Преносна среда: Разлика между версии
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Ред 1:
'''Преносна среда''' в разпространението на [[сигнал]]и се нарича всяко [[вещество]] (независимо от [[Агрегатно състояние|агрегатното състояние]]
[[Файл:RG-59.jpg|thumb|right|[[Коаксиален кабел]], един от примерите за '''преносна среда''']]
[[Електромагнитно излъчване|Електромагнитното излъчване]] може да се пренася по оптична среда като [[оптично влакно]] или по [[проводник]] (усукана двойка), [[коаксиален кабел]] или по [[диелектрик|диелектричен]] [[вълновод]]. То може да преминава и през вещество, прозрачно за конкретната [[дължина на вълната]], например [[вода]], [[въздух]], [[стъкло]] (за видимата светлина) или [[бетон]]. Науката е разглеждала и теоретични преносни среди като [[етер]]а, с който през 19 век учените са се опитвали да обяснят преноса на енергия. В днешно време е установено, че електромагнитната енергия може да се пренася във вакуум или в [[Космос]]а.
Ред 12:
# Данните се [[Кодиране|кодират]] в [[двоичен код]] от страната на изпращача
# Носещият сигнал се [[Модулация (техника)|модулира]] според двоичното представяне на данните
# От страната на получателя пристигащият сигнал се демодулира и се превръща в двоични числа
# Извършва се декодиране<ref>{{cite book | last = Agrawal | first = Manish | title = Business Data Communications | publisher = John Wiley & Sons, Inc. | year = 2010 | page = 54 | ISBN = 0470483369}}</ref>
==Телекомуникации==
При пренос на данни с цел комуникация под преносна среда се има пред вид физическата среда на разпространение на сигнали по [[Комуникационен канал|комуникационни канали]].
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For telecommunications purposes in the United States, [[Federal Standard 1037C]], transmission media are classified as one of the following:
* Guided (or bounded)—waves are guided along a solid medium such as a [[transmission line]].
* Wireless (or unguided)—[[transmitter|transmission]] and [[Receiver (radio)|reception]] are achieved by means of an [[antenna (radio)|antenna]].
One of the most common physical medias used in networking is [[Copper wire and cable|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 = 0470483369}}</ref>
Another example of a physical medium is [[optical fiber]], which has emerged as the most commonly used transmission medium for long-distance communications. Optical fiber 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 = 0470483369}}</ref>
Multimode and 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.
Wireless media may carry [[surface waves]] or [[skywave]]s, either [[Longitudinal wave|longitudinal]]ly or [[Transverse wave|transverse]]ly, and are so classified.
In both communications, 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.
=== Types of transmissions ===
A transmission may be simplex, 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 same time.
There are two types of transmission media:
*Guided
*Unguided
Guided Media:
*Un-shielded Twisted Pair (UTP)
*Shielded Twisted Pair
*Coaxial Cable
*Optical Fiber
Unguided Media: Transmission media then looking at analysis of using them unguided transmission media is data signals that flow through the air. They are not guided or bound to a channel to follow. Following are unguided media used for data communication:
*Radio Transmission
*Microwave -->
== Източници ==
<references />
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