Telecommunication systems have many key concepts with it. First, the basic system consists of three primary units that are always present in some form. There should be a transmitter, an electronic device which, with the aid of an antenna, produces radio waves. It also takes information and converts it to a signal, or any stream of quantities in time or spatial sequence.
A telecommunication system should also have a transmission medium, also called the “physical channel” that carries the signal. It could be any material (solid, liquid, gas, or plasma) as long as it can propagate energy waves. An example is the “free space channel,” an optical communication technology that uses light propagating in free space to transmit data for telecommunications or computer networking. A receiver, an electronic device that receives radio waves and converts the information carried by them to a usable form, of course, is also needed.
Second, communication signals can be either by analog signals or digital signals. Analog signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity like analogous to another time varying signal. Digital signal, on the other hand, is a physical signal that is a representation of a sequence of discrete values, a quantified discrete-time signal, like an arbitrary bit stream, or of a digitized (sampled and analog-to-digital converted) analog signal.
Third, there is a communications network which is a collection of transmitters, receivers, and communication channels, referring either to a physical transmission medium like a wire, or to a logical connection over a multiplexed medium such as a radio channel, that send messages to one another. It is a collection of terminals, links and nodes which connect to enable telecommunication between users of the terminals.
Fourth is the communication channels with two different meanings. The first meaning is that it is the physical medium that carries a signal between the transmitter and the receiver. The second one is seen in the phrase communications channel, which is a subdivision of a transmission medium so that it can be used to send multiple streams of information simultaneously. This refers either to a physical transmission medium like a wire, or to a logical connection over a multiplexed medium such as a radio channel.
Lastly, telecommunication system’s last key concept is modulation, or the shaping of a signal to convey information. It is the process of varying one or more properties of a high-frequency periodic waveform, called the “carrying signal,” with a “modulating signal” which typically contains information to be transmitted. Source