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I have done multiple searches to figure out if fsk encoding of binary data has ever been used in a "wired" communication scheme but have come up with very little.

It seems, at least to me, that utilizing multiple, thin frequency bands to encode essentially parallel pieces of information, cabled internet links could dramatically increase their bandwidth.

Does anyone know if research related to this has been done?

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  • \$\begingroup\$ Err, fsk as in frequency shift keying? It's used for HART communication to transfer data really really slow over long cables in industry automation. (I'm not implying that it can't be faster, just reporting that it has been done) \$\endgroup\$ – Arsenal Nov 2 '15 at 14:25
  • \$\begingroup\$ You are asking for opinions and these sort of questions are generally closed. Also think about this question... the only answer has to be "yes" - nobody would be dumb enough to say "no" because it would just be a guess. \$\endgroup\$ – Andy aka Nov 2 '15 at 15:44
  • \$\begingroup\$ The twenty something odd characters in landline caller ID are sent in FSK format (same as Bell 202 1200 baud modem). So it is still widely used while the modem application is all but dead. \$\endgroup\$ – rioraxe Nov 2 '15 at 23:29
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Yes, this is how V21 low-speed modems work. Here's a good short description and example implementation from TI.

(The fastest available wired communications links just send raw unmodulated bit edges, possibly with varying threshold voltages).

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  • \$\begingroup\$ Also, abandoned in favor of PSK for later modem standards. There are some fundamental bandwidth problems wit FSK, but I don't remember the details. \$\endgroup\$ – Fizz Nov 2 '15 at 14:31
  • \$\begingroup\$ Were receivers more often implemented by using a bandpass filter and then measuring the frequency, or did they treat the signal as a pair of anti-phase-OOK signals and compare the strength thereof? I would think the latter would be more noise-resistant, but transmitting an unmodulated reference tone along with an OOK tone would seem simpler. Maybe FSK was chosen because it can work well with either style of receiver? \$\endgroup\$ – supercat Nov 2 '15 at 18:43
  • \$\begingroup\$ According to this: edge.rit.edu/edge/P09141/public/FSK.pdf something like the latter ("matched filter decoding"), although it can also be treated as an FM signal. \$\endgroup\$ – pjc50 Nov 2 '15 at 19:37
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It seems, at least to me, that utilizing multiple, thin frequency bands to encode essentially parallel pieces of information, cabled internet links could dramatically increase their bandwidth.

That's how broadband ADSL works: -

enter image description here

See this interesting article also.

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    \$\begingroup\$ Yes, but DMT is not [M]FSK; each bin uses QAM and PSK. And splitting between bins is done by ODFM. \$\endgroup\$ – Fizz Nov 2 '15 at 15:53
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    \$\begingroup\$ @RespawnedFluff, even if DMT is not the same as FSK, it does match the description that Andy quoted. As such, it makes sense that OP would be interested in an answer explaining it. \$\endgroup\$ – The Photon Nov 2 '15 at 16:40
  • \$\begingroup\$ @The Photon: Yes, I realize that. But the OP might think it's the same thing... and it's not. \$\endgroup\$ – Fizz Nov 2 '15 at 17:01
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A form of FSK called phase−coherent Frequency Shift Keying was used in the mostly defunct IEEE 802.4, aka token bus. There were actually several flavors of token bus:

  • using phase-continous FSK it ran up to 1Mbit on 75-ohm coax
  • using phase-coherent FSK it ran up to 10Mbit on something [to be determined]
  • there was a third version using AM/PSK (amplitude modulation and phase shift)

In the phase-coherent form the frequency transitions are made only at zero-crossings, but I'm not sure what the additional titbid that "the two singalling frequencies are integrally related to the data rate" means. You can probably read US patent 4799239 and figure that out.

Also, what you are asking about in your second paragraph is not just FSK (as used in the ancient modems) but MFSK, also called M-ary FSK. I suppose one could use this to code multiple bits, like [say] Gigabit Ethernet does with its 5 voltage levels over twisted pairs [1000BASE-T]. Given the general death of FSK over cables though, I'm not sure this was ever done [in commercial products]. Google Scholar is your friend for finding more or less obscure academic attempts at practical products and theoretical background on it.

I don't remember exactly what caused the demise of FSK in modems (and transition to PSK etc. after the 2400 baud ones), but a certain cheekily titled paper notes with respect to the [wireless] 802.11 protocol that:

FSK was abandoned in favor of QAM when the 802.11 standards committee deemed it impossible to implement in 1997. This was mainly due to the limitations of the technology available at the time, one of which is the poor tracking bandwidth of the traditional Phase Locked Loop (PLL). The second issue was due to the numerous noise sources associated with down conversion (dominant channel noises).

So you may need more expensive electronics for high-speed MFSK.

Also from this textbook

Binary and M-ary FSK have the poorest bandwidth efficiency of all the digital modulation techniques used.

Efficiency is on the horizontal axis in the following graph (from the same book); the vertical axis there is the power required to get a BER of 10^-4. So, on the plus side, FSK is thrifty in terms of transmitter energy/power requirements.

enter image description here

After reading Andy's answer, I realize that you may have been thinking just in broad terms of using sub-carriers, i.e. frequency division multiplexing in your 2nd paragraph. But [M]FSK is not one of those schemes.

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    \$\begingroup\$ Yes, "sub carriers" is what I was trying to get at. \$\endgroup\$ – ElecEng2016 Nov 3 '15 at 16:26
  • \$\begingroup\$ @ElecEng2016: In that case, there is prior art that long predates DSL, e.g. en.wikipedia.org/wiki/L-carrier \$\endgroup\$ – Fizz Nov 3 '15 at 16:32
  • \$\begingroup\$ Well, those L-carriers were analog channels being multiplexed in frequency, and were abandoned to time-division multiplexing once digital trunks (T-carrier) were introduced. But since fiber on the last mile took much longer to get introduced as [GPON etc.], you basically had DSL fill the gap by a similar technique over copper. \$\endgroup\$ – Fizz Nov 3 '15 at 16:43
  • \$\begingroup\$ Also the Wikipedia article on GPON is rather confused. If you're interested in that electronicdesign.com/what-s-difference-between/… is a better read. \$\endgroup\$ – Fizz Nov 3 '15 at 16:51
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In the 80's there were a technology called FMVFT which used FSK modulation over a telephone channel. Baudrates of 50bps or 200bps are standardised in ITU-T R.35 and R.38.

No more than 4 years ago I implemented these protocols on an FPGA board for the revamping of a railway diagnostic equipment. So I can say they are still used...

Anyway, to answer your second question, you can be sure that every single Hz that a wire can transport has been exploited to increase the bandwidth of the channel. You cannot FSK modulate two DSL signals over a twisted wire, just because the DSL signal is yet using all of the available bandwidth of that wire (well maybe you can still improve something... but not using FSK).

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