Atransmission medium is a system or substance that can mediate thepropagation ofsignals for the purposes oftelecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulate sound, and a transmission medium forsounds may beair, but solids and liquids may also act as the transmission medium.Vacuum or air constitutes a good transmission medium forelectromagnetic waves such aslight andradio waves. While a material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission medium they pass through, for instance, byabsorption orreflection orrefraction at theinterfaces between media. Technical devices can therefore be employed to transmit or guide waves. Thus, an optical fiber or a copper cable is used as transmission media.
Electromagnetic radiation can be transmitted through anoptical medium, such asoptical fiber, or throughtwisted pair wires,coaxial cable, ordielectric-slabwaveguides. It may also pass through any physical material that is transparent to the specificwavelength, such aswater,air,glass, orconcrete.Sound is, by definition, the vibration of matter, so it requires a physical medium for transmission, as do other kinds of mechanical waves andheat energy. Historically, science incorporated variousaether theories to explain the transmission medium. However, it is now known that electromagnetic waves do not require a physical transmission medium, and so can travel through the vacuum offree space. Regions of theinsulative vacuum can becomeconductive forelectrical conduction through the presence of freeelectrons,holes, orions.
Inoptics, anoptical medium is material through whichlight and otherelectromagnetic waves propagate. It is a form of transmission medium. Thepermittivity andpermeability of the medium define how electromagnetic waves propagate in it.
A physical medium in data communications is the transmission path over which a signal propagates. Many different types of transmission media are used ascommunications channel.
In many cases, communication is in the form of electromagnetic waves. With guided transmission media, the waves are guided along a physical path; examples of guided media include phone lines,twisted pair cables,coaxial cables, and optical fibers. Unguided transmission media are methods that allow the transmission ofdata without the use of physical means to define the path it takes. Examples of this includemicrowave,radio orinfrared. Unguided media provide a means for transmitting electromagnetic waves but do not guide them; examples are propagation through air, vacuum and seawater.
The term direct link is used to refer to the transmission path between two devices in which signals propagate directly from transmitters to receivers with no intermediate devices, other than amplifiers or repeaters, used to increase signal strength. This term can apply to both guided and unguided media.
Asignal transmission may besimplex, half-duplex, or full-duplex.
In simplex transmission, signals are transmitted in only one direction; one station is a transmitter and the other is the receiver. In the half-duplex operation, both stations may transmit, but only one at a time. In full-duplex operation, both stations may transmit simultaneously. In the latter case, the medium is carrying signals in both directions at the same time.
In general, a transmission medium can be classified as
There are two main types of transmission media:
One of the most common physical medium used in networking iscopper wire. Copper wire to carry signals to long distances using relatively low amounts of power. Theunshielded twisted pair (UTP) is eight strands of copper wire, organized into four pairs.[1]
Twisted pair cabling is a type of wiring in which two conductors of a singlecircuit are twisted together for the purposes of improvingelectromagnetic compatibility. Compared to asingle conductor or an untwistedbalanced pair, a twisted pair reduceselectromagnetic radiation from the pair andcrosstalk between neighboring pairs and improves rejection of externalelectromagnetic interference. It was invented byAlexander Graham Bell.[2]
Coaxial cable, orcoax (pronounced/ˈkoʊ.æks/) is a type ofelectrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket. The termcoaxial comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English physicist, engineer, and mathematicianOliver Heaviside, who patented the design in 1880.[3]
Coaxial cable is a type oftransmission line, used to carry highfrequencyelectrical signals with low losses. It is used in such applications astelephone trunk lines,broadband internet networking cables, high-speed computerdata busses, carryingcable television signals, and connectingradio transmitters andreceivers to theirantennas. It differs from othershielded cables because the dimensions of the cable and connectors are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a transmission line.
Optical fiber, which has emerged as the most commonly used transmission medium for long-distance communications, is a thin strand of glass that guides light along its length. Four major factors favor optical fiber over copper: data rates, distance, installation, and costs. Optical fiber can carry huge amounts of data compared to copper. It can be run for hundreds of miles without the need for signal repeaters, in turn, reducing maintenance costs and improving the reliability of the communication system because repeaters are a common source of network failures. Glass is lighter than copper, allowing for less need for specialized heavy-lifting equipment when installing long-distance optical fiber. Optical fiber for indoor applications costs approximately a dollar a foot, the same as copper.[4]
Multimode andsingle mode are two types of commonly used optical fiber. Multimode fiber uses LEDs as the light source and can carry signals over shorter distances, about 2 kilometers. Single mode can carry signals over distances of tens of miles.
