Hi,

I just recently discovered this wonderful forum and i'm not yet accustomed to the way things are posted here so forgive me for any mistakes.

I was pondering on the following:
Why can a digital signal not be sent over a telephone line?
What are the main differences between an analog signal and a digital one?
Does a different data link layer protocol always entail a different physical line ?

I will be extremely happy is someone could provide me with an answer to these basic but yet essential questions.

Thank you in advance.
 

Part of the difference between analogue and digital signals has to do with the signal format, frequency, and modulation. DS1 and E1 are digital signals that ride phone line pairs, in addition to all the xDSL formats. In some cases they can travel the same pair, for example when a POTS line is shared with ADSL.

....JIM....
 

So some signals aren't modulated before being put on the telephone line? I was always under the impression that whenever a signal needed to traverse a telephone line it needed to be an analog signal. Take for instance a T1 connection to your provider.
The traffic is routed trough your provider in digital format then is modulated to be put on the pots and then when it reaches your modem it's demodulated to a digital signal ?

Does T1 have it's own physical layer or can it travel across like telephone wires or cables. This is al very confusing because it has never been properly explained to me. Can anyone perhaps recommend literature on this ?

Try this link: http://en.wikipedia.org/wiki/T-carrier. It should help, that way I won't have to write a book. You might try using Google search for some reference material that may help understanding all of this...

....JIM....
 

Thank you for the information. I've been searching on google for a while and i've found most of the information in needed but one thing i haven't found yet.

Why can't a digital signal be sent over the POTS or over cable ???


If you could answer those questions i would greatly appreciate it.

What kind of digital signal are to trying to send on POTS or cable?

....JIM....
 

I'm not trying to send a digital signal. I would just like to know if it can be done and why it can or can't be done.
Like for instance digital signals have a frequency that is too high to be put on the pots or something like that.
I want to understand why it is neccesary to modulate the digital signal into an analog signal.

I guess it is a matter of understanding the different methods and protocols of transmission and bandwidth used for different physical media, copper versus fiber, for instance. But as far as POTS on a copper pair is concerned, they can both utilize that medium. DSL is a digital transmission on copper, and POTS is analogue. They can share the same medium, but they use different portions of bandwidth on that physical medium to send and receive. There are all kinds of modulation techniques that were developed in order to make the thing work.

You might want to get some basic education about electronics, radio frequency and multiplexing systems. If Google does not work, there are lots of libraries that have basics books on these subjects. There are also a lot of the "'blank' for dummies" books available too.
 
....JIM....