Is a cable modem really a modem?
March 4, 2023 5:37 PM Subscribe
A modem (modulator/demodulator) is a device that converts analog to digital signals and vice versa. Is the coax cable bringing internet into my house really carrying an analog signal? I find that surprising in 2023.
If it is indeed analog, why is that better than running a digital signal from the cable company to all the end points? If it's digital, then is the thing we call a cable modem really some kind of router?
If it is indeed analog, why is that better than running a digital signal from the cable company to all the end points? If it's digital, then is the thing we call a cable modem really some kind of router?
If it's digital, then is the thing we call a cable modem really some kind of router?
Once the analog signal has been converted to digital bytes the cable modem frequently has some minor routing abilities (and always switching) to get the resulting packets out the right port.
posted by Tell Me No Lies at 6:23 PM on March 4, 2023
Once the analog signal has been converted to digital bytes the cable modem frequently has some minor routing abilities (and always switching) to get the resulting packets out the right port.
posted by Tell Me No Lies at 6:23 PM on March 4, 2023
Yeah, classical cable is/was analog and so something has to modulate and demodulate the signal.
As I understand it the general term if it were digital would be Network Interface Device (NID), and the analogous term for 'cable modem' on digital fiber is Optical Network Terminal (ONT).
posted by SaltySalticid at 6:32 PM on March 4, 2023 [2 favorites]
As I understand it the general term if it were digital would be Network Interface Device (NID), and the analogous term for 'cable modem' on digital fiber is Optical Network Terminal (ONT).
posted by SaltySalticid at 6:32 PM on March 4, 2023 [2 favorites]
Just to add some more color here, indeed every form of "digital" communications is still analog at the very bottom. USB, Ethernet, HDMI, Wifi, 5G, RS232, telegraph ... all digital communications ultimately involve analog voltage waveforms on physical wire (or radio waves in airspace). The natural world is analog, and unless you are inside a simulation, you are in an analog world.
In the case of a cable modem, it's useful to think of a modem as not just a MODulator / DEModulator, but a device that converts information between MODES. Cable coax carries digital data in QAMx carriers (or whatever, see the modulation section of the DOCSIS page already mentioned above) that are very much analog, and then the cable modem converts that digital data over to the Ethernet and Wifi you use in your house, also analog at the very bottom.
Even fiber optics ultimately uses analog at the very bottom, but it's a totally different kind of "modulation" ...
posted by intermod at 7:14 PM on March 4, 2023 [8 favorites]
In the case of a cable modem, it's useful to think of a modem as not just a MODulator / DEModulator, but a device that converts information between MODES. Cable coax carries digital data in QAMx carriers (or whatever, see the modulation section of the DOCSIS page already mentioned above) that are very much analog, and then the cable modem converts that digital data over to the Ethernet and Wifi you use in your house, also analog at the very bottom.
Even fiber optics ultimately uses analog at the very bottom, but it's a totally different kind of "modulation" ...
posted by intermod at 7:14 PM on March 4, 2023 [8 favorites]
The short answer: "no"
The long answer: "no, not really"
Modem was supposed to be digital->analog->digital converter. But modern digital transmitters like ISDN, DSL, Cable "modem" do not do the multi-step conversion... Because they already transmit over a digital medium.
But what we call "modems" nowadays are access device plus router combinations.
posted by kschang at 7:24 PM on March 4, 2023
The long answer: "no, not really"
Modem was supposed to be digital->analog->digital converter. But modern digital transmitters like ISDN, DSL, Cable "modem" do not do the multi-step conversion... Because they already transmit over a digital medium.
But what we call "modems" nowadays are access device plus router combinations.
posted by kschang at 7:24 PM on March 4, 2023
Modem was supposed to be digital->analog->digital converter. But modern digital transmitters like ISDN, DSL, Cable "modem" do not do the multi-step conversion... Because they already transmit over a digital medium.
This is flat out wrong. Both DSL and cable modems use orthogonal frequency-division multiplexing extensively - which is a form of modulation that generates an analog signal at various frequencies. Each side has a corresponding demodulator for the signal from the other side. Hence modulator-demodulator - a modem. Here is information about the modulation standards for DSL. Here is information about the modulation standards for DOCSIS for cable.
If it is indeed analog, why is that better than running a digital signal from the cable company to all the end points?
Defining "analog" vs "digital" is surprisingly tricky. Many "analog" electrical engineers would point out all digital signals are analog - and I would tend to agree with them.
