Channels can be divided into wired channels and wireless channels. Wired channels: such as twisted pair, cable, fiber, waveguide, etc.; Wireless channel: An electromagnetic wave propagation channel of various frequency bands or wavelengths provided by free space. Signals are transmitted within the channel and are subject to a wide variety of interference from the channel. Interference is roughly divided into 4 categories: 1. Radio interference From a variety of wireless transmitters. It is characterized by a wide frequency range covering almost all frequency bands used. However, the frequency of a particular station is generally fixed, so protection can be applied. In addition, due to the perfect radio frequency management, such interference can be minimized. 2. Industrial interference From a variety of electrical equipment, such as motors, power lines, power switches, electric ignition (such as car ignition) devices. Such interference is generally in the lower frequency range, such as car ignition interference in the range of tens of megahertz. The use of shielding and sophisticated filtering measures can largely avoid industrial interference. 3. Sky interference From lightning, magnetic storms, sunspots, and cosmic rays, they have a lot to do with seasonal and climate changes. Different regions are also very different, such as near the equator and in the polar regions. In the year when sunspots change (about 11 years and one cycle), the interference from the sky is increased, and short-wave communication is sometimes interrupted for a long time. 4. Internal interference From the various internal device resistances, antennas, transmission lines, etc. inside the channel. The random thermal motion of molecules or electrons in these electronic devices creates so-called undulating noise that produces additive interference to the communication signal. The various communication systems involved in this book are mainly such noises, called thermal noise. From the mechanism, it is a Gaussian statistical feature and an important factor in communication system interference. There are four main types of channels commonly used for communication: 1. Telephone channel A telephone channel generally refers to a channel based on a conventional analog telephone or low-speed data transmission provided by a large public switched telephone network (PSTN). The communication channel is mostly composed of a user terminal to a local exchange (node) and then to a call link established by another user. Once the call (ie, the call) ends, the link is disconnected in time. The telephone channel is generally a linear system with a band limit of 300~3400Hz. When used for data transmission, it is necessary to add a modem to the user terminal, and transmit the modulated wave using a relatively flat frequency band of 600~3000 Hz. At present, the user line from the user to the node (switch) can be equipped with a broadband modem (such as ADSL), which can extend the communication bandwidth to 2~6MHz within a few kilometers, and supports broadband Internet access and multimedia services. 2, fiber An optical fiber is a physical medium that converts an electrical signal into an optical signal (electrical/optical conversion) and then transmits the optical signal. The optical cable is composed of a cladding covering the optical fiber core. The optical signal propagates in the core of the fiber in the form of an electromagnetic field. Since the fiber was put into use in the 1970s, it has quickly shown many outstanding advantages, such as its extremely wide bandwidth (2&TImes; above 1014 Hz), through the currently available technology - Dense Wavelength Division Multiplexing (DWDM), one fiber It can support a transmission rate of 1600Gb/s; experiments show that based on a single wave 160Gb/s rate of 1024 waves, up to about 160Tb/s point-to-point transmission traffic (1Tb/s = 1012b/s); fiber transmission loss Very low, less than 0.2dB/km, free from electromagnetic interference, extremely light weight (one fiber core 27g/km), resistant to bending, heat and corrosion resistance, easy to lay and flexible, can be erected on poles, and the price of fiber is extremely low At present, domestic production exceeds demand. 3. Mobile wireless channel Mobile wireless communication was initially developed to extend the coverage and communication capabilities of the telecommunications network. So far, the development of mobile networks has been amazing, and has expanded from cities to rural and remote areas. As of May 2009, the number of mobile users in China has reached 687 million, twice the number of traditional fixed-line (PSTN) users. The next generation of mobile communications will be accessed in a broadband manner, and the transmission rate will increase by several tens of times and hundreds of times compared with the current GSM system, support multimedia services, and widely implement personal communication systems (PCS). Mobile communication is networked in a cellular manner, and the channel has multipath fading and time varying characteristics. At present, GSM mobile communication utilizes the 900/1800 MHz frequency band, and the bidirectional spectrum is 2 & TImes; 25 MHz, each providing 125 carriers, and each carrier includes 8 time division multiple access (TDMA) channels. Thus, each carrier has a 200 kHz bandwidth, including eight 25 kHz channels, and the user's digital telephone rate is 13 kb/s. When the error protection bit with high error correction capability is used, the net rate is as high as 9.8 kb/s. The mobile network has one base station in each cell, and up to thousands of users in the forwarding cell simultaneously communicate. The radiated power of each channel of mobile communication should be controlled within its 25 kHz bandwidth. The attenuation of out-of-band radiation should be at least -40 dB, and the quality system should reach -70 dB, so as not to significantly interfere with adjacent channels. Isolating 1 to 2 cells can reuse frequencies, so the entire mobile network can support a very large number of users to communicate with each other. National and worldwide mobile communications can be achieved through handoffs and inter-network roaming. 4, satellite channel A satellite channel is a special type of wireless channel. At 35,978 km (approximately 36,000 km) above the Earth's equator, three simultaneous satellites are evenly distributed, allowing them to communicate between two earth stations on the Earth through their transponders. Since the advent of the early 1960s (1962), the system has been used up to 6 GHz upstream and 4 GHz downstream, with a total bandwidth of 500 MHz, and provides 12 transponders each with a bandwidth of 36 MHz, each capable of accommodating 1200 digital telephones or 25~150 narrowband conference TVs. One transponder can support five or six HDTVs (High Definition Digital TV). Because transoceanic satellite communications (such as between China and the United States) need to communicate with the two earth stations via two satellite transponders, the distance is as high as 150,000 km, and the communication delay is as high as 0.5 seconds. The feeling of delay. At present, domestic satellite communications have opened a large number of services, such as satellite TV programs and distance education. In order to use frequency resources reasonably, the various communication channels in each region do not interfere with each other, save the frequency band as much as possible, and meet the requirements of validity and reliability transmission. The International Telecommunications Union (ITU) scientifically allocates the entire communication frequency band, as shown in Table 1. Show. Table 1 Communication band division Hdmi Cables,Hdmi Cord,Micro Hdmi Cable,Extra Long Hdmi Cable UCOAX , https://www.ucoax.com
First, several common channel characteristics 