Anoptical fiber is a flexible,transparent fiber made bydrawingglass (silica) or plastic to a diameter slightly thicker than that of ahuman hair.[5] Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage infiber-optic communications, where they permit transmission over longer distances and at higherbandwidths (data rates) than electrical cables. Fibers are used instead ofmetal wires because signals travel along them with lessloss; in addition, fibers are immune toelectromagnetic interference, a problem from which metal wires suffer excessively.[6] Fibers are also used forillumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of afiberscope.[7] Specially designed fibers are also used for a variety of other applications, some of them beingfiber optic sensors andfiber lasers.[8]
Optical fibers typically include acore surrounded by a transparentcladding material with a lowerindex of refraction. Light is kept in the core by the phenomenon oftotal internal reflection, which causes the fiber to act as awaveguide.[9] Fibers that support many propagation paths ortransverse modes are calledmulti-mode fibers, while those that support a single mode are calledsingle-mode fibers (SMF). Multi-mode fibers generally have a wider core diameter[10] and are used for short-distance communication links and for applications where high power must be transmitted.[citation needed] Single-mode fibers are used for most communication links longer than 1,000 meters (3,300 ft).[citation needed]
Being able to join optical fibers with low loss is important in fiber optic communication.[11] This is more complex than joining electrical wire or cable and involves carefulcleaving of the fibers, precise alignment of the fiber cores, and the coupling of these aligned cores. For applications that demand a permanent connection, afusion splice is common. In this technique, an electric arc is used to melt the ends of the fibers together. Another common technique is amechanical splice, where the ends of the fibers are held in contact by mechanical force. Temporary or semi-permanent connections are made by means of specializedoptical fiber connectors.[12]
The field of applied science and engineering concerned with the design and application of optical fibers is known asfiber optics. The term was coined by Indian physicistNarinder Singh Kapany, who is widely acknowledged as the father of fiber optics.[13]
Radio propagation is the behavior ofradio waves as they travel, or arepropagated, from one point to another, or into various parts of theatmosphere.[14] As a form ofelectromagnetic radiation, like light waves, radio waves are affected by the phenomena ofreflection,refraction,diffraction,absorption,polarization, andscattering.[15] Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for internationalshortwavebroadcasters, to designing reliablemobile telephone systems, toradio navigation, to operation ofradar systems.
Different types of propagation are used in practical radio transmission systems.Line-of-sight propagation means radio waves that travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used to medium-range radio transmission such ascell phones,cordless phones,walkie-talkies,wireless networks,FM radio andtelevision broadcasting andradar, andsatellite communication, such assatellite television. Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas. It is the only propagation method possible atmicrowave frequencies and above. At microwave frequencies, moisture in the atmosphere (rain fade) can degrade transmission.
At lower frequencies in theMF,LF, andVLF bands, due todiffraction, radio waves can bend over obstacles like hills, and travel beyond the horizon assurface waves which follow the contour of the Earth. These are calledground waves.AM broadcasting stations use ground waves to cover their listening areas. As the frequency gets lower, the attenuation with distance decreases, sovery low frequency (VLF) andextremely low frequency (ELF) ground waves can be used to communicate worldwide. VLF and ELF waves can penetrate significant distances through water and earth, and these frequencies are used for mine communication and military communication with submerged submarines.
Atmedium wave andshortwave frequencies (MF andHF bands) radio waves can refract from a layer ofcharged particles (ions) high in the atmosphere, called theionosphere. This means that radio waves transmitted at an angle into the sky can be reflected back to Earth beyond the horizon, at great distances, even transcontinental distances. This is calledskywave propagation. It is used byamateur radio operators to talk to other countries and shortwave broadcasting stations that broadcast internationally. Skywave communication is variable, dependent on conditions in the upper atmosphere; it is most reliable at night and in the winter. Due to its unreliability, since the advent ofcommunication satellites in the 1960s, many long-range communications that previously used skywaves now use satellites.
In addition, there are several less common radio propagation mechanisms, such astropospheric scattering (troposcatter) andnear vertical incidence skywave (NVIS), which are used in specialized communication systems.
Transmission and reception of data is typically performed in four steps:[16]
