Science Questions

How can one cable carry many signals?

Sat, 6th Aug 2011

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Question

Oliver Liu asked:

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

Answer

Dave -   There are a variety of different ways of doing this:  In some of the older ones, you can basically mix your signal with a radio frequency and essentially send lots of different radio frequencies down the same wire, the same piece of copper, in the same way as you can send lots of different radio frequencies through space.  Each one of those can have a different conversation on it, and you can send lots of phone calls down one piece of copper like that. 

You can also do something which is called Ďtime division multiplexingí.  So that's whereby you send a hundredth of a second of one personís conversation, a hundredth of a second of another personís conversation, a hundredth of a second of another personís conversation... and you interleave them, and then you have some electronics which you can take those back out again.

Chris -   That puts it all back together.

 

Dave -   ...Back together again.  Or more recently, in the last 20 years, you basically digitise everything and a phone signal can probably take maybe 10 kilobits of information.  An optic fibre can take gigabits of information, so you can get thousands and thousands of phone calls down the same wire.

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