Networking: physical layer

I have a couple of basic questions/statements which I'm not sure about because I can't find a simple and comprehensive description of some of the basic stuff regarding the physical layer. (I've read the relevant chapters in a couple books like Tanenbaum's, and Kurose & Ross's one, but I couldn't find the pertinent information.)

I'm assuming a usual network topology involving a PC that's connected to modem A which in turn is connected to another modem, B, basically, that resides in an end office (local central office, CO), which (modem B) is connected to a router, which is connected to the internet.

1. Is it true that digital modulation (e.g., FDM, TDM) occurs only between modem A and modem B, wherein a digital signal (in terms of 0's and 1's) coming from the PC is converted to an analog signal, and at modem B it's translated back to those same original bits, propagated across the internet in that form, and then converted back to the analog equivalent at the receiving local loop and at the very end to the digital signal and bits for the receiving PC?
2. During this modulation, let's say FDM, multiple digital signals (coming from multiple modems in the neighborhood of modem A) are multiplexed into one signal until they reach the CO?
3. Supposing the above is true and that FDM is used, do different modems have to use different frequencies? How is this determined?

Wikipedia on multiplexing:
In telephony, a customer's telephone line now typically ends at the remote concentrator box, where it is multiplexed along with other telephone lines for that neighborhood or other similar area. The multiplexed signal is then carried to the central switching office on significantly fewer wires and for much further distances than a customer's line can practically go. This is likewise also true for digital subscriber lines (DSL).

So I assume there's a device there somewhere that "concentrates" these signals into one and decides which signal will have which frequency.

4. If the above is true, I assume the reverse is also true, so when signals are sent back from modem B to the rest of the modems in the neighborhood, among which there's modem A – how does modem A know which signal is intended for it? Does it use a filter corresponding to... which frequency (again not sure where the frequency comes from)?

I apologize if my questions are too stupid; I've been rereading stuff about this online for some time now and I can't find a normal description of this anywhere. If you have any suggestions in that regard (a book or something), let me know.

Thank you in advance for your answers.
 

berkeman

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I apologize if my questions are too stupid; I've been rereading stuff about this online for some time now and I can't find a normal description of this anywhere.
No need to apologize at all. Your questions are fine, and it looks like you have been putting in the time trying to figure them out on your own. It's all good. :smile:
1. Is it true that digital modulation (e.g., FDM, TDM)
Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) are multiplexing techniques, not modulation techniques. Modulation takes an input signal from some source, and converts it into a form that will propagate through some channel well. Modulation techniques like AM, FM, QAM, ODM, etc. Wikipedia is a good source for learning about various modulation techniques.
digital signal (in terms of 0's and 1's) coming from the PC is converted to an analog signal, and at modem B it's translated back to those same original bits, propagated across the internet in that form, and then converted back to the analog equivalent at the receiving local loop and at the very end to the digital signal and bits for the receiving PC?
Yes, pretty much, but don't get too hung up in "analog" versus "digital" representations. Pretty much everything in communication channels is analog, but still represents "1s" and "0s". There is also some extra overhead for addressing information, CRC reliability checks, timestamps, etc.
4. If the above is true, I assume the reverse is also true, so when signals are sent back from modem B to the rest of the modems in the neighborhood, among which there's modem A – how does modem A know which signal is intended for it? Does it use a filter corresponding to... which frequency (again not sure where the frequency comes from)?
It sounds like a lot of this question revolves around DSL modem access to the Internet from your home:

https://whatis.techtarget.com/reference/Fast-Guide-to-DSL-Digital-Subscriber-Line

DSL is a point-to-point analog modem technology -- each of your neighbors has their own twisted pair cable connecting their DSL modem to the CO's modem for that address. My cable modem access, by contrast, only has a single coax from my house to a local concentrator box by the street, and from there to the main office, the traffic is multiplexed onto a single fiber optic cable (using FDM and TDM together, most likely).

I hope that helps some. Please read a bit more about multiplexing and modulation, and post more questions. We are happy to help on this. :smile:

EDIT/ADD -- I just saw this in your post. I'm no expert in DSL (a good friend of mine was one of the analog EEs who helped to design the modems for early DSL systems), but this is not my experience with DSL (including a recent experinece). The distance from the CO determines whether your DSL connection has a good enough S/N ratio to give you viable Internet speed, not the distance from your home to some wonderful concentrator near you. DSL works okay until you move into the hills, far away from the nearest CO, in my experience...

Wikipedia on multiplexing:
In telephony, a customer's telephone line now typically ends at the remote concentrator box, where it is multiplexed along with other telephone lines for that neighborhood or other similar area. The multiplexed signal is then carried to the central switching office on significantly fewer wires and for much further distances than a customer's line can practically go. This is likewise also true for digital subscriber lines (DSL).
 
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