VSAT communication is a kind of satellite communication. VSAT is the abbreviation of very small aperture terminal, which means very small aperture satellite communication terminal, and usually refers to the satellite communication earth station with the terminal antenna aperture of1.2-2.8m.. It is a kind of satellite communication terminal equipment developed in the early 1980s, and has been widely developed in recent years.
VSAT communication system can generally work in two frequency bands, namely 14 (uplink)/1(downlink) GHz Ku band and 6 (uplink) /4 (downlink) GHz C band. C-band developed earlier with less rain attenuation, but the space resources are crowded, and the antenna aperture and terminal equipment are larger. At present, the VSAT system in Ku band develops faster than that in C band, because the air resources in Ku band are relatively loose, the antenna aperture is small and it is easy to install. The Usat ultra-small aperture terminal, known as the new talent of Vast, that is, the satellite communication earth station with antenna aperture of 0.3-0.5 meters, works in Ku band.
In addition to the inherent advantages of satellite communication, the development of large-scale communication has two main characteristics:
1. The equipment of the invention is simple in structure, all-solid, small in size, low in energy consumption and convenient for system integration and installation.
Most stations usually have only two units (cabinets) indoors and outdoors, which is very convenient to install and can be installed at the user's location. The well-known and widely used satellite TV receiving station is actually a one-way (only receiving and not sending) station. Vast station equipment is light and flexible, which is suitable for building rolling satellite communication ground station. For example, during the Gulf War, the exclusive coverage of Baghdad by CNN reporters in the United States was carried out by the vast number of communication ground stations. As we all know, satellite communication system consists of space station (communication satellite), earth station and transmission channel. The earth station also includes a signaling system and a ground relay system. Huge equipment can be installed at the location of the user terminal without tandem transfer, directly connected to the communication terminal, controlled by the user himself, and no relay system is needed, which greatly facilitates the user, greatly reduces the equipment cost and has obvious economic effect.
Second, the vast satellite communication network is flexible and convenient. In a huge system, communication network structure can be divided into three types: star network, mesh network and hybrid network, each of which has its own characteristics.
1. Star network consists of a main station (usually a hub station in a central city) and several small stations (remote user terminal stations). The main station has a large-aperture antenna (generally 10-20m) and a high-power transmitter. Network is not only responsible for network management, but also for sending and receiving information between various large substations, that is, providing transmission channels and exchange functions between substations. Therefore, the master station has the control function. A star network system can accommodate hundreds or thousands of small stations. All the small stations in the network establish direct communication links with the main station, and the main station can communicate directly by satellite (small station-satellite-main station). Small stations can't communicate directly, but must transit through the main station to form a communication link in the form of "small station-satellite-main station-satellite-small station". It can be seen that the links between small stations are connected by satellite twice and by "double hop", so there is a large transmission delay of about 0.45 seconds, and users between small stations will feel a little uncomfortable or unaccustomed when talking. This is a defect that the huge star-shaped network link is used for calling, so this "double-hop" transmission is suitable for data service or recording telephone, but not for real-time voice service. Otherwise, the direct communication between users needs an adaptation process, so star network is especially suitable for transmitting high-speed data services between small stations and main stations.
2.Mesh network link has changed the star network link mode, and it is also composed of a main station and several small stations, but the small stations can realize "single hop" communication according to the "small station-satellite-small station" communication link without going through the main station. Thus, the transmission delay is reduced by half compared with the star network, only 0.27 seconds, and users can adapt to the call. At this time, with the help of the network management system, the master station is responsible for allocating channels for each huge substation and monitoring its working status.
3. Hybrid network is a kind of network that combines star network and mesh network and concentrates their respective advantages to complete the link. The vast number of stations in the network can communicate directly in both directions without switching through the main station.
