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I was helping a friend set up a simple home network after a move recently. He referred to his ONT as a modem, and I was tempted to say "well, actually, that's not a modem..."

I didn't, because I realize that from a non-technical perspective, "modem" effectively means "the box from my ISP that I plug the wireless router into".

But from a technical perspective, is an ONT appropriately described as a (type of) modem? I'm not a big hardware person, but my instinct is no, because there's no carrier wave with fiber. The modem article on wikipedia has a section on optical modems that talks about optical network units... so, bonus question: are ONU and ONT synonymous?

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  • Home networking and consumer-grade devices are explicitly off-topic here. You could try to ask this question on Super User.
    – Ron Maupin
    Oct 17 '20 at 16:43
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Short answer: no, an ONT is no modem.

Modem is short for MOdulator/DEModulator and the term is only used for three device classes: telephone modems, DSL modems, and cable modems (thx Cody). These modulate digital data on to voice-grade phone lines or (possibly former) television cable. DSL and cable "modems" often include a router, but technically these are routers that include a modem.

An ONT is an Optical Network Terminator, it connects the ISP's (passive) optical network with your LAN. Normally, it includes a router as well but in theory it could just be media converter.

Optical networks work by sending light pulses over optical fiber, so you actually could argue that they use amplitude modulation of the used wavelength. Accordingly, calling an ONT "modem" isn't completely wrong from the physics POV but in a networking context, that term isn't used by people in the know.

The rule of thumb is that "modem" is only used for devices coding digital data onto an "analog" medium - in the sense of its normal or traditional use. Of course, technically all media are actually analog in their physical nature...

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  • 1
    and cable modems?
    – Cody
    Jan 10 '20 at 22:57
  • @Cody Yes, and cable modems - forgot about them!
    – Zac67
    Jan 10 '20 at 23:01
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    yeah, in the last year i struggled with the same question without coming to a satisfying answer. Normally i would argue as the same manner as you. But you'll find the term "optical modem" in a lot of technical papers. You said it already... there is always some kind of analogue below the digital, it depends on how deep you dive into the matter. - My mind is not at peace on this subject. Jan 14 '20 at 20:08
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An ONT has some sort of modem inside, as the box people normally call a modem does. I mean, modem is short for modulator-demodulator, that is, something that takes a digital signal and modulates into an analog one, and demodulates an analog signal to convert it into digital stream of 0s and 1s

In the case of the ONT the analog signal it's modulated light, while in the case of a PSTN modem it's modulated current/voltage.

So, technically and in a precise and "picky" way, neither boxes are modems, but they contains modems in the inside.

Whether this distinction is useful at all, I don't know.

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So modem conceptually is generally used when you take a digital signal and apply a frequency modulation (pam or qam for example) that encodes multiple bits per symbol/signal. Optical modulations exist for the same reason as in the RF domain to increase the spectral capacity of a given band. 100G/200/400g modems do exist in the optical domain and are used to multiplex lower rate ethernet or other media in the channel spacing of a single, or fewer wavelengths then would otherwise be required.

AN ONT for a PON deployment may include a modem of some variety, especially if it is 100Ggigabit or RFOG (cable over optical), but might also not include what we think of as a modem if for example it's just gigabit ethernet and TDM.

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  • So, PAM-4 qualifies a modem while PAM-2 doesn't? Just asking. ;-)
    – Zac67
    Jan 14 '20 at 21:53
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    I'd have said that you don't need multiple bits per symbol to count as the modulation for a modem. Those 300-baud things were definitely modems, no?
    – jonathanjo
    Jan 14 '20 at 22:54

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