Telecommunications is the scienze that studies electonic communicarions systems, and the devices that transmit and receive messages.
Modern telecommunications systems are based on devices such as telephones, radios, computers, which can send and receive images, sound, and other kinds of data.
These devices transmit messages through wires or cables or by radio waves; for example, the telephone sends and receives signals through cables, that are laid underground.
Moreover, telephone communications are also transmitted using microwaves, that are sent from one satellite to another.
There are two different way to transmit information: analogue and digital.
Analogue transissions use signals without modifications; for example, an analogue telephone system transmits and receives an electric current that copies the pattern of the sound waves of the speaker's voice. This current travels over the wires and at the end it is converted back to sounds waves for the receiver.
On the contrary, in digital transmissions, the signals are converted into a code, that consists in two elements: 0 and 1, which can be represented by the on-off flashing of a led.
In most digital telecommunications systems, the coded signals are transmitted by a beam of light, which travels through an optical fibre cable, and at the end the code is converted for the receiver.
Digital systems are more efficient than analogue because they produces less noise and less distortion.