A computer transmission medium
A transmission medium is something that can mediate the propagation of signals 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 sounds may be air , but solids and liquids may also act as the transmission medium.
Vacuum or air constitutes a good transmission medium for electromagnetic waves such as light and radio waves . While 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 or by reflection or
refraction at the interfaces between media. Technical devices can therefore be employed to transmit or guide waves. Thus, an optical fiber or a copper cable are 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 waveguides . 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 insulative vacuum can become
conductive for electrical conduction through the presence of free electrons , holes , or ions .
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 forms of 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 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 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.
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
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, transparent fiber made by drawing glass ( silica ) or plastic to a diameter slightly thicker than that of a human hair . 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 communications, where they permit transmission over longer distances and at higher bandwidths (data rates) than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference , a problem from which metal wires suffer excessively. Fibers are also used for 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 . Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers
Optical fibers typically include a core surrounded by a transparent 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. Fibers that support many propagation paths or
transverse modes are called multi-mode fibers , while those that support a single mode are called
single-mode fibers (SMF). Multi-mode fibers generally have a wider core .