The elements of a telecommunication system are a transmitter, a channel, a line and a receiver. The transmitter is a device that transforms or codes the message into a physical phenomenon called the signal. The transmission channel, by its physical nature, is likely to modify or degrade the signal on its path from the transmitter to the receiver. The receiver has a decoding mechanism capable of recovering the message within certain limits of signal degradation by the communication channel.
Telecommunication can be point-to-point or point-to-multipoint in which case it is called conferencing. Broadcasting is a particular form of telecommunication that is point-to-multipoint and goes predominantly from the transmitter to the receivers.
The art of the telecommunications engineer is to analyse the physical properties of the transmission channel, and the statistical properties of the message, in order to design the most effective coding and decoding mechanisms.
When systems are designed to communicate through human sense organs (mainly vision and hearing), physiological and psychological characteristics of human perception will be taken into account. This has important economical implications and engineers will research how far defects may be introduced to the signal, that are perceptible to the user, but do not affect the viewing or hearing experience too badly.
Take a normal conversation between you and a friend as an example. The message is the sentence your mind decides to communicate to your friend. The transmitter is the language areas in your brain, the motor cortex, your vocal cords, the larynx, and your mouth that produce those sounds called speech. The signal is the sound waves that can be identified as speech. The channel is the air carrying those sound waves, and all the acoustic properties of the space you are in : echoes, ambient noise, reverberation. The receiver is your friend's ear, the auditory nerve, the language areas in your friend's brain that will make the difference between your voice and the sound of a car passing by, and decode your speech into, hopefully, the same sentence.
The car passing by is an example of an important property of the channel called noise. Another important aspect of the channel is called the bandwidth, and you would become very aware of the effects of a limited bandwidth if you were now talking to your friend on a telephone or a walkie-talkie.
Bell Labs scientist Claude E. Shannon published A Mathematical Theory of Communication[?] in 1948. This landmark publication was to set the mathematical models used to describe communication systems called information theory. Information theory enables us to evaluate the capacity of a communication channel according to its bandwidth and signal-to-noise ratio.
At the time of publication, telecommunication systems were predominantly based on analog electronic circuit design. The introduction of mass-produced digital integrated circuits that has enabled telecom engineers to take full advantage of information theory. From the demand of telecom circuitry, a whole specialist area of integrated circuit design has emerged, called digital signal processing.
More recent telecommunications systems, take advantage of some of these imperfections to actually improve the quality of the channel.
Modern telecommunication systems make extensive use of time synchronization, and there is a link between the development of communications and time-keeping technology. Most modern wide-area communications systems are synchronised to atomic clocks, or to secondary clocks synchronised to atomic time.
See modulation for examples of techniques for encoding information into analog signals.
Examples of telecommunications systems: