Signal refers to both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology.
In The Signal by William Powell Frith, a woman sends a signal by waving a white handkerchief.
A Finnish distant signal at the western approach to Muhos station is displaying Expect Stop.
A woman hailing a cab is sending a signal of availability to be picked up.
A flare is a common means to signal during dark or smoke-filled conditions.
A transmission medium is a system or substance 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 a 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 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 is used as transmission media.
RG-59 flexible coaxial cable composed of: Outer plastic sheath Woven copper shield Inner dielectric insulator Copper core
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