Transmission Media and Electromagnetic Signals

The transmission media is considered lower than the physical layer and is directly controlled by it. Transmission media is said to be at layer zero. Computers and other devices send data using electromagnetic signals which travel through the media.

Electromagnetic signals are a combination of electric and magnetic fields which oscillate perpendicular to one another. Electromagnetic signals or waves are named according to frequency – radio waves, infrared, visible light, microwaves, ultraviolet light, X rays, gamma rays and cosmic rays. All electromagnetic waves of all frequencies comprise the Electromagnetic Spectrum. Not all portions of the electromagnetic spectrum are usable for telecommunication. It should be noted that all electromagnetic signals can travel through a vacuum. In other words, these waves do not require media. Electromagnetic signals also travel the fastest in a vacuum at exactly 299,792,458 m/s which is known as the speed of light.

The speed of electromagnetic waves that travel through a medium is described by the refractive index (RI). RI = velocity of light in a vacuum / velocity of light in medium

Data communication authors classify transmission media into two types – guided and unguided media. Guided media force electromagnetic waves to travel through a conductor by encasing the conductor with an insulator. Guided media include twisted-pair cable, coaxial cable and fiber-optic cable. Unguided media transport signals without conductors and are normally broadcasted through air. It should be noted that ‘unguided media’ does not mean that the signals sent through them cannot be directed. Electromagnetic waves used in unguided media are microwaves, radio waves and infrared.

Transmission through unguided media is also referred to as wireless communication. Wireless communication may also refer to electromagnetic signals that are transmitted through vacuum.