If you define digital as representing a binary signal, then sending many bits per second requires changing a voltage on a wire very frequently. Simple digital signals switch between a 0 and a 1 very fast. The combination of changing voltage frequently, and doing so with a very fast transition from 0 to 1 (and vice versa) is uniquely bad for long-distance communication. It might seem obvious that a 1 Mb/s digital signal takes 1 MHz of electrical bandwidth (the frequencies contained in the signal) - but that would definitely not be the case. In fact, an ideal square wave (instantaneous switching from 0 to 1) requires an infinite amount of electrical bandwidth to transmit.
Having a signal with very high bandwidth is disadvantageous for a number of reasons. Handwaving here a bit, such a signal would require very expensive/bulky cable (to maintain consistent dielectric and impedance characteristics), would likely violate FCC regulations on emitting excessive electrical noise, and would require expensive electronics to receive/transmit (due to the bandwidth required). In practice, it's easier to modulate the signal - use various forms of converting binary signals into analog voltages (and generally, many analog voltages) with filtering that reduces the electrical bandwidth of the signal. As a very hand-wavey example again, one could transmit 256 bits by sending 256 successive 0 or 1 voltages - or send a single voltage that has 256 different possible levels for 256 different values to transmit. Further, there are techniques to "slow down" the transitions in the signals generated - this reduces the frequency content of the signals produced, which helps the problems listed above.
The above is a very high level overview that tries to avoid getting into electrical specifics. If you're curious about this, I would start with an understanding of Fourier/frequency domain mathematics. The style of transmission used in cable modems is designed to be tolerant of frequency-specific signal attenuation, which is common in long distance wiring.
posted by saeculorum at 8:55 PM on March 4, 2023 [13 favorites]
This is flat out wrong. Both DSL and cable modems use orthogonal frequency-division multiplexing extensively - which is a form of modulation that generates an analog signal at various frequencies. Each side has a corresponding demodulator for the signal from the other side. Hence modulator-demodulator - a modem. Here is information about the modulation standards for DSL. Here is information about the modulation standards for DOCSIS for cable.
If it is indeed analog, why is that better than running a digital signal from the cable company to all the end points?
Defining "analog" vs "digital" is surprisingly tricky. Many "analog" electrical engineers would point out all digital signals are analog - and I would tend to agree with them.
If you define digital as representing a binary signal, then sending many bits per second requires changing a voltage on a wire very frequently. Simple digital signals switch between a 0 and a 1 very fast. The combination of changing voltage frequently, and doing so with a very fast transition from 0 to 1 (and vice versa) is uniquely bad for long-distance communication. It might seem obvious that a 1 Mb/s digital signal takes 1 MHz of electrical bandwidth (the frequencies contained in the signal) - but that would definitely not be the case. In fact, an ideal square wave (instantaneous switching from 0 to 1) requires an infinite amount of electrical bandwidth to transmit.
Having a signal with very high bandwidth is disadvantageous for a number of reasons. Handwaving here a bit, such a signal would require very expensive/bulky cable (to maintain consistent dielectric and impedance characteristics), would likely violate FCC regulations on emitting excessive electrical noise, and would require expensive electronics to receive/transmit (due to the bandwidth required). In practice, it's easier to modulate the signal - use various forms of converting binary signals into analog voltages (and generally, many analog voltages) with filtering that reduces the electrical bandwidth of the signal. As a very hand-wavey example again, one could transmit 256 bits by sending 256 successive 0 or 1 voltages - or send a single voltage that has 256 different possible levels for 256 different values to transmit. Further, there are techniques to "slow down" the transitions in the signals generated - this reduces the frequency content of the signals produced, which helps the problems listed above.
The above is a very high level overview that tries to avoid getting into electrical specifics. If you're curious about this, I would start with an understanding of Fourier/frequency domain mathematics. The style of transmission used in cable modems is designed to be tolerant of frequency-specific signal attenuation, which is common in long distance wiring.
posted by saeculorum at 8:55 PM on March 4, 2023 [13 favorites]
As a very hand-wavey example again, one could transmit 256 bits by sending 256 successive 0 or 1 voltages - or send a single voltage that has 256 different possible levels for 256 different values to transmit.
