I understand that the name "Modem" stands for Modulator/Demodulator, which means that it works on the analogic singal coming from the backbone and translates it into digital.

However, I was just thinking: does the network card on the PCs work as Modems as well? I have checked and noticed that, somehow, UTP cables transfer inforation through digital signals.

How does that work? Are the digital signals the same that the CPU transmits to other componentes in the computer via circuits?

From what I see, there's 3 types of signal transfer:

  1. From CPU to other components via Circuits
  2. Something else via Ethernet Cables
  3. Via internet

Why we need to modulate signal to send it through the backbone and do not need to modulate it to transfer data via UTP cables?


EDIT: Got some interesting answers (thank you very much @rickybeam and @user2964971) but it still isn't clear for me why we require mod/demod for the backbone, but we don't need it to Ethernet.

I think that a better way of asking the same would be: via internet (say dial up) we convert electronic signals into waves and back, and Ethernet just get electronic signals and send it through just like in a circuit, would that be the case?


I think it reallly comes down to how you define "modulation"

One definition (taking from the first paragraph of the wikipedia "modulation" article):

In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted.

By that definition the line coding schemes used by modern network cards would not qualify as "modulation" (you could argue that the Manchester Encoding used by 10 megabit ethernet does count as modulating a carrier). On the other hand the schemes used by modern dialup modems wouldn't either

On the other hand if we take a broader view of the term modulation as covering any scheme designed to take a signal and encode it as a waveform whose voltage and frequency characteristics suitable for a given channel then the coding schemes used by ethernet would count.

In practice the term "modem" is typically used for devices used over channels that were not specifically built to carry data. That doesn't imply much at a technical level though.

  • Thanks, @peter-green, I guess that this pretty much outline the green area that I was exploring with the question. It's clear to me that the terminology was created during a time in which it was much more precise, but that the names of the components currently stick more to the usage than to the original technical reason they were created for. Mar 2 '17 at 12:58

Unless we're talking about dialup, most things today aren't technically a "modem". An RS-232 modem really is modulating and demodulating. The +/-25VDC serial signal (as per spec, that few systems provide) is literally translated to/from the audio signal. (the encoding at higher speeds is much more complex, but in the beginning (Bell 103) it was 4 distinct tones.)

What we call a modem today -- cablemodem, DSL modem, etc. -- are protocol translators. They interconnect two layer-1 technologies.

  • 2
    This currently doesn't seem to answer the bottom-line question: "Why we need to modulate signal to send it through the backbone and do not need to modulate it to transfer data via UTP cables?"
    – This
    Oct 15 '14 at 1:29
  • Answers the last question just fine. If there's any confusion it's in the last question itself. The signal is modulated because the "backbone" is often a different medium than UTP.
    – Rumblebutt
    Oct 15 '14 at 20:29

Why we need to modulate signal to send it through the backbone: When modems were used as modulators/demodulators, a phone call was done. A modem dial-up really means dialing up. A modem accesses the internet by dialing a number at the ISP's site. Once, the connection is established the line is occupied. When then someone else tries to make a voice call the internet connection breaks or the caller hears a screechy sound and finally the internet access connection breaks. The experience depends on the telco's wiring. Then, DSL came. From a consumer's perspective it did one very special thing: Talking over the phone while surfing on the net.

not need to modulate it to transfer data via UTP cables: Twisted pairs of copper wire are designed for local networks (a room, between rooms, a floor, between floors). The signal from a CPU sockets runs on a bus and after 10 or more centimeters it reaches a slot or onboard device. Nonetheless are both the cable and the bus manufactured for being a medium for a certain signal. The modem as in mo-/demodulator was invented to send data via a signal over a wire that once was used for carrying a human voice sampled by a speaker. Yes, the voice was sampled and the signal then amplified. The connection of two endpoints was switched by a company or a state facility. Hence, PSTN.

Are there other modems? For examples, a DOCSIS modem. Its uses a coaxial cable for transmission. Although only a portion within the frequency spectrum is used, that spectrum could be endlessly subdivided. Therefore, it's analog as well. Its definitely not digital. (Look up "coaxial antenna" to get the picture)

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