Transmission medium

Source: Wikipedia, the free encyclopedia.
This article is about transmission of signals. For biological transmission, see Vector (molecular biology).
Cutaway diagram of Coaxial cable, one example of a transmission medium

A transmission medium is a system or substance that can mediate the

interfaces
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.

electrical conduction through the presence of free electrons, holes, or ions
.

Optical medium

This section is an excerpt from Optical medium.[edit]
In
electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability
of the medium define how electromagnetic waves propagate in it.

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 cables, 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

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 the same time.

Types

In general, a transmission medium can be classified as

There are two main types of transmission media:

  • guided media—waves are guided along a solid medium such as a transmission line;
  • unguided media—
    reception are achieved by means of an antenna
    .

One of the most common physical media used in networking is

unshielded twisted pair (UTP) is eight strands of copper wire, organized into four pairs.[1]

Guided media

Twisted pair

Main article: Twisted pair

Twisted pair cabling is a type of wiring in which two conductors of a single

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.[2]

Coaxial cable

Main article: Coaxial cable
RG-59 flexible coaxial cable composed of:
  1. Outer plastic sheath
  2. Woven copper shield
  3. Inner dielectric insulator
  4. Copper core

Coaxial cable, or coax (pronounced /ˈkoʊ.æks/) 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.[3]

Coaxial cable is a type of

radio transmitters and receivers to their antennas. It differs from other shielded 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

Main article: Optical fiber
A bundle of optical fiber
Fiber crew installing a 432-count fiber cable underneath the streets of Midtown Manhattan, New York City
A TOSLINK fiber optic audio cable with red light being shone in one end transmits the light to the other end
A wall-mount cabinet containing optical fiber interconnects. The yellow cables are single mode fibers; the orange and aqua cables are multi-mode fibers: 50/125 µm OM2 and 50/125 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.[4]

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.

An optical fiber is a flexible,

fiber optic sensors and fiber lasers.[8]

Optical fibers typically include a

single-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 careful 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 connectors.[12]

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.[13]

Unguided media

Radio

Main article: Radio propagation

Radio propagation is the behavior of

shortwave 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.

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 MF, LF, and VLF bands, due to diffraction radio waves can bend over obstacles like hills, and travel beyond the horizon as surface waves which follow the contour of the Earth. These are called ground waves. 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

communication satellites
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:[16]

  1. At the transmitting end, the data is encoded to a binary representation.
  2. A carrier signal is modulated as specified by the binary representation.
  3. At the receiving end, the carrier signal is demodulated into a binary representation.
  4. The data is decoded from the binary representation.

See also

References

  1. ISBN 978-0470483367
    .
  2. ISBN 9780782143317
    .
  3. ISBN 0-8018-6909-9
    .
  4. ISBN 978-0470483367
    .
  5. ^ "Optical Fiber". www.thefoa.org. The Fiber Optic Association. Archived from the original on 24 January 2009. Retrieved 17 April 2015.
  6. ISBN 978-0130326812
    .
  7. ^ "Birth of Fiberscopes". www.olympus-global.com. Olympus Corporation. Archived from the original on 9 May 2015. Retrieved 17 April 2015.
  8. doi:10.1016/s1068-5200(02)00527-8
    .
  9. ^ Senior, pp. 12–14
  10. ^ The Optical Industry & Systems Purchasing Directory. Optical Publishing Company. 1984.
  11. ^ Senior, p. 218
  12. ^ Senior, pp. 234–235
  13. ^ "Narinder Singh Kapany Chair in Opto-electronics". ucsc.edu. Archived from the original on 2017-05-21. Retrieved 2019-05-06.
  14. ISBN 0-672-20678-1
    , Library of Congress Card No. 43-14665 page 26-1
  15. ISBN 0-07-048470-8
    , Chapter 8
  16. ISBN 978-0470483367
    .
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