Correcting myself here, this should say: one could transmit 8 bits by sending 8 successive 0 or 1 voltages - or send a single voltage that has 256 different possible levels for 256 possible different values to transmit.
posted by saeculorum at 9:11 PM on March 4, 2023 [1 favorite]
Correcting myself here, this should say: one could transmit 8 bits by sending 8 successive 0 or 1 voltages - or send a single voltage that has 256 different possible levels for 256 possible different values to transmit.
posted by saeculorum at 9:11 PM on March 4, 2023 [1 favorite]
Defining "analog" vs "digital" is surprisingly tricky. Many "analog" electrical engineers would point out all digital signals are analog - and I would tend to agree with them.
Sorta yeah pretty much. But y'know USB and I do believe twisted pair ethernet are of the streams of 1/0 that are sent and picked by clocking. The bandwidth and frequency and distance stuff still applies, but these are typically shorter and lower speed connections that use voltage differences, not any sort of multiple frequency stacking.
How does a USB keyboard work? - YouTube
Coaxial cable that your talking about (and very old thic and thin ethernet) work differently from the twisted pair types of connections.
But you can still argues that every 1/0 ends up being a voltage or a light or a propagating electro-magnetic field so.... some things are basically square-waves of 1/0, some aren't and turn a bunch of 1/0's into a smaller bunch of frequency or even phase shift. The latter are the true modems.
posted by zengargoyle at 11:51 PM on March 4, 2023 [2 favorites]
Sorta yeah pretty much. But y'know USB and I do believe twisted pair ethernet are of the streams of 1/0 that are sent and picked by clocking. The bandwidth and frequency and distance stuff still applies, but these are typically shorter and lower speed connections that use voltage differences, not any sort of multiple frequency stacking.
How does a USB keyboard work? - YouTube
Coaxial cable that your talking about (and very old thic and thin ethernet) work differently from the twisted pair types of connections.
But you can still argues that every 1/0 ends up being a voltage or a light or a propagating electro-magnetic field so.... some things are basically square-waves of 1/0, some aren't and turn a bunch of 1/0's into a smaller bunch of frequency or even phase shift. The latter are the true modems.
posted by zengargoyle at 11:51 PM on March 4, 2023 [2 favorites]
twisted pair ethernet are of the streams of 1/0 that are sent and picked by clocking
1000BaseT Ethernet - the type your desktop has - uses PAM5 encoding with symbols consisting of five discrete voltage levels. Error correction is built into the encoding as well.
posted by saeculorum at 11:57 PM on March 4, 2023 [1 favorite]
1000BaseT Ethernet - the type your desktop has - uses PAM5 encoding with symbols consisting of five discrete voltage levels. Error correction is built into the encoding as well.
posted by saeculorum at 11:57 PM on March 4, 2023 [1 favorite]
1000BaseT Ethernet - the type your desktop has
There is still a surprising amount of 100BASE-TX stuff out there, especially on routers/switches or boxes that aren't either pretty new, or at least enthusiast-grade. Its encoding is logically digital, as I understand it (even if yes, it will ultimately be a voltage in a cable which is inherently analogue).
posted by Dysk at 2:23 AM on March 5, 2023 [1 favorite]
There is still a surprising amount of 100BASE-TX stuff out there, especially on routers/switches or boxes that aren't either pretty new, or at least enthusiast-grade. Its encoding is logically digital, as I understand it (even if yes, it will ultimately be a voltage in a cable which is inherently analogue).
posted by Dysk at 2:23 AM on March 5, 2023 [1 favorite]
These days most things that transmit data over a wire (or over the air) could be described as modems since they don't use simple on off keying, frequency shift keying, or phase shift keying, possibly with some error correction. I don't consider simple single bit modulation to be worthy of the name except maybe if it's doing some filtering to limit the passband like dialup modems had to do.
Even modern fiber optics are using advanced modulation schemes to transmit multiple bits per baud. 400ZR optics are most definitely modems by any reasonable definition of the word, though nobody is really going to call it that any more than someone would say that a 1Gbps+ Ethernet NIC is a modern. Technically they are, but calling them that reduces understanding, it doesn't increase it.
posted by wierdo at 3:13 AM on March 5, 2023 [2 favorites]
Even modern fiber optics are using advanced modulation schemes to transmit multiple bits per baud. 400ZR optics are most definitely modems by any reasonable definition of the word, though nobody is really going to call it that any more than someone would say that a 1Gbps+ Ethernet NIC is a modern. Technically they are, but calling them that reduces understanding, it doesn't increase it.
posted by wierdo at 3:13 AM on March 5, 2023 [2 favorites]
Don't want to get into a "philosophical" argument here, but this "analog vs digital" debate is a bit like "is light a wave or a particle" debate.
