The most commonly used channels in modern communication networks
synonymTelephone cable(Telephone cable) generally refers to wired channel
Wired channel refers to visible media such as open wire, symmetrical cable, coaxial cable, optical cable and waveguide.Wired channel is one of the most commonly used channels in modern communication networks.For example, symmetrical cable (also called telephone cable) is widely used for short distance transmission.
Open line refers to the overhead line that is erected on the poles in parallel.It is conductive bare wire or wire with insulating layer.Although its transmission loss is low, it is sensitive to external noise interference due to its vulnerability to weather and environment, and has been gradually replaced by cables.
Symmetrical cable
There are two types of cables, namely symmetrical cables and coaxial cables.Symmetrical cable is made of several pairs of twin wires called core wires in a protective sleeve. In order to reduce the interference between each pair of wires, each pair of wires is made into a twisted shape, called twisted pair. The pairs of wires in the same cable are also twisted together according to certain rules. In telecommunications networks, there are usually 25 pairs of twisted pairs in a symmetrical cable,The core diameter of symmetrical cable is 0.4mm~1.4mm, with large loss but stable performance.Symmetrical cable is widely used in subscriber access circuit in wired telephone network. Each subscriber telephone is connected toTelephone exchangeUsually, the twisted pair with wire gauge of 22~26 is used.Twisted pair has also been widely used in computer LAN. The super five types of wires used in Ethernet are composed of four twisted pairs.
Coaxial cable
The coaxial cable is composed of two concentric cylinders inside and outside, and the two conductors are separated by insulators.The inner conductor is mostly a solid conductor, and the outer conductor is a hollow conductive pipe or metal woven mesh. There is an insulation protective layer outside the outer conductor. The solid dielectric material fire insulation support can be filled between the inner and outer conductors to support and insulate.Since the outer conductor is usually grounded, it can play a very good shielding role.With the wide application of optical fiber, most trunk lines for long-distance signal transmission use optical fiber instead of coaxial cable. In CATV (Cable Television), coaxial cable is also widely used to provide TV signals for usersSPC telephone exchangeThe PCM group signal in the middle still uses coaxial cable to transmit signal, and the coaxial cable is also used as the transmission medium for the internal IF and RF parts of the communication equipment, such as the feeder connecting the wireless communication transceiver and the antenna.
Optical fiber
The wired channel for transmitting optical signals islight-guide fiber, referred to as optical fiber.Modern optical fiber was improved by Charles Kuen, a Chinese scientist, who is considered as the "father of optical fiber".In 1970, Corning Corporation of the United States produced the first practical optical fiber in the world. With the continuous improvement of processing and manufacturing technology, the attenuation of optical fiber continues to decline. Trunk transmission networks around the world are mainly composed of optical fibers.
Optical signal transmission in optical fiber is based onPrinciple of total reflectionThe optical fiber can be divided into multimode fiber (MMF) and single mode fiber (SMF). The optical signal in multimode fiber has multiple propagation modes, while there is only one propagation mode in single-mode fiber.The signal light source of optical fiber can include LED (Light Emitted Diode) and laser.The optical wavelength used in practical applications is mainly in the 1.31 and 1.55 low loss wavelength windows. For example, the 1000Base LX physical interface in the Ethernet network uses 1.31 wavelength optical signals.There are also 850 nm signal light sources in the computer LAN, such as the 1000Base SX physical interface in the Ethernet network.The spectral purity of LED light source is low, and optical signals of different wavelengths travel at different speeds in the optical fiber. Therefore, with the increase of distance, optical signal propagation will be dispersive, causing signal distortion and limiting the distance of optical fiber transmission. Therefore, for long-distance transmission, signals need to be relayed every other distance.The dispersion of single-mode fiber is much smaller than that of multimode fiber (there is modal dispersion in multimode fiber), so the transmission distance without repeater is longer, and the dispersion caused by the transmission of laser source with high spectral purity is smaller.[1]
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Wired channels use wires as the transmission medium, and signals are transmitted along the wires. The energy of signals is concentrated near the wires, so the transmission efficiency is high, but the deployment is not flexible enough.The transmission media used in this type of channel include overhead open wires, telephone lines, twisted pairs, symmetrical cables and coaxial cables, etc., which transmit electrical signals by wires, as well as those used to transmit modulated optical pulse signalslight-guide fiber。[2]
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The transmission medium is wire (twisted pair or optical fiber, etc.), the signal is transmitted along the wire, and the energy is relatively concentrated near the wire, so it has high transmission efficiency.[3]High signal-to-noise ratio, narrow bandwidth resources, echo andnonlinear distortion 。[4]
