Know the transmission medium, the medium of network communication

Purple represents the primary directory
Pink represents the secondary directory
Blue represents the third level directory
Red represents keywords
Orange represents description

Common transmission media
　　 Twisted pair
　　 Coaxial cable
　　 optical cable
　　 Wireless and satellite communication channels
Twisted pair
　　 The most commonly used transmission medium .
　　 Two copper insulated conductors , The two wires are twisted together , It can reduce the interference to adjacent line pairs . The cable composed of several pairs of twisted pair is called twisted pair cable .
　　 Can be used for Analog transmission , It can also be used for The data transfer .
　　 Unshielded twisted pair UTP（Unshielded Twisted Pair）（ Yes 3 Classes and 5 class ）, Shielded twisted pair （STP）.
　　 Sensitive to electromagnetic interference , also You can't Support very high-speed data transmission .
Coaxial cable
　　 Good anti-interference , Higher bandwidth . The inner and outer conductors are coaxial , All of them are made of copper .
　　 Baseband coaxial cable
　　　　50 ohm , For digital data transmission , Such as computer LAN , It was widely used in the early stage of LAN development .
　　　　 The bandwidth depends on the cable length .
　　 Broadband coaxial cable
　　　　75 ohm , For analog transmission .
　　　　Cable TV technology ,300MHz or 450MHz, the height is 900MHz.
　　　　 It is mainly used in residential areas of cable TV Networks .
Optical fiber
　　 Optical waveguide material （ Fiber core , Cladding , coat ）, One optical cable can contain multiple optical fibers .
　　 The refractive index of the core higher than Cladding , When the incident angle of light on the fiber core Greater than Critical value occurs Total reflection phenomenon .
　　 Composition of optical transmission system
　　　　 Optical fiber transmission medium 、 The light source 、 detector .
　　　　 The light source ：
　　　　　　 Light emitting diode or laser diode , Light pulses are emitted when power is on .
　　　　 detector ：
　　　　　　 photodiode , When exposed to light , Generate an electrical pulse .
　　　　 Expressed by the appearance of light pulses “1”, The absence of indicates “0”.
　　　　 at present , In the laboratory, the optical fiber bandwidth exceeds 50Tbps; Practical optical fiber 100 Gbps.
　　 How optical fibers work
　　
　　 Optical fiber classification ： Single mode fiber and multimode fiber
　　　　 Pattern （mode）： Is a quantity related to many parameters , Different light rays propagate in different modes inside the medium
　　　　 Multimode fiber
　　　　　　 The core is thick , There may be Varied The mode of simultaneous transmission along different routes .
　　　　 Single mode fiber
　　　　　　 When reducing the core diameter to the wavelength of light wave , Transmission of light in optical fiber No reflection , And travel in a straight line .
　　　　 Comparison between single-mode fiber and multimode fiber
　　　　　　
　　　　 At present, the single-mode optical fiber system actually used can 100Gbps Rate transmission of 100km, And there is no need to relay .
　　 Advantages of optical fiber transmission
　　　　 Wide frequency band , Large transmission capacity .
　　　　 Low attenuation , Long transmission distance （ about 100km, Copper wire approx 5km）.
　　　　 Light weight , Small size .
　　　　 Free from electromagnetic interference and electrostatic interference .
　　　　 Strong security and confidentiality .
　　　　 Rich in raw materials , Low production cost .
Wireless and satellite communication
　　 Propagation mode of electromagnetic wave ：
　　　　 One is to spread in free space , That is, through Wireless mode spread ; The other is to spread in a limited space , That is, through Wired mode spread .
　　 From the electromagnetic spectrum ：
　　　　 wireless , microwave , Infrared , Visible light , Ultraviolet (uv) ,x- ray ,γ- ray
　　　　（ frequency ） low 　　　　　　　　　　　　　　　　　　 high
　　 Different transmission media can transmit electromagnetic wave signals of different frequencies , Twisted pair It can transmit low frequency – If signal , Coaxial cable It can transmit low frequency – UHF signal , Optical fiber It can transmit near-infrared and visible light signals .
　　 Wireless communication system By the sending device 、 Receiving equipment 、 Wireless channel composition , utilize Wireless electromagnetic wave , Realize data transmission . It is divided into medium wave communication according to the use frequency 、 Short wave communication 、 Ultrashort wave communication 、 Microwave and satellite communications .
　　 Communication between fixed objects can be used Wired transmission media , You can also use Wireless communication system To achieve .
　　 Realizing mobile communication requires Wireless communication system .
　　 Packet radio network It's a use of Wireless channel Conduct Packet switching Communication network of .
Microwave communication
　　 microwave ： The wavelength is 1m—1mm, Or the frequency is 300MHz—300GHz Electromagnetic waves in the range .
　　 Microwave communication ： Communication with electromagnetic waves in the microwave band .
　　 Characteristics of microwave communication ：
　　　　 The bandwidth of microwave band . Large communication capacity , There are many communication devices that can work at the same time , And it can reduce the interference between devices .
　　　　 Suitable for transmitting broadband signals . A set of microwave communication equipment can hold thousands of sessions working at the same time , Or used to transmit broadband signals such as TV images .
　　　　 High antenna gain . When the antenna area is given , Its gain is inversely proportional to the square of the wavelength . The wavelength of microwave band is short , It's easy to make a high gain antenna .
　　　　 Small external interference . The frequency is higher than 100MHz when , All kinds of interference, such as industrial interference 、 The interference of celestial electricity and the change of sunspots basically do not work . The stability and reliability of communication transmission are guaranteed .
　　　　 Less investment , Be quick , Flexibility .
　　　　 Relay communication mode .（ There are many relay stations ）
　　　　 It can transmit digital and analog signals .
Satellite communications
　　 Between the microwave communication stations on the earth, artificial earth satellites are used for Relay station And communication .
　　 There are many on the satellite Transponder The function of is to receive 、 Zoom in and send messages . Several transponders have one 36MHz Channel of , Different transponders use different frequencies , The ground transmitting station uses uplink Transmit microwave signals to communication satellites ; Satellite reception 、 Zoom in 、 frequency conversion , Reuse downlink Send back to the ground receiving station . uplink and downlink Use different frequencies .
　　 Characteristics of satellite communication ：
　　　　 The coverage area is large , Communication distance is far .
　　　　 It has multiple access connection characteristics .
　　　　 Satellite communication is flexible .
　　　　 Wide frequency band , Large communication capacity .
　　　　 The communication line is stable , Quality , High reliability , The operation rate of the system is high .
　　　　 It can be collected spontaneously , Conducive to detection .
　　　　 Cost has nothing to do with distance .
　　 Disadvantages of satellite communication ：
　　　　 Be highly reliable 、 Long life communication satellite .
　　　　 The earth station is required to have a high-power transmitter , High sensitivity receiver and high gain antenna .
　　　　 There is large signal delay and echo interference .
　　　　　　 one-way 8 Thousands of kilometers , Back and forth 16 Thousands of kilometers , About demand 0.5 About seconds . Electromagnetic waves travel in vacuum at the speed of light per second 30 Thousands of kilometers .
Wireless LAN uses ISM Frequency band