The huge communication system integrates many advanced communication technologies such as packet information transmission, switching, multiple access protocol and spectrum expansion, and transmits many kinds of information such as data, voice, video images, faxes and random information. Generally speaking, star network is mainly based on data communication and is compatible with voice services. Mesh networks and hybrid networks are mainly based on voice communication and are compatible with data transmission services. Like general satellite communication, a basic advantage of vast communication is that it can realize simultaneous communication between multiple earth stations, that is, vast small stations, by using the same satellite. This communication is called "multiple access connection". The key to realize multiple access connection is that the signals sent by each earth station can be recognized by the corresponding opposite station after being mixed and forwarded by the satellite transponder, and the interference between the signals of each earth station should be as small as possible. The technical basis of realizing multiple access connection is signal division. As long as there are differences between signals in a certain parameter, such as different signal frequencies, different time when signals appear, or different spaces where signals are located, they can be separated. In order to achieve this goal, it is necessary to adopt a certain multiple access connection mode.
In the huge communication network, there are three main multiple access modes:
1, the "single carrier" (SCPC) in frequency division multiple access (FDMA) refers to the different frequencies of transmission signals to distinguish or represent the site addresses of different stations. In other words, each large substation uses different signal frequencies to distinguish different large substations.
2. Time Division Multiple Access (TDMA) mode. That is to say, different time intervals of transmission signals are used to distinguish or represent the site addresses of different large stations. That is to say, each huge station transmits signals at different time intervals for receiving and sending digital codes, so as to distinguish different stations.
3. Code Division Multiple Access (CDMA). That is, different symbols of the transmission signal are used to distinguish or represent the site addresses of different large stations.
In a huge communication system, because of the different types of services transmitted, different multiple access connection methods are often adopted. For example, in the same ground station, frequency division multiple access (FDMA) is used for voice transmission, while time division multiple access (TDMA) is used for data transmission. There is also a channel allocation problem closely related to multiple access, that is, how to allocate frequency bands, time slots and address codes to stations in an orderly manner, which is the channel allocation technology.
There are many technical methods for channel allocation in multiple access mode. In large communication systems, pre-allocation mode and on-demand allocation mode are often used. There are two pre-allocation methods: fixed pre-allocation method and on-time pre-allocation method. The former is to assign a certain number of carrier frequencies to each station according to the prior agreement, and the station can only use the dedicated frequencies assigned to it to communicate with the related stations, and other stations cannot occupy these frequencies. Because each huge station has a dedicated carrier frequency, it is faster to establish communication. However, because each vast station always occupies a carrier frequency regardless of whether it works or not, the frequency utilization rate is low. Therefore, this method is suitable for lines with large traffic volume, and the latter is a pre-allocation method proposed according to the traffic volume of the stations in different periods in order to improve the channel utilization rate.
On-demand channel allocation, also known as application-based channel allocation, overcomes the shortcomings of pre-allocation channel method, but applies for channels when needed. After the communication is completed, the channel returns to the management control center for distribution, which greatly improves the utilization rate.
At present, the vast communication technology is relatively mature, and new technologies and products are gradually enriching the vast communication, making it more perfect and more convenient to operate.
The development of vast communication is only fifteen or sixteen years. Since 1984, Hughes network systems developed the first set of wide-area communication equipment and put it into commercial operation, the global wide-area communication has developed vigorously. At present, there are more than 100000 Vast stations in the United States alone, and American Vast technology is introduced to produce Vast equipment in Asia and India, with an annual output of more than1000. At one time, the number of global stations reached several hundred thousand. It can be seen that the output is large and the application has reached a considerable level.
With the deepening of China's reform and opening-up, Vast Communication has also achieved good development in China. China Telecom has been involved in a wide range of communication services for a long time, and 1995-1998 is the period of great development of paging industry in China. In order to expand the coverage and realize national networking, mass communication is widely used in paging industry to achieve this goal. During this period, almost all paging stations used Haote communication, which made the development of Haote communication market form a small climax.
At present, China's financial banking, petroleum, earthquake, civil air defense, civil aviation, meteorology, news, newspapers, military and other departments have established their own huge communication networks. All these have formed a broad industrial application market.
With the demand of communication, Internet and multimedia for broadband, it will provide channels and opportunities for the development of large-scale communication to transform the single TV broadcast reception into comprehensive multimedia reception such as TV broadcast, distance education, telemedicine, rural telephone, satellite Internet access and video broadcast.