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 computernetworking .
The elements of a telecommunication system are a transmitter , a medium( line ) and possibly a channel imposed upon themedium (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 fromthe transmitter to the receiver. The receiver has a decoding mechanism capable of recovering the message within certain limits ofsignal 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 .)
One of the roles of the telecommunications engineer is to analyse the physical properties of the line or transmission medium , and thestatistical 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 (sense) ), physiological and psychologicalcharacteristics 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 affectthe viewing or hearing experience too badly.
Examples of Human (tele)communications
In a simplistic example, take a normal conversation between you and a friend. The message is the sentence your mind decides tocommunicate 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 aircarrying those sound waves , and all the acoustic properties of the space youare in: echoes , ambient noise, reverberation . Between you and your friend (the receiver), may be other technologies that do or do notintroduce their own distortions of the original vocal signal (e.g. telephone, HAM radio , IP phone ,etc.) The penultimate receiver is your friend's ear, the auditorynerve , the language areas in your friend's brain that will make the difference between your voice and the sound of a carpassing 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 ofthe channel is called the bandwidth , and you would become very aware of theeffects 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 ofCommunication in 1948 . This landmark publication was to set the mathematicalmodels used to describe communication systems called informationtheory . Information theory enables us to evaluate the capacity of acommunication channel according to its bandwidth and signal-to-noise ratio . Original theory on communication principles was provided by Harry Nyquist and ÉmileBaudot after whom the term Baud was conceived to represent a single piece oftransmitted information.
Early telecommunication systems were predominantly based on analog electronic circuit design and used a single encodingtechnique. The introduction of mass-produced digital integratedcircuits has enabled telecom engineers to take full advantage of information theory and simultaneously use multiple encodingtechniques. From the demands of telecom circuitry, a whole specialist area of integrated circuit design has emerged called digital signal processing .
The development of the computer modem from 1980 is a clear testimony of increases in information transfer capability through theuse of multiple mechanisms. A modem today uses frequency, phase and data compression techniques to squeeze data through whatoriginally seemed an impossibly small bandwidth.
Possible imperfections in a communication channel are: shot noise , thermal noise , latency , non-linearchannel transfer function , sudden signal drops, bandwidth limitations, signalreflections (echos). More recent telecommunications systems take advantage of some of these imperfections toactually 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 itbecame 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 newstandard, 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 intoanalog signals.
Examples of telecommunications systems:
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