Cable Modem Turned Spectrum Analyzer

Hopefully by now most of us know better than to rent a modem from an internet service provider. Buying your own and using it is almost always an easy way to save some money, but even then these pieces of equipment won’t last forever. If you’re sitting on an older cable modem and thinking about tossing it in the garbage, there might be a way to repurpose it before it goes to the great workbench in the sky. [kc9umr] has a way of turning these devices into capable spectrum analyzers.

The spectrum analyzer feature is a crucial component of cable modems to help take advantage of the wide piece of spectrum that is available to them on the cable lines. With some of them it’s possible to access this feature directly by pointing a browser at it, but apparently some of them have a patch from the cable companies to limit access. By finding one that hasn’t had this patch applied it’s possible to access the spectrum analyzer, and once [kc9umr] attached some adapters and an antenna to his cable modem he was able to demonstrate it to great effect.

While it’s somewhat down to luck as to whether or not any given modem will grant access to this feature, for the ones that do it seems like a powerful and cheap tool. It’s agnostic to platform, so any computer on the network can access it easily, and compared to an RTL-SDR it has a wider range. There are some limitations, but for the price it can’t be beat which will cost under $50 in parts unless you happen to need two inputs like this analyzer .

Thanks to [Ezra] for the tip!

19 thoughts on “Cable Modem Turned Spectrum Analyzer

  1. “thinking about tossing it in the garbage”

    If the readership of Hackday is like me, this is probably not only the last thing on our minds but also borderline unthinkable.

    1. Sods law, if you keep it, it’ll be sat in a junk box and never used, if you throw it away, it’ll be needed a few months from now. Although granted, before throwing it away, I’ll at least have the LEDs, oh and the cable, and the… And, it’s in the junk bin anyway. I need a clear out at least every other year.

  2. My Arris SB6183 has this feature but I don’t find it useful. Instead, I periodically scrape the modem signal data with Perl and write that to a text file. I then use Excel to plot/analyze the data: downstream power & SNR, upstream power, and correctable/uncorrected data. It provides more information to me than the spectrum analyzer and quite fortunately for me, so does the ISP maintenance engineer who receives my analysis when things are acting badly in my neighborhood. There are way more elegant methods I’ve seen in Python and Grafana.

  3. Renting a modem? It that a US thing?
    I am getting my modem on loan from my ISP.
    They work with Fritzboxen, which are quite reliable, secure and full featured.
    On average I have gotten a new model every two years.
    If I ever terminate my subscription, I have to return it, that is all.
    I pay 68 euros per month for a 40/4 connection, TV (settop box also on loan) and IP-telephony (no handset supplied).
    Fixed IPv4 and IPv6 addresses.
    Often they have nice free extras: a WiFi extender, a SIM-card for mobile data (500 MB/month).
    And they don’t freak out when they hear the word Linux.
    At the moment I have a bit of fiberglass sticking out of the pavement in front of the house, can’t wait for it to be extended inside.

    1. 35 euros a month for optical fiber and blazing fast internet, 4k TV, telephone. Well and where I live university is free, fast trains everywhere,health insurance is including everybody, food is local and nobody has guns except some hunters.

      1. Thinking more not as many malicious penal colony industry networks, i.e. malicious attorney, law enforcement, healthcare, courts, jails and prisons desperate for “work” and “paychecks” to the point where they have “strike” and “assault” teams that are far worse than the union strike operations whether official or unofficial. Now days with hostile takeovers of whatever using remote sensing concealed wireless sound, body and mind control assault weapons operations and operators that are most likely the reason why the main stream media is playing dumb regarding the “Havana Syndrome Health Attacks” that are far more common than most will admit and desperately aggressively compounded and concealed by full time obstructionists obstructing justice and not justifiable serial killing whatever they want and can.

        The causation is not the guns, the causation is the malicious intent in those remote sensing concealed wireless sound, body and mind control assault weapons operations. The guns are not the imminent threat. What brain washes and Mann Act violates those to use guns maliciously are the imminent threat causation. Granted, there may be a few narcotics/poisons, sexual deviants & sodomizers and/or inbreeds that may without nurturing natural maliciously assault, batter, maim and mass murder.

  4. “Hopefully by now most of us know better than to rent a modem from an internet service provider. Buying your own and using it is almost always an easy way to save some money, but even then these pieces of equipment won’t last forever.”

    No charge on this end. Coincidentally they upgraded me and I’m returning the old one. So no e-waste.

  5. I’m wondering if, with the right software, it would be possible to fully bend the broad spectrum ability to our will add an amplifier and use it as a fully fledged software defined radio. If it can display the frequency plot and then receive and transmit internet once one is selected, then I guess, without knowing the inner workings, it might be possible, unless they have some hardware channel hopping or encryption that can’t be bypassed or accounted for in software.

    1. And if it can be configured to operate as a remote software defined radio, maybe with a man in the middle acting as a web host to redirect it. Would the hacker community create a massive global network that pools all that spectrum data into a single source for triangulating signals across the spectrum. That’s a lot of power for anyone who can wield it.

  6. Another conspiracy against rights and deprivation of rights under color of law not allowing the access to the spectrum analyzer. Complete B.S.! Granted the developer tools console access while online is a concern when malicious acts performed.

    However, seems a firmware work around is possible to allow the 8888 (or your modem) SA access offline maybe as a way?

    Like the comment above by @CRJEEA & @tekkieneet, I’m feeling some hack ability capability.

    Can be a great advance from the cable box spectrum analyzers.

  7. Hmm as above I have a cable modem never thrown away, it’s coming back with “961”,”682.543353″,”ArrisGro_07:43:8b”,”Dell_66:b3:d9″,”ARP”,”60″,””,” is at 00:e0:6f:07:43:8b”

    But I wonder if it’s too old for this stunt, the main chip being a lbt4030bpce

    Mind you now I’ve opened the box there are a bunch of 0.1″ headers including 2 2×20’s so I wonder what can be connected to these….

  8. Encountered a couple of old cable modems, both acted like they didn’t know port 8080. Suspect they had the ‘cable haunt’ vulnerability patch pushed into them from some ISP. “Factory reset” didn’t fix it. There really ought to be a way to reflash these things (apart from being an ISP) even if it means soldering wires onto the board to get at JTAG or whatever.

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