telecommunications :: Modem Spy :: phone call recording

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telecommunications
The basic elements of a simple telecommunications system are: a.) a transmitter that operates upon the information to be communicated in such a way that some type of signal suitable for transmission is produced (this includes any encoding, compression, and error correcting operations, modulation processes if some type of carrier is to be used, or virtually any other operation that is to be performed upon the information in order to prepare it for transmission in the form of a signal); b.) a signal that is transmitted by the transmitter; c.) a transmission medium, which constitutes a communications channel over which the signal is transmitted; d.) a receiver (such as a radio receiver) that receives the signal and reverses any and all of the operations that were performed upon the information by the transmitter in order to convert it into the form of a signal (e.g., the receiver decodes, decompresses, compares error correction codes, demodulates the signal to separate it from any carrier that was used, etc. and generally performs these operations in the exact reverse order in which they were performed by the transmitter); Note: The transmission medium and the communications channel may also be considered to exist separately - i.e., the communications channel may be considered to consist of some limitation imposed upon the transmission medium, either by the physical nature of the transmission medium with respect to the type of signal energy that will be used to transmit the information over it (for example transmission lines, which have a finite bandwidth, create a 'channel' for electromagnetic energy because of this frequency limitation), or by some process designed explicitly for that purpose (such as multiplexing). However, this distinction is not always made (since virtually any practical transmission medium has some form of physical limitation associated with it), and the transmission medium 'connecting' the transmitter to the receiver is itself considered to be a communications channel (which, of course, may be further subdivided into other 'channels' by multiplexing). 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 may therefore require a decoding mechanism to recover the message from the received signal. This mechanism can be designed to tolerate a significant degree of signal degradation. Sometimes, the final "receiver" is the human eye, ear (or other sensory organ) 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 (see simplex). 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 (see Information theory) in order to design the most effective encoding and decoding mechanisms. When systems are designed to communicate through human sensory organs (mainly those for vision and hearing), physiological and psychological characteristics of human perception must be taken into account. Certain types of defect, while objectively measurable, are not readily apparent to human perception while others are disproportionately apparent. The cost of a system can therefore be reduced by choosing to omit certain information. There is clearly a tradeoff between reduced cost and user demand for higher quality, and this is an important economic consideration for those who plan systems. The field of telecommunications is no doubt one of the most exciting occupational fields that modern society has to offer. New technology is constantly being developed and finds its applications in the technical systems that make up a telecommunications network. This creates opportunities for developing existing services further, and introducing completely new ones. 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 consists of 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 consists of the sound waves (pressure fluctuations in air particles) that can be identified as speech when properly decoded. The communications channel consists of the air (transmission medium) carrying those sound waves, and the limitations of the 'channel' include all of the acoustic properties of the surrounding space: echoes, ambient noise, reverberation. Between the speaker and the listener, there might be other devices that do or do not introduce their own distortions of the original vocal signal (for example a telephone, a HAM radio, an IP phone, etc.), although for the sake of the example, each of these would technically be considered to be a separate telecommunication system. The 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 information and filter out background noise, echos, and any other interference or distortions introduced by the physical properties of the channel. 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.
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