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A transmission medium (plural transmission media) is a material substance (solid, liquid, gas, or  plasma) that can propagate energy waves. For example, the transmission medium for  soundreceived by the ears is usually air, but solids and liquids may also act as transmission media for sound.

The absence of a material medium in vacuum may also constitute a transmission medium for 

electromagnetic waves such as light and radio waves. While material substance is not requiredfor electromagnetic waves to propagate, such waves are usually affected by the transmissionmedia they pass through, for instance by absorption or by reflection or refraction at the interfaces  between media.

The term transmission medium also refers to a technical device that employs the materialsubstance to transmit or guide waves. Thus, an optical fiber or a copper cable is a transmissionmedium. Not only this bt also is able to guide the transmission of networks.

A transmission medium can be classified as a:

•  Linear medium, if different waves at any particular point in the medium can besuperposed;

•  Bounded medium, if it is finite in extent, otherwise unbounded medium;

• Uniform medium or homogeneous medium, if its physical properties are unchanged atdifferent points;

•  Isotropic medium, if its physical properties are the same in different directions.

Coaxial cable, one example of a transmission medium

Electromagnetic radiation can be transmitted through an optical media, such as optical fiber , or through twisted pair wires, coaxial cable, or  dielectric-slab waveguides. It may also pass throughany 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 does other kinds of mechanical waves and heat energy. Historically, science

incorporated various aether theories to explain the transmission medium. However, it is nowknown that electromagnetic waves do not require a physical transmission medium, and so cantravel through the "vacuum" of free space. Regions of the insulative vacuum can becomeconductive for electrical conduction through the presence of free electrons, holes, or ions.

[edit] TelecommunicationsMany transmission media are used as communications channels.

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For telecommunications purposes in the United States, Federal Standard 1037C, transmissionmedia are classified as one of the following:

• Guided (or bounded)—waves are guided along a solid medium such as a transmissionline.

• Wireless (or unguided)— transmission and reception are achieved by means of an

antenna.

Wireless media may carry surface waves or skywaves, either longitudinally or  transversely, andare so classified.

In both cases, communication is in the form of electromagnetic waves. With guided transmissionmedia, the waves are guided along a physical path; examples of guided media include phonelines, twisted pair cables, coaxial cables, and optical fibers. Unguided transmission media aremethods that allow the transmission of data without the use of physical means to define the pathit takes. Examples of this include microwave, radio or infrared. Unguided media provide a meansfor transmitting electromagnetic waves but do not guide them; examples are propagation throughair, vacuum and seawater.

The term direct link is used to refer to the transmission path between two devices in whichsignals propagate directly from transmitters to receivers with no intermediate devices, other thanamplifiers or repeaters used to increase signal strength. This term can apply to both guided andunguided media.

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 onlyone 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.

[edit] See also• Vacuum permittivity

• Transmission (telecommunications)

• Excitable medium

• Duplex (telecommunications)

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