Editing Transmission medium

{{short description|Conduit for signal propagation}}
{{about|transmission of signals|biological transmission|Vector (molecular biology)}}

[[File:Coaxial cable cutaway.svg|thumb|Cutaway diagram of Coaxial cable, one example of a transmission medium]]

A '''transmission medium''' is a system or substance that can mediate the [[wave propagation|propagation]] of [[signal]]s for the purposes of [[telecommunication]]. 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 for [[sound]]s may be [[air]], but solids and liquids may also act as the transmission medium. [[Vacuum]] or air constitutes a good transmission medium for [[electromagnetic wave]]s such as [[light]] and [[radio wave]]s. While a material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance, by [[absorption (electromagnetic radiation)|absorption]] or [[Reflection (physics)|reflection]] or [[refraction]] at the [[interface (chemistry)|interface]]s 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 an [[optical medium]], such as [[optical fiber]], or through [[twisted pair]] wires, [[coaxial cable]], or [[dielectric]]-slab [[waveguide]]s.  It may also pass through any physical material that is transparent to the specific [[wavelength]], such as [[water]], [[air]], [[glass]], or [[concrete]]. [[Sound]] is, by definition, the vibration of matter, so it requires a physical medium for transmission, as do other kinds of mechanical waves and [[heat]] energy. Historically, science incorporated various [[aether 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 of [[free space]]. Regions of the [[Electrical insulation|insulative]] vacuum can become [[conductive]] for [[electrical conduction]] through the presence of free [[electron]]s, [[Electron hole|holes]], or [[ion]]s.

==Optical medium==
{{excerpt|Optical medium}}

==Telecommunications==
A physical medium in data communications is the transmission path over which a signal propagates. Many different types of transmission media are used as [[communications 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 cable]]s, and optical fibers. Unguided transmission media are methods that allow the transmission of [[data]] without the use of physical means to define the path it takes. Examples of this include [[microwave]], [[radio]] or [[infrared]]. 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.

== Simplex versus duplex ==
A [[signal transmission]] may be [[Simplex communication|simplex]], half-[[Duplex (telecommunications)|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.

== Types ==
In general, a transmission medium can be classified as
* ''linear'', if different waves at any particular point in the medium can be [[superposition principle|superposed]];
* ''bounded'', if it is finite in extent, otherwise ''unbounded'';
* ''uniform'' or ''homogeneous'', if its physical properties are unchanged at different points;
* ''isotropic'', if its physical properties are the same in different directions.

There are two main types of transmission media:
* ''guided media''—waves are guided along a solid medium such as a [[transmission line]];
* ''unguided media''—[[Transmitter|transmission]] and [[Receiver (radio)|reception]] are achieved by means of an [[Antenna (radio)|antenna]].

One of the most common physical media used in networking is [[copper wire]]. Copper wire to carry signals to long distances using relatively low amounts of power. The [[unshielded twisted pair]] (UTP) is eight strands of copper wire, organized into four pairs.<ref>{{cite book | last = Agrawal | first = Manish | title = Business Data Communications | publisher = John Wiley & Sons, Inc. | year = 2010 | page = 37 | isbn = 978-0470483367}}</ref>

=== Guided media ===
==== Twisted pair ====
{{main|Twisted pair}}
''Twisted pair'' cabling is a type of wiring in which two conductors of a single [[Electronic circuit|circuit]] are twisted together for the purposes of improving [[electromagnetic compatibility]]. Compared to a [[Single-ended signaling|single conductor]] or an untwisted [[balanced pair]], a twisted pair reduces [[electromagnetic radiation]] from the pair and [[crosstalk]] between neighboring pairs and improves rejection of external [[electromagnetic interference]]. It was invented by [[Alexander Graham Bell]].<ref>{{cite book |last1=McBee |first2=David |last2=Barnett |first3=David |last3=Groth |first1=Jim |title=Cabling : the complete guide to network wiring |date=2004 |publisher=SYBEX |location=San Francisco |isbn=9780782143317 |page=11 |edition=3rd |url=https://books.google.com/books?id=AKDSTYu3nl4C&pg=PA11}}</ref>

==== Coaxial cable ====
{{main|Coaxial cable}}
[[File:RG-59.jpg|thumb|right|{{center|[[RG-59]] '''flexible coaxial cable''' composed of:}}<ol type="A" style="list-style-type: upper-latin;"><li>Outer plastic sheath</li><li>Woven copper shield</li><li>Inner dielectric insulator</li><li>Copper core</li></ol>]]
''Coaxial cable'', or ''coax'' (pronounced {{IPAc-en|'|k|oʊ|.|æ|k|s|}}) is a type of [[electrical 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 term [[coaxial]] comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English physicist, engineer, and mathematician [[Oliver Heaviside]], who patented the design in 1880.<ref>{{cite book|last=Nahin|first=Paul J.|title=Oliver Heaviside: The Life, Work, and Times of an Electrical Genius of the Victorian Age|year=2002|isbn=0-8018-6909-9}}</ref>

