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When it comes to emergency communications, most people may think of dedicated communication equipment or networks. This is indeed the case. Satellite communications, microwave communications, radio stations and other communication methods have always been the main technical means of emergency communications because they are inherently strong in terms of anti-destruction capabilities. These communication systems are generally not open to the general public and are dedicated emergency communication systems.

Compared with dedicated systems, providing emergency communication services through public telecommunications networks is also a method of providing emergency communications, and has gradually become a focus of research in the field of international emergency communications.

From the overall technical level, emergency communications are divided into two main methods: wired and wireless.

Wired emergency communications refer to conventional domestic and international telephone networks, the Internet, etc. In particular, the wired public telecommunications network is the most widely distributed information exchange network in the country. It has a wide range of access, strong adaptability, and low cost. It is the most basic means of information transmission in natural disaster emergency communications. By using the voice channel of the wired public telephone exchange network and the integrated communication terminal equipment, it is easy to realize the transmission of comprehensive information such as telephone, fax, computer data, etc. between the national rescue command center and the rescue command centers in various places. However, wired communications are mainly transmitted through optical cables and cables, which are restricted by geographical conditions and have poor anti-destruction capabilities. Once destroyed, communications are immediately blocked and difficult to restore.

Wireless emergency communication is the transmission of information by electromagnetic waves. In the early days, medium and short wave communications were the main methods. After the 1940s, ultra-short wave and microwave communication services developed rapidly. In particular, the emergence of satellite communications made the wish of “communication without time and space restrictions” a reality. Wireless communication has strong anti-destruction capabilities, flexibility, and convenient networking, and is an effective means of emergency communication.

At present, the specific emergency communication methods available in China include: fixed telephones, mobile phones, Ku-band satellite communication vehicles, C-band satellite communication vehicles, 100W single-sideband communication vehicles, one-point multi-address microwave communication vehicles, user wireless loop equipment, maritime satellite type A stations, type B stations, type M stations, 24-channel UHF communication vehicles, 1000-line line-controlled switching vehicles, mobile phone communication vehicles, adaptive radio stations, walkie-talkies , the Internet, etc.

Fixed telephone network has the advantages of wide coverage and large audience. Generally speaking, fixed telephone network is the preferred network for emergency communication because of its low cost and large capacity. The emergency calls such as 110, 119, 122, etc., which we usually call, are actually a traditional means of emergency communication. The characteristics of emergency calls are: they are open to all the public and anyone can use this means of communication; the communication flow is convergent, and a large number of communication flows will point to a few limited points, such as the 110 alarm center. The services provided by emergency calls to the public can be divided into two categories. The first category is personal assistance. When an individual needs medical or safety assistance, these services can be obtained through emergency calls. This type of service belongs to the category of social welfare and strictly speaking does not belong to emergency communication. The second type of service is to serve as a channel for the public to report information in an emergency, and at the same time as a means for the public to report conditions and locations, and to achieve self-rescue or rescue others during disaster relief and disaster recovery. This is the main way that emergency calls are used in emergency communications.

However, fixed-line phones also have their shortcomings. They are limited by cables and cannot be used at any time and any place as we expect. The original emergency communication architecture did not consider that the fixed-line network should be responsible for emergency communications. Once an emergency occurs, the government’s decision-making bodies and functional departments will use dedicated emergency communication systems (such as satellite, microwave and other communication systems), while the emergency communication means available to corporate users and individual users are very limited and can only rely on emergency call services. When the fixed-line network is responsible for emergency communications, the emergency communication services available to various users will be greatly enhanced.

Regarding the provision of emergency communication services through public telecommunication networks, the current research directions mainly include three: telecommunication network emergency communication capabilities, emergency calls, and Internet emergency response. The first direction studies how to improve the emergency communication capabilities of telecommunication networks so that they can undertake emergency communication tasks; the second direction studies where traditional emergency calls will develop; and the third direction studies how to ensure smooth network operation in emergencies.

The biggest advantage of mobile communication is its mobility. Communication is not limited by time and place. As long as it is within the signal coverage area, it can communicate freely. Therefore, since its advent, it has played a huge role in emergency communication. At present, in emergency communication, mobile communication is not only used for simple applications such as making calls, but also can use the positioning and location services of mobile communication to carry out safety rescue, location tracking and traffic navigation. The commonly used mobile communication systems are digital trunking communication system, Global System for Mobile Communication (GSM), 3G, 4G, etc. With the development of mobile communication, as long as people are within the coverage area of ​​mobile communication, they can communicate while moving, which is very flexible and convenient. In addition, the price of mobile phones and the cost of mobile communication have gradually dropped to a level that ordinary people can accept, which makes mobile phone networks gradually occupy a dominant position in emergency communication.

Mobile networks often use emergency communication vehicles in emergency communications. These vehicles are actually special base stations that connect end users to the mobile network via microwaves or satellites. This emergency communication method can be used in disasters where base stations are destroyed, such as earthquakes, and can also be used to temporarily expand capacity in some occasions where the traffic volume increases sharply (such as large gatherings). Emergency communication vehicles can be seen as a means for mobile networks to enhance their survivability/resilience and improve their recovery capabilities.

Satellite communication has a long range and is not limited by ground conditions. It has the unique advantage of flexibility and mobility, and can achieve communication between places that ground transmission means cannot meet with excellent performance and fast speed, which is very suitable for emergency communication needs. Especially in areas with large areas and underdeveloped ground communication lines, satellite communication means can provide the most cost-effective solution.

During the 2008 Sichuan Wenchuan earthquake, a large number of satellite phones and maritime satellite phones were deployed nationwide, which played an important role in disaster relief. However, due to weather conditions and the excessive increase in the number of users, satellite resources were tight and it was difficult to get through. In addition, satellite calls were very expensive, so it was difficult to promote their use at the grassroots level. Of course, under war conditions, there was also the issue of the enemy destroying communication satellites.

Shortwave radio is transmitted through the ionosphere, and the communication autonomy is stronger than satellite. In addition, the communication is free, which is conducive to large-scale promotion. Its unsatisfactory features are that the antenna is large and sometimes noisy. The development and application of shortwave frequency adaptive technology have greatly improved the reliability and effectiveness of shortwave communication. Adaptive emergency radio equipment is small in size, easy to transport and install, and simple to operate, making it more suitable for emergency communication.

The emergency communications of the Sichuan Wenchuan earthquake left a profound inspiration to the industry in two aspects: first, the earthquake damaged almost all daily communication systems, including wired telephones and the Internet, mobile phones, and ultra-short wave clusters; second, the communication and contact in the early stage of disaster relief mainly relied on two tools, namely shortwave radio and satellite mobile phones. The reason why daily communication systems are paralyzed in major disasters is that disasters often destroy a large area of ​​the basic conditions on which they rely for operation: optical cables and microwave lines, power systems, communication hub buildings, antenna towers, etc. This shows that the anti-destruction ability of these daily communication systems is very poor and they are often powerless in the face of disasters.

The reason why shortwave radio can serve as a backbone emergency communication tool in the event of a sudden disaster is essentially its irreplaceable independent communication capability and anti-destruction capability. They work independently without relying on ground communication networks and power systems (car radios can work for one to several days on car batteries). In disasters and wars, the probability of all shortwave radio stations being destroyed is extremely low. As long as a few of these devices survive, they can report the disaster in a timely manner, and initial disaster relief communications will not be a problem.