1. Classified by communication service
According to the different types of communication services, communication systems can be divided into telegraph communication systems, telephone communication systems, data communication systems, image communication systems, etc. Because the telephone communication network is the most developed and popular, some other communication services are often transmitted through the public telephone communication network, such as telegraph communications and long-distance data communications can be transmitted through the telephone channel. Integrated service digital communication network is suitable for information transmission of various types of services.
2. Classified by modulation mode
According to whether the signal transmitted in the channel is modulated or not, the communication system can be divided into baseband transmission system and bandpass transmission system. Baseband transmission is the direct transmission of unmodulated signals, such as local telephone, cable broadcast; Bandpass transmission is a general term for the modulated transmission of various signals. There are many modulation methods, and Table 1-1 lists some common modulation methods.
3. Classification according to signal characteristics
According to the transmission channel is analog signal or digital signal, the communication system is divided into analog communication system and digital communication system.
4, according to the transmission medium classification
According to the transmission medium, the communication system can be divided into wired communication system and wireless communication system. Wired communication is the use of wire (such as overhead open wire, coaxial cable, optical fiber, waveguide, etc.) as a transmission medium to complete the communication, such as city telephone, cable TV, submarine cable communication. Wireless communication relies on electromagnetic wave propagation in space to achieve the purpose of transmitting information, such as short-wave ionospheric propagation, microwave line-of-sight propagation, satellite relay and so on.
5, according to the working band classification
According to the working frequency or wavelength of the communication equipment, it is divided into long wave communication, medium wave communication, short wave communication, far infrared communication and so on.
6, according to the signal multiplexing classification
There are three basic multiplexing modes for transmitting multiplex signals, namely frequency division multiplexing, time division multiplexing and code division multiplexing. Frequency division multiplexing is to make different signals occupy different frequency ranges by means of spectrum shifting. Time division multiplexing uses pulse modulation to make different signals occupy different time intervals. Code division multiplexing is the use of orthogonal encoding to carry different signals. Frequency division multiplexing is used in traditional analog communication. With the development of digital communication, time division multiplexing is used more and more widely. Code division multiplexing is used in space communication spread spectrum communication and mobile communication system. In addition, there are wavelength division multiplexing, space division multiplexing.