Coaxial cable is a type of [[transmission line]], used to carry high [[frequency]] [[electrical signal]]s with low losses.  It is used in such applications as [[trunking|telephone trunk lines]], [[broadband internet]] networking cables, high-speed computer [[bus (computing)|data busses]], carrying [[cable television]] signals, and connecting [[radio transmitter]]s and [[radio receiver|receivers]] to their [[antenna (radio)|antenna]]s. It differs from other [[shielded cable]]s 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 ====
{{main|Optical fiber}}
[[File:fibreoptic.jpg|thumb|right|A bundle of optical fiber]]
[[File:Stealth Fiber Crew installing fiber cable underneath the streets of Manhattan.jpg|thumb|Fiber crew installing a 432-count fiber cable underneath the streets of Midtown Manhattan, New York City]]
[[File:Fiber optic illuminated.jpg|thumb|A [[TOSLINK]] fiber optic audio cable with red light being shone in one end transmits the light to the other end]]
[[File:Optical-fibre-junction-box.jpg|thumb|right|A [[19-inch rack|wall-mount cabinet]] containing optical fiber interconnects. The yellow cables are [[single-mode optical fiber|single mode fibers]]; the orange and aqua cables are [[multi-mode optical fiber|multi-mode fibers]]: 50/125&nbsp;μm OM2 and 50/125&nbsp;μm OM3 fibers respectively.]]

''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 cost approximately a dollar a foot, the same as copper.<ref>{{cite book | last = Agrawal | first = Manish | title = Business Data Communications | publisher = John Wiley & Sons, Inc. | year = 2010 | pages = 41–43 | isbn = 978-0470483367}}</ref>

[[Multi-mode optical fiber|Multimode]] and [[Single-mode optical fiber|single 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.

An ''optical fiber'' is a flexible, [[transparency and translucency|transparent]] fiber made by [[Drawing (manufacturing)|drawing]] [[glass]] ([[silica]]) or plastic to a diameter slightly thicker than that of a [[Hair's breadth|human hair]].<ref>{{cite web|title=Optical Fiber|url=http://www.thefoa.org/tech/ref/basic/fiber.html|website=www.thefoa.org|publisher=[[The Fiber Optic Association]]|access-date=17 April 2015|archive-date=24 January 2009|archive-url=https://web.archive.org/web/20090124144105/http://www.thefoa.org/tech/ref/basic/fiber.html|url-status=live}}</ref> Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in [[fiber-optic communication]]s, where they permit transmission over longer distances and at higher [[Bandwidth (computing)|bandwidths]] (data rates) than electrical cables. Fibers are used instead of [[metal]] wires because signals travel along them with less [[Attenuation|loss]]; in addition, fibers are immune to [[electromagnetic interference]], a problem from which metal wires suffer excessively.<ref>{{cite book|ref=Senior|last1=Senior|first1=John M.|last2=Jamro|first2=M. Yousif|title=Optical fiber communications: principles and practice|date=2009|publisher=Pearson Education|isbn=978-0130326812|pages=7–9}}</ref> Fibers are also used for [[Illumination (lighting)|illumination]] 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 a [[fiberscope]].<ref>{{cite web|title=Birth of Fiberscopes|url=http://www.olympus-global.com/en/corc/history/story/endo/fiber/|website=www.olympus-global.com|publisher=Olympus Corporation|access-date=17 April 2015|archive-date=9 May 2015|archive-url=https://web.archive.org/web/20150509005616/http://www.olympus-global.com/en/corc/history/story/endo/fiber/|url-status=live}}</ref> Specially designed fibers are also used for a variety of other applications, some of them being [[fiber optic sensor]]s and [[fiber laser]]s.<ref>{{cite journal|last1=Lee|first1=Byoungho|title=Review of the present status of optical fiber sensors.|journal=Optical Fiber Technology|date=2003|volume=9|issue=2|pages=57–79 |doi=10.1016/s1068-5200(02)00527-8|bibcode=2003OptFT...9...57L}}</ref>

Optical fibers typically include a [[Core (optical fiber)|core]] surrounded by a transparent [[Cladding (fiber optics)|cladding]] material with a lower [[index of refraction]]. Light is kept in the core by the phenomenon of [[total internal reflection]] which causes the fiber to act as a [[waveguide]].<ref>[[#Senior|Senior]], pp. 12–14</ref> Fibers that support many propagation paths or [[transverse mode]]s are called [[multi-mode fiber]]s, while those that support a single mode are called [[single-mode fiber]]s (SMF). Multi-mode fibers generally have a wider core diameter<ref>{{Cite book|url=https://books.google.com/books?id=HFMiAQAAMAAJ&q=Multi-mode+fibers+generally+have+a+wider+core+diameter+and+are+used+for+short-distance+communication+links|title=The Optical Industry & Systems Purchasing Directory|date=1984|publisher=Optical Publishing Company|language=en}}</ref> and are used for short-distance communication links and for applications where high power must be transmitted.{{citation needed|date=April 2015}} Single-mode fibers are used for most communication links longer than {{convert|1000|sp=us|m|ft}}.{{citation needed|date=April 2015}}