I grant that I may have simplified it a bit TOO MUCH, but to the average consumer, these broadband options at home, i.e. cable, DSL, and so on, are "digital" mediums. Heck, it's even in the acronym DSL.
Of course, you're still transmitting over a physical medium, and when it involves the EM spectrum, it's a continuous spectrum, there are no clear steps unless you want to get down into the level of individual photons. Modern data transmission is a mixed-mode operation, but that's a "black box" to the average consumer. It's like making sausage: they don't know or care about the actual details. All they really want to know is they plug this in, they get digital signals out.
The term "modem" simply was genericized to mean ANY sort of "access box" that puts out "digital" stuff for network or computer. Yes, it may do modulation at a DEEP hardware level to some extent, but is that really worthy of a "debate"?
posted by kschang at 5:51 AM on March 5, 2023
I grant that I may have simplified it a bit TOO MUCH, but to the average consumer, these broadband options at home, i.e. cable, DSL, and so on, are "digital" mediums. Heck, it's even in the acronym DSL.
Of course, you're still transmitting over a physical medium, and when it involves the EM spectrum, it's a continuous spectrum, there are no clear steps unless you want to get down into the level of individual photons. Modern data transmission is a mixed-mode operation, but that's a "black box" to the average consumer. It's like making sausage: they don't know or care about the actual details. All they really want to know is they plug this in, they get digital signals out.
The term "modem" simply was genericized to mean ANY sort of "access box" that puts out "digital" stuff for network or computer. Yes, it may do modulation at a DEEP hardware level to some extent, but is that really worthy of a "debate"?
posted by kschang at 5:51 AM on March 5, 2023
zengargoyle: Coaxial cable that your talking about (and very old thic and thin ethernet) work differently from the twisted pair types of connections.
Um, thick and thin Ethernet carry signal levels that map 1:1 to a stream of bits, just as 10Mbit and 100Mbit twisted pair does. If you hang an oscilloscope off a coax Ethernet cable you'll see it alternating between two signal levels, no more. Cable internet uses AHellOfALot[tm] signals of different frequencies, which makes it essentially analog.
The related terms, somewhat opaque to outsiders, are 'baseband' and 'broadband'. Coax Ethernet is baseband, cable internet is broadband.
posted by Stoneshop at 6:46 AM on March 5, 2023 [2 favorites]
Um, thick and thin Ethernet carry signal levels that map 1:1 to a stream of bits, just as 10Mbit and 100Mbit twisted pair does. If you hang an oscilloscope off a coax Ethernet cable you'll see it alternating between two signal levels, no more. Cable internet uses AHellOfALot[tm] signals of different frequencies, which makes it essentially analog.
The related terms, somewhat opaque to outsiders, are 'baseband' and 'broadband'. Coax Ethernet is baseband, cable internet is broadband.
posted by Stoneshop at 6:46 AM on March 5, 2023 [2 favorites]
Sure, you can debate about ''digital' signals being reallyanalog, or 'analog' signals being actually digital due to the Planck length and quantum effects.
But what's not philosophically or operationally difficult is knowing what computing paradigm a thing is built on/operating in.
And something like a fiber optic 'modem' is not really a modem in the sense of de/modulating analog signals, while a cable modem is.
posted by SaltySalticid at 6:49 AM on March 5, 2023 [3 favorites]
But what's not philosophically or operationally difficult is knowing what computing paradigm a thing is built on/operating in.
And something like a fiber optic 'modem' is not really a modem in the sense of de/modulating analog signals, while a cable modem is.
posted by SaltySalticid at 6:49 AM on March 5, 2023 [3 favorites]
Response by poster: Thank you all for this enlightening discussion. Very interesting. I'm going to mark this resolved, but feel free to add more if there's more to say.
posted by Winnie the Proust at 4:40 PM on March 5, 2023
posted by Winnie the Proust at 4:40 PM on March 5, 2023
This thread is closed to new comments.
You may be confusing different layers of communication, e.g. the OSI model; almost everything is analog at the physical level, including WIFI and Ethernet, and signal conditioning and transformation is required to convert it into pure digital for computer logic to process it.
Perhaps the simplest way to think of your cable modem is that it is basically WIFI (a high frequency analog radio transmission) operating over a physical wire.
posted by seanmpuckett at 5:59 PM on March 4, 2023 [10 favorites]