'clock on July 7, outside the test center of the first-level senior middle school in Chibi City, when candidates were entering the test room, a citizen used a drone to take pictures of the test center’s delivery scene, and the test center was against high-tech fraud. After the patrol police found it, they used drone counter-guns to drive away and make a forced landing.

　　If you are near the test room, you will definitely receive a text message, which roughly means: In order to create a fair and just environment, the relevant examination authorities will turn on the communication signal shielding device at all test sites in the city during the test period. The quality of calls and Internet access is affected, such as frequent failures to make calls, failure to connect, intermittent voice calls, slow Internet access on mobile phones, or frequent disconnections. So, do you know the principle of shielding wireless mobile phone signals? What is the function of turning on the signal jammer during the college entrance examination? The invigilator at the entrance of the examination room is waving a metal detector. Does it really work?

　　To know how the cell phone signal is shielded, let's first understand the working principle of mobile communication. As a mobile communication device, a mobile phone needs to establish a connection with a nearby base station when communicating, and then access the core network to find the communication destination. In the above process, all information is carried and transmitted through electromagnetic waves. The electromagnetic wave frequency band used by the mobile phone is related to the system used by the mobile phone and the operator where it is located.

　　But in general, the electromagnetic wave frequency bands used by the three operators are distributed in the range of 850-4900MHz. If the college entrance examination organizer wants to shield the mobile phone signal in the examination room, it is impossible to make the 2G/3G/4G/5G base stations near the examination room not work, otherwise all the communication equipment in this area will not be able to communicate normally. Then the only way is to shield the mobile phone signal in the examination room.

　　The principle of shielding mobile phone signals The mobile phone signal jammer scans from the low frequency band of the forward channel (the channel from the base station to the mobile phone) to the high frequency band at a certain speed during the working process, and forms interference when the mobile phone receives the message signal. The normal data sent from the base station cannot be detected, so that the connection with the base station cannot be established, and there is no signal and no service state. For example, when the mobile phone communicates with the base station, the signal jammer is like a messenger, deliberately making noise around you. The noise signal is so large that the mobile phone can't understand what the base station is saying, and the mobile phone can't hear what the base station is saying. In fact, this is not to shield the mobile phone signal, but to obliterate the signal sent by the mobile phone in a large number of noise signals, so as to achieve the purpose of interference.

　　At present, the mobile phone signal jammers used in the examination room are all full frequency bands, which can interfere with all the frequency bands of the three operators 2G/3G/4G/5G, and WIFI signals and Bluetooth signals can also be shielded. The above is the principle of mobile phone signal jammer. However, the frequency range of electromagnetic waves ranges from several Hz to several hundreds of GHz, and each segment may be used as a communication signal. Will there be some special frequency bands that are used in the examination room? This is true for other radio communication tools, and radio communication tools with dedicated frequency bands "came into being" in this context. The working frequency band of these devices is selected outside the frequency range covered by the mobile phone jammer, and the mobile phone signal jammer will not work on them. For this special radio communication tool, the examination room is specially equipped with a monitoring vehicle equipped with a radio monitor for real-time monitoring of radio wave signals from unknown sources.

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