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=-=-=-=-=-=-=-=-=-=-=-=-=-=-= - P.I.S.S. Philez Number 48 = = - - Understanding Phones = = - - by Skrike = =-=-=-=-=-=-=-=-=-=-=-=-=-=-= For those of you first entering the "scene" welcome. In this article I aim my attentions, at you, who do not understand many basic principles of our lovely phone system. All modern communication devices rely on the electromagnetic spectrum to send and manipulate data. In order to understand how this works first picture a wave. A starting point, rising to a crest and then dipping into a trough and rising again. These waves can be described in 2 different ways, the key characteristics of an analog signal are made up of this. 1. strength, or amplitude of the wave (the vertical distance between a wave trough and crest) and the frequency (the number of times per second the wave cycle repeats) Most telephones work by copying the variations of sound waves generated at the transmitting end onto analog electromagnetic waves, which are converted back into sound waves when they arrive at the receiving end. The specific qualities of a sound -- its loudness and its pitch, for example -- depend on the waves' amplitude, or strength, and on their frequency, or how closely together waves are spaced (measured in cycles per second, called hertz). The electromagnetic spectrum ranges from extremely low-frequency radio waves of 30 hertz -- with a wave length of the Earths diameter -- to high frequency cosmic rays of more than 10 million trillion hertz, with wavelengths smaller than the nucleus of an atom. The range of frequencies making up a signal is called a bandwidth. The human voice, for example, can typically generate frequencies from 100 to 10,000 hertz, for a bandwidth of 9,900 hertz; laser optical fibers, in contrast, operate over a band of 200 trillion hertz. Because the ear does not require a vast range of frequencies to elicit meaning from ordinary speech, the phone company typically allots a total bandwidth of 4,000 hertz for voice transmission.The wide bands of fiber optics and other high frequency media, such as microwaves, can thus accomodate many phone conversations, once the signal has been translated to a higher frequency. Phone transmissions are implemented by multiplexing. This multiplexing allows multiple streams of electronic messages to be transmitted over the same connection in the time otherwise required for one message. Multiplexing is effected in two major ways: frequency-division multiplexing (FDM) and time-division multiplexing (TDM). The older of the two, FDM, is used with analog transmission. The analog signal is impressed on another analog signal of different frequency -- a carrier -- altering the carriers shape so that it bears the pattern of the message. The carrier frequency generally remains constant; only its amplitude varies, at a rate corresponding to that of the message signal. Since each carrier has a different frequency, carriers can be stacked one atop the other and sent together over a cable or microwave radio link capable of carrying a broad range of frequencies; the carriers are then separated at the other end. The greater the mediums bandwidth, the more carriers it can transmit, and the more messages it can handle simultaneously. Time division multiplexing operates by a kind of round robin, employing a device that scans individual channels in rotation -- taking a byte from each channel and transmitting the bytes in a string, according to a sequence determined at the outset. In the TDM method, each byte is also condensed so that many can be sent during the time ordinarily required for one: Each byte is briefly stored in a buffer, then released as a series of much shorter pulses, leaving a space of unused time between series. Into these spaces, similarly condensed bytes from other channels can be inserted. TDM is implemented on digital transmissions. It works on a principle similar to a token ring network. I hope this helped you a little. More on phones next month. -- Skrike skrike@ida.net http://compound.dyn.ml.org/skrike shoutouts to akseez who got me started in all this ---------------------------------------------------------------------- PISS - People into Serious Shit Founders - Defenestrator, PhrostByte Members - Author Parselon Wu Forever kQs Extinction Grench Rhodekyll Dial Tone Psycho Phreak Djdude Circular Reclusion Havok Luther AT2Screech Phantom Operator Apocalypse Skrike Contributors- Sameer Ketkar The Axess Phreak PISS, the author, and anyone else does not take responsibility for what you do with the stuff contained in this file. If you get busted, don't cry to us. We don't care. We have never done any of this. Really. And we don't condone it. Uh-huh. Want more stuff? Go to http://piss.home.ml.org E-mail the group at davematthews@rocketmail.com (C) Copyright 1997 PISS Publications and also copyrighted by the author. This file may be posted freely as long as this notice stays on the end. All rights reserved. Or something like that.