Telecommunication is the extension of communication over a distance. In practice it also recognizes that something may be lost in the process; hence the term 'telecommunication' covers all forms of distance and/or conversion of the original communications, including radio, telegraphy, television, telephony, data communication and computer networking.
The elements of a telecommunication system are a transmitter, a medium (line) and possibly a channel imposed upon the medium (see baseband and broadband as well as multiplexing), and a receiver. The transmitter is a device that transforms or encodes the message into a physical phenomenon; the signal. The transmission medium, 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. In some cases, the final "receiver" is the human eye and/or ear (or in some extreme cases other sense organs) and the recovery of the message is done by the brain (see psychoacoustics.)
Telecommunication can be point-to-point, point-to-multipoint or broadcasting, which is a particular form of point-to-multipoint that goes only from the transmitter to the receivers.
One of the roles of the telecommunications engineer is to analyse the physical properties of the line or transmission medium, and the statistical properties of the message in order to design the most effective encoding 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 economic implications and engineers will research what defects may be tolerated in the signal yet not affect the viewing or hearing experience too badly.
Examples of human (tele)communications
In a simplistic example, consider a normal conversation between two people. The message is the sentence that the speaker decides to communicate to the listener. The transmitter is the language areas in the brain, the motor cortex, the vocal cords, the larynx, and the mouth that produce those sounds called speech. The signal is the sound waves (pressure fluctuations in air particles) that can be identified as speech. The channel is the air carrying those sound waves, and all the acoustic properties of the surrounding space: echoes, ambient noise, reverberation. Between the speaker and the listener (the receiver), might be other devices that do or do not introduce their own distortions of the original vocal signal (e.g. telephone, HAM radio, IP phone , etc.) The penultimate receiver is the listener's ear and auditory system, the auditory nerve, and the language areas in the listener's brain that will "decode" the signal into meaningful information and filter out background noise.
All channels have noise. Another important aspect of the channel is called the bandwidth. A low bandwidth channel, such as a telephone, cannot carry all of the audio information that is transmitted in normal conversation, causing distortion and irregularities in the speaker's voice, as compared to normal, in-person speech.
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. Original theory on communication principles was provided by Harry Nyquist and Émile Baudot after whom the term Baud was conceived to represent a single piece of transmitted information.
Early telecommunication systems were predominantly based on analog electronic circuit design and used a single encoding technique. The introduction of mass-produced digital integrated circuits has enabled telecom engineers to take full advantage of information theory and simultaneously use multiple encoding techniques. From the demands of telecom circuitry, a whole specialist area of integrated circuit design has emerged called digital signal processing.
Early phone systems used analog transmission lines between central offices, but in the 1960s digital multiplexed circuits were used to send voice calls over a Time Division Multiplexed (TDM) circuit. This was done at speeds of either 1.544 Mbit/s (a T1), or at 2.04 Mbit/s (an E1). A T1 circuit was capable of carrying 24 voice channels while an E1 was capable of carrying 30 voice channels. Each voice channel uses 64 kbit/s worth of digital bandwidth to convey the analog waveform.
The development of the computer modem from 1980 is a clear testimony of increases in information transfer capability through the use of multiple mechanisms. A modem today uses frequency, phase and data compression techniques to squeeze data through what originally seemed an impossibly small bandwidth.
Possible imperfections in a communication channel are: shot noise, thermal noise, latency, non-linear channel transfer function, sudden signal drops, bandwidth limitations, signal reflections (echos). More recent telecommunications systems take advantage of some of these imperfections to actually improve the quality of the channel.
Modern telecommunication systems often make extensive use of a clock signal which is used to decode a transmitted data stream, synchronization. In order to accumulate and manage such streams a telco always provided the clock signal. With the advent of global communications it became necessary to have a single worldwide standard derived from a master atomic clock, or to secondary clocks synchronised to that clock. Synchronous circuits are often used between routers. Asynchronous Transfer Mode, ATM is a relatively new standard, operating at very high bit rates where synchronization outside of the data stream can result in errors.
See modulation for examples of techniques for encoding information into analog signals.
Examples of digital channel coding systems: Hamming coding, Gray coding, Binary coding, Turbo coding.
Examples of telecommunications systems: