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Author Topic: How can one cable carry many signals?  (Read 4080 times)

system

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How can one cable carry many signals?
« on: 10/08/2011 01:01:02 »
Hello,

I was wondering how many separate and distinct signals are sent simultaneously along the same wire  as with telephone calls or internet data. It is my understanding that all signals are sine waves. How is it that a cable is able to carry all these messages at once. I am having difficulty understanding how two or more transmitters attached to a single cable won't transmit a jumbled mess to all the receivers.

To be clear, with telephone and the internet, I understand that the signals from each transmitter have an identity and are routed to their destination.  I also understand that different frequencies of waves can be sent simultaneously as with different musical instruments playing at once. My question is exclusive to how one cable (copper or fiberoptic) is able to carry so many waves at once while still being able to faithfully reproduce them at their different intended receivers. These cables must carry unthinkable amounts of data at once.

It is simply beyond me how so many sine waves can go over a single wire simultaneously.

Thank you for the time you spent reading this. I hope I haven't made some shameless error in thought and the answer is truly quite obvious.

Sincerely,

Oliver Liu
Asked by Oliver Liu

                                        Visit the webpage for the podcast in which this question is answered.
 ...or Listen to the Answer or [download as MP3]
« Last Edit: 10/08/2011 01:01:02 by _system »


 

Offline graham.d

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How can one cable carry many signals?
« Reply #1 on: 10/08/2011 09:31:42 »
There is some differences in signalling methods that are used. Sometimes signals can be digitised to produce a stream of 1s and 0s and then specific codes added to differentiate between blocks of a signal intended for receiver A vs receiver B etc. This is simply multiplexing data on to a line and is not what you are asking about. This is sometimes referred to as Time Division Multiplexing and is quite a simple and common method used on transmissions over lines with a high bandwidth capability (Ethernet is an example).

What you are trying to understand is another method whereby different signals may be encoded on to different frequency carriers and then decoded to recover the signals. This is a form of Frequency Division Multiplexing. Basically the digital (or analog) data is "put" on to a high frequency carrier. There are many methods how this can be done - one method is to adjust the phase of the carrier in a way related to the data amplitude (Phase modulation), another would be to vary the frequency slightly (Frequency Modulation) or yet another to vary the amplitude of the carrier. Combinations of phase and amplitude can also be used together (Quadrature Amplitude Modulation) to get more bits of data without spreading the carrier frequency too much.

What you then have is a fairly narrow frequency band dedicated to one data transmission. You can then add other carriers at different frequencies as long as they don't overlap (at least more than a certain amount). Basically the individual bands can then be selectively band filtered  and then each band decoded to recover the data encoded on to it. This method is used in wireless applications too.

In fibre communications this is effectively employed also. Mostly only one band is used for simplicity but the bandwidth is so high that many signal can be multiplexed (as in the first method) before being used to modulate the light (usually infra red) signal. There are some systems that use multiple frequencies also, and the light output is "filtered" into bands optically, but these systems are more expensive. There are also other considerations regarding the amount of signal loss that is acceptable and the effects of dispersion of the signal in transit - over long distances the signals have to be detected, error corrected and retransmitted (in devices known as repeaters).

I hope this helps.
 

Offline graham.d

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How can one cable carry many signals?
« Reply #2 on: 10/08/2011 09:33:36 »
Oops, I did not spot that this had been answered in a podcast. I hope I did not contradict the undoubtedly erudite anser.
 

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How can one cable carry many signals?
« Reply #2 on: 10/08/2011 09:33:36 »

 

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