Piss Issue 48

eZine lover (@eZine)

Published in 

Piss

 · 5 years ago

  

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- P.I.S.S. Philez Number 48 = 
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-   Understanding Phones    = 
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-        by Skrike          = 
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	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 

---------------------------------------------------------------------- 
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