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This question already has an answer here:

I just wondering how modems work.

I know the process of "modulation" and "demodulation".

So I wanto know is "demodulation" are simple revese of "modulation" in real world.

The PC can got an IP address when connect as type 1

What will happen when I connect them like type 2?

type 1 :

[PC] ================= [router]

type 2 :

[PC] === [m] ------- [m] === [router]

[m] : modem(exactly same)

=== : Ethernet cable

--- : DSL cable (phone line, maybe optical fiber, or something else?)

ISP has a large number of model, Is them same thing as the one in my home(but with diffrent size/speed/price...etc)? Or it's completely different thing?

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marked as duplicate by sawdust, Kevin Panko, DavidPostill, Matthew Williams, Dave Dec 9 '14 at 12:12

This question was marked as an exact duplicate of an existing question.

It's not clear what you're trying to do here. What sort of wire is "===" and what sort of wire is "-----"? – Daniel R Hicks Sep 17 '13 at 12:36
are we talking dialup modems or something more modern? – Journeyman Geek Sep 18 '13 at 10:11
up vote 0 down vote accepted

If I understand your question correctly, your type 1 and type 2 connections will function identically.

As long as your modems satisfactorily communicate with each other, which shouldn't be a problem as long as both modems comply with accepted standards, they will be transparent to the PC and router.

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Ah, but the devil is in the details. I seriously doubt that two DSL customer-end modems will communicate with each other. – Daniel R Hicks Sep 17 '13 at 12:43
If there is "customer-end" modems, then the "ISP-side" is different with "customer-end" one? – GongT Sep 18 '13 at 11:15
@GongT yes, usually on the ISP side there will be a DSLAM‌​, two consumer modems will likely not have the internal firmware to talk to another consumer modem, however the modem's firmware may be flashable or configurable to allow it, but I doubt you will find one that would work with default configurations. – Scott Chamberlain Oct 23 '13 at 3:30

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