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Anything that travels over wire, through air, via light is analog. Digital is simply 1’s and 0’s, the devices on either side of the “connection” convert analog (in its many forms) back into digital information.
To put it in perspective, even your Cat5 or network cable, is an analog medium where digital information is changed into signals are transferred to pairs of wires, and then back to digital by network cards or routers.
Hope this makes things clearer.
No. A digital signal, technically speaking, is a composite of analog signals. To make a square wave, representing zeroes and ones, you will add sine-waves with different harmonics. A digital signal can also have multiple signal levels - agan, this is not zeroes and ones. A signal can go upto +5V, 0V and -5V (but these signals will ONLY represent 0s and 1s). Read up on different line-coding schemes and Fourier Transform.
In case anyone is interested in DSL, the technique used for the transmission is called 256-QAM (Quadrature Amplitude Modulation).
To the layman, there might not seem to be a difference. Afterall, adding oranges will just get you more oranges. But it is important and necessary to differentiate between digital signals and analog signals.
If we continue with your analogy, the data stored on a hard-drive is analog as well. But we know thats not the case; its not classified as analog, it is always treated as digital.
But without analog, digital cannot exist. As digital is just an approximation.
So to put it in perspective, lets say your DSL modem is connected to your computer through Ethernet. The data your PC sends to the modem is sent using a line-coding scheme. The modem understands this line-coding scheme because of the Ethernet standard. The modem then converts this received data into an analog signal, based on 256-QAM.
Like I mentioned before, we convert your digital data into an analog signal because of bandwidth constraints, your telephone line does not have the required bandwith.
I realize you might be thinking “but they are both electrical signals, so what does it matter?!”. The thing is, the electrical signal in Ethernet is discrete, it only represents 0s and 1s whereas the signal in your telephone represents a multitude of values.