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# Telecommunication

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Telecommunication is the technique of transmitting a message, from one point to another, knowing how much information, if any, is likely to be lost in the process.

The elements of a telecommunication system are a transmitter?, a channel? 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 reciever. The reciever 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 recievers.

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 enginneers will research how far defects may be introduced to the signal, that are perceptible to the user, but do not affect his 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 chord]?s, 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.

In 1948 a Bell Labs scientist called Claude E. Shannon published A Mathematical Theory of Communication. 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]?.

Possible imperfections in a communication channel are: [shot noise]?, [thermal noise]?, latency, non linear channel [transfer function]?, sudden signal drops, bandwidth limitations, [signal reflection]?s.

Examples of analog coding systems: Amplitude modulation, Frequency modulation, [Quatrature amplitude modulation]?, [Phase keying]?.

Examples of digital coding systems: [Huffman coding]?, [Gray coding]?, [Binary coding]?.

/Talk

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Last edited August 11, 2001 9:37 am (diff)
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