The structure of communication system can be point-to-point or point-to-multipoint, and broadcasting is a special form of point-to-multipoint communication.
What is telecommunications?
Telecommunications is an important pillar of the information society. No matter in human social and economic activities, or in all aspects of people's daily life, it is inseparable from telecommunications, an efficient and reliable means.
What is "telecommunications"? The definition of ITU is: "Communication using wired electricity, radio, light or other electromagnetic systems." According to this definition, the sender uses any electromagnetic system, including wired telecommunication system, wireless telecommunication system, optical communication system and other electromagnetic systems, and adopts any forms of expression, including symbols, characters, sounds, images and various visible, audible or usable signals composed of these forms. The process of sending information to one or more recipients is called telecommunications. It includes not only traditional telecommunication media such as telegraph and telephone, but also modern telecommunication media such as optical fiber communication, data communication and satellite communication, including both the above-mentioned two-way information transmission media and unidirectional information transmission media such as radio and television.
The history of transmitting information by electricity began with telegrams. Telegraph is a way of transmitting information by symbols, which is also called digital mode. However, after the invention of telephone by 1876, analog mode has occupied an absolute advantage in the field of telecommunications. In 1950s, the appearance of PCM technology brought a ray of light to digital communication. But its real growth and development, and even the unification of the world, mainly rely on the power of electronic computers. Therefore, some people regard "C&C" (the integration of communication and computer) as an important symbol of a new revolution in the history of human information. Today, when we talk about "telecommunications", we have to talk about the wide application of computers in various telecommunications fields. It not only plays a key role in realizing the intelligence of telecommunications, making telecommunications no longer simply "transmit" information, but also increases the functions of information processing and storage.
Telecommunications started with wired communication with metal wires, and by the end of 19, wireless telegrams appeared, which made wireless communication methods rise for a period of time. However, due to some inherent weaknesses of wireless communication, it was difficult to overcome under the technical conditions at that time, so for a long time, except for navigation and disaster relief, it was generally in a "standby" state. Due to the development of technology, not only the wireless mode has made a comeback, but also a brand-new communication mode with optical fiber as the transmission medium has emerged. This not only brings the diversification of telecommunication transmission modes, but also lays a material foundation for the high-speed and personalization of telecommunications.
As can be seen from the above, although the basic concept of telecommunications 100 has not changed for many years, its extension has experienced vicissitudes. The dominant position of telephone has been shaken, and the arrival of information expressway and the Internet has brought unprecedented impact to the concept and mode of traditional telecommunications. [Edit this paragraph] was formed in 1948, when Shannon, who worked in Bell Laboratories, published a paper "Mathematical Principles of Communication" and established a mathematical theoretical method for analyzing communication systems, namely information theory. Information theory enables us to calculate the channel capacity according to the bandwidth and signal-to-noise ratio characteristics of the channel.
In the era when the paper was published, the telecommunication system was mainly based on analog circuit system. Since then, with the popularization of digital integrated circuits, the design of telecommunication system can make full use of the theoretical guidance of information theory to improve performance. Therefore, digital signal processing has also become a new field.
There are inevitably various defects in the actual communication channel, including shot noise, thermal noise, delay, nonlinear transfer function, rapid fading, bandwidth limitation and signal reflection. In some current telecommunication system designs, these defects can be used to improve the quality of channel transmission.
Modern communication systems generally put forward high requirements for time synchronization. Accurate timing technology is closely related to the development of communication technology. Most modern wide-area communication systems use atomic clocks as a time reference.
Modulation refers to the process of converting information into analog signals suitable for long-distance propagation.
The main digital channel coding methods: Hamming code, Gray code, binary code and Turbo code. [Edit this paragraph] The telecommunications industry in China has developed for 20 years, 1984, 10. 10, the Ministry of Finance decided to use 19% of the profits of enterprises affiliated to the Ministry of Posts and Telecommunications to support the development of communications.
Postal savings business resumed in April, 1986. /kloc-in October/February, the Postal Law of People's Republic of China (PRC) was promulgated.
1987165438+10, Guangzhou opened the first mobile phone office in China.
1993 September19, China's first digital mobile phone communication network was opened in Jiaxing, Zhejiang.
On April 20th, 1994, the first 64 KB/s international dedicated line connected to the Internet was opened for the first time. China public computer network construction began.
1in March, 1994, the State Council requested to further reform the performance management system and change the General Post Office and the Telecommunications Bureau into separate accounting enterprises.
In July, China United Communications Co., Ltd. was established and began to break the monopoly of the telecommunications industry.
1April, 1995, the General Administration of Telecommunications of the Ministry of Posts and Telecommunications was officially registered as an enterprise legal person.
In September, the world's first commercial CDMA mobile communication network was opened in Hong Kong.
1997 1 In June, the Ministry of Posts and Telecommunications decided to separate posts and telecommunications nationwide.
1In March, 1998, on the basis of the former Ministry of Electronics Industry and the Ministry of Posts and Telecommunications, the State Council began to set up a new Ministry of Information Industry.
In April, the newly established Ministry of Information Industry issued the Guiding Opinions on Separation of Posts and Telecommunications. The State Post Bureau was officially listed, and posts and telecommunications began to separate.
In September, the divestiture of paging service was basically completed, and Guo Xin Communication Co., Ltd. was established and then transferred to China Unicom.
1In February, 1999, the State Council established China Mobile and China Telecom Group Company through the reorganization plan of China Telecom.
In April, China Network Communication Co., Ltd. was established.
On June 5438+1October 10, 2004, China Satcom signed an agreement with Guo Xin Paging, and Unicom began to quit the paging industry.
On June 29th, 2004, Tietong Company was handed over from the Ministry of Railways to the State-owned Assets Supervision and Administration Commission, renamed as "China Tietong", and operated as a wholly state-owned basic telecom operator.
On July 20th, 2005, the State Council executive meeting discussed and approved in principle the postal system reform plan.
The Ministry of Information Industry and the National Development and Reform Commission jointly informed that some telecom tariffs will be subject to upper limit management from 10 to 1.
IPTV commercial pilot started in Shanghai; 96% of administrative villages in China have telephones.
On June 5th+February, 2006, China Mobile and China Unicom jointly launched mobile TV services with CCTV.
In February 2007, China Telecom and China Netcom signed a cooperation agreement, and the two sides stopped competing in each other's territory.
On March 7, China Internet Information Center officially launched the "CN Domain Name Unitary Experience Activity".
From June+10 in 5438, the total number of telephone users in China exceeded 900 million, of which mobile phones accounted for 60% of the total number of telephone users.