Being able to join optical fibers with low loss is important in fiber optic communication.<ref>[[#Senior|Senior]], p. 218</ref> This is more complex than joining electrical wire or cable and involves careful [[Cleave (fiber)|cleaving]] of the fibers, precise alignment of the fiber cores, and the coupling of these aligned cores. For applications that demand a permanent connection a [[fusion splice]] is common. In this technique, an electric arc is used to melt the ends of the fibers together. Another common technique is a [[mechanical splice]], where the ends of the fibers are held in contact by mechanical force. Temporary or semi-permanent connections are made by means of specialized [[optical fiber connector]]s.<ref>[[#Senior|Senior]], pp. 234–235</ref>

The field of applied science and engineering concerned with the design and application of optical fibers is known as '''fiber optics'''. The term was coined by Indian physicist [[Narinder Singh Kapany]], who is widely acknowledged as the father of fiber optics.<ref name="southasia.ucsc.edu">{{cite web|url=http://southasia.ucsc.edu/endowed-chairs/narinder-singh-kapany.html|title=Narinder Singh Kapany Chair in Opto-electronics|publisher=ucsc.edu|access-date=2019-05-06|archive-date=2017-05-21|archive-url=https://web.archive.org/web/20170521222508/http://southasia.ucsc.edu/endowed-chairs/narinder-singh-kapany.html|url-status=live}}</ref>

=== Unguided transmission media ===
==== Radio ====
{{main|Radio propagation}}

''Radio propagation'' is the behavior of [[radio wave]]s as they travel, or are [[wave propagation|propagated]], from one point to another, or into various parts of the [[atmosphere]].<ref>H. P. Westman et al., (ed), ''Reference Data for Radio Engineers, Fifth Edition'', 1968, Howard W. Sams and Co., {{ISBN|0-672-20678-1}}, Library of Congress Card No. 43-14665 page 26-1</ref> As a form of [[electromagnetic radiation]], like light waves, radio waves are affected by the phenomena of [[reflection (physics)|reflection]], [[refraction]], [[diffraction]], [[absorption (electromagnetic radiation)|absorption]], [[polarization (waves)|polarization]], and [[scattering]].<ref>Demetrius T Paris and F. Kenneth Hurd, ''Basic Electromagnetic Theory'', McGraw Hill, New York 1969 {{ISBN|0-07-048470-8}}, Chapter 8</ref>   Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international [[shortwave]] [[Broadcasting|broadcasters]], to designing reliable [[mobile telephone]] systems, to [[radio navigation]], to operation of [[radar]] 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 as [[cell phone]]s, [[cordless phone]]s, [[walkie-talkie]]s, [[wireless network]]s, [[FM radio]] and [[television broadcasting]] and [[radar]], and [[satellite communication]], such as [[satellite 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 at [[microwave]] frequencies and above. At microwave frequencies, moisture in the atmosphere ([[rain fade]]) can degrade transmission.

At lower frequencies in the [[medium frequency|MF]], [[low frequency|LF]], and [[VLF]] bands, due to [[diffraction]] radio waves can bend over obstacles like hills, and travel beyond the horizon as [[surface wave]]s which follow the contour of the Earth. These are called [[ground wave]]s.  [[AM broadcasting]] stations use ground waves to cover their listening areas. As the frequency gets lower, the attenuation with distance decreases, so [[very low frequency]] (VLF) and [[extremely 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.

At [[medium wave]] and [[shortwave]] frequencies ([[medium frequency|MF]] and [[high frequency|HF]] bands) radio waves can refract from a layer of [[charged particle]]s ([[ion]]s) high in the atmosphere, called the [[ionosphere]]. 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 called [[skywave]] propagation. It is used by [[amateur 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 of [[communication satellite]]s in the 1960s, many long-range communication that previously used skywaves now use satellites.

In addition, there are several less common radio propagation mechanisms, such as [[tropospheric scattering]] (troposcatter) and [[near vertical incidence skywave]] (NVIS) which are used in specialized communication systems.

==Digital encoding==
Transmission and reception of data is typically performed in four steps:<ref>{{cite book | last = Agrawal | first = Manish | title = Business Data Communications | publisher = John Wiley & Sons, Inc. | year = 2010 | page = 54 | isbn = 978-0470483367}}</ref>
# At the transmitting end, the data is encoded to a binary representation.
# A carrier signal is modulated as specified by the binary representation.
# At the receiving end, the carrier signal is demodulated into a binary representation.
# The data is decoded from the binary representation.

==See also==
*[[Excitable medium]]
*[[Luminiferous aether]]

==References==
{{Reflist}}

{{Telecommunications}}

[[Category:Electromagnetic radiation]]