DOCSIS 3 cable modem

DOCSIS 3 Cable Modems

The internet is relied upon for many different functions. One thing that I see more of is STL via the public network. There are many ways to accomplish this using Comrex Bric links, Barix units, or simply a streaming computer.

We often can take for granted the infrastructure that keeps our connection to the public network running. Cable modems are very common as either primary or backup devices at transmitter sites, homes, offices, etc. The basic cable modem uses some type of DOCSIS (Data Over Cable Service Interface Specification) modulation scheme. This system breaks up the bandwidth on the coaxial cable into 6 MHz channels for downstream and upstream transmission. Generally, downstream transmission is 16 channels of 256-QAM signals. Upstream is 4 channels of QPSK or up to 64-QAM signals. Depending on your traffic shaping plan with the cable company, this will allow up to 608 Mbps down and 108 Mbps up. Those speeds also can change due to network congestion, which is the bane of coaxial cable-based internet service.

The internet should now be considered a public utility. Especially after the COVID-19 emergency, distance learning, telecommuting at all the other changes we are experiencing. I know in the past, ISPs were reluctant to accept that role, as there are many responsibilities. That being said, when the public network goes down, many things grind to a halt.

Sometimes the problem is at the cable office or further upstream. Loss of a backbone switch, trunk fiber, or DOCSIS equipment will cause widespread outages which are beyond anything a field engineer can deal with.

Then there are the times when it is still working, but not working right. In that situation, there are several possible issues that could be creating a problem. A little information can go a long way to returning to normal operation. One thing that can be done with most newer cable modems, log into the modem itself and look at the signal strength on the downstream channels. Again, most cable modems will use as their management IP address. The username and password should be on the bottom of the modem. I also Googled my modem manufacturer and model number and found mine that way.

Navigate around until you find a screen that looks like this:

DOCSIS 3.0 Downstream Channel Statistics

There is a lot of helpful information to look at. The first thing is the Pwr (dBmV) level. DOCSIS 3 modems are looking for -7 dBmV to +7 dBmV as the recommended signal level. They can deal with -8 to -10 dBmV / +8 to +10 dBmV as acceptable. -11 to -15 dBmV / +11 to + 15 dBmV is maximum and greater than -15/+15 dBmV is out of tolerance.

The next column to look at is the SNR (Signal to Noise Ratio). DOCSIS 3 needs to be greater than 30 dB and preferably 33 dB or greater.

The last two columns are the codeword errors. This is a Forward Error Correction (FEC) system that verifies the received data and attempts to correct any corrupted bits. The lower the codeword error number, the better the data throughput. Codeword errors are often due to RF impairments and can be a strong indicator of cable or connector issues. Another possible cause is improper signal strength, which can be either too high or too low.

Upstream data is transmitted on 4 channels.

DOSSIS 3.0 Upstream Channels

The only statistic that is useful on the upstream channels is the Pwr, which should be between 40 and 50 dBmV.

I have found a few simple parts and tools that can sometimes restore a faltering cable connection. First, I have several attenuator pads; 3dB, 6dB and 10 dB with type F connectors. This has actually cured an issue where the downstream signal was too hot causing codeword errors. Next, some good Ideal weatherproof crimp-on F connectors for RG-6 coax and a good tool should also be in the tool kit. I have had to replace mouse-chewed RG-6 from the outside cable drop into the transmitter building. Fortunately, there was some spare RG-6 in the transmitter room.

If these attempts do not fix the issue, then of course, be prepared to waste a day waiting for the cable company to show up.

Print Friendly, PDF & Email

5 thoughts on “DOCSIS 3 Cable Modems”

  1. Nice writeup, Paul!

    For me two biggest problem with cablemodems is one: infant mortality. These things are very cheaply made, and it’s not uncommon at all to get a “bad” one right out the box.

    But two, and more important, is that cablemodems usually require “power on the poles” to function. That is, if the power goes out in your area, it doesn’t matter if you have a generator! The cablemodem still won’t work. I don’t know why this is; I assume there’s some kind of signal boosters on the lines/poles that have to be powered to work properly.

    That’s why I’ve come around to never relying solely on a cablemodem for an STL. There needs to be something else that’ll work during a regional power outage (i.e. after a severe thunderstorm, hurricane or snowstorm). Verizon FiOS usually does. A lot of copper-based services will (T-1, DSL, frame-relay, etc). You can use 4G LTE based internet but obviously that’s playing with fire; there’s no guarantee a regional power outage won’t knock out enough cellphone towers/facilities around your broadcast tower to also deprive your tower location of 4G service.

    One thing you can do, that surprisingly doesn’t occur to a lot of people, is to use cheap wireless ethernet radios (like Ubiquiti/UBNT) not just to be a direct STL, but also to extend internet service from a distant location to your tower. This can get you a lot of flexibility since perhaps you have a more reliable ISP (like FiOS) a given distance away from your tower, and those UBNT radios – even the cheap ones – are often good for 10 or 15 miles.

  2. To Brad’s question, it depends on how the cable system operator has built up their Outside Plant. Where I live, all the power supply boxes have a concrete pad nearby, and a generator power input, and after a power-loss event, they roll the trucks with small portable gennies to those points. Now, if they forget to check the fuel levels, oil, etc., that’s another story! DSLAMs have the same generator provisions, for AT&T UVerse and DSL.

    Here’s a link to a very useful site that explains DSL and Cable Outside Plant hardware…

  3. Oh great another link to follow. So down this rabbit hole interesting site.

    The cable company uses by directional amplifiers on the polls and prior to data service they were one Direction. after they had added data service and consequently phone service they fell under the regulations that required them to have battery backed up infrastructure for emergency Communications which the phone company is required to have. I don’t know which came first on that, as the phone network originally ran off a battery system so independently had redundancy off the grid. I don’t know at what point it became a requirement.

    At one point the FCC was trying to make Internet considered a utility but got shot down. Then we had a different administration and what little power they had was revoked.

    I’m just a crazy person who thinks all technologies AM in FM can coexist with Internet radio and future tech. It’s nice to have some thing that’s extremely basic and easy to implement and receive especially in emergencies.
    I just wish we mandated cell phone companies to have FM radios in the phones accessible to the users, like they do in many other countries.

  4. > The internet should now be considered a public utility.

    Yup, that’d be great.

    But one of the reasons the cableco can do it so much cheaper than the ILEC is precisely because the cableco isn’t *bound* by all those tariffs and standards that made it possible to treat the telco as a Public Utility.

    I’d love to be able to treat cablemodem internet as a Public Utility.

    But I can’t — it doesn’t live up to those standards, and it probably never will. So I have to build around it. And lemme tell you, getting physical diversity of cable internet to a building is … well, let’s say it can go up into 5 figures all by itself, even if you *have* a competitive provider, and weren’t smart enough to build in both footprints.

    No, I must respectfully disagree. We’d *like* to be able to trust cable internet to live up to Public Utility standards, but I went through Hurricane Irma 3 years ago, and I can tell you: Verizon/Frontier never went out for more than maybe an hour.

    Spectrum was down for 4 or 5 *days*.

    Even Sprint cell was down for 48 hours.

  5. Just noticed the reply thanks. I’d like to clarify I don’t mean cable Internet I mean all Internet, we certainly learned that in 2020. They need enough room to do what is needed but need to be regulated as being required as so many rural areas don’t get the opportunity unless they’re willing to pay lots of money to get the cable line extended or deal with DSL if they’re close enough or dial up. and on the greedy maybe that’s not the right word satellite providers that give you a capped data plan but don’t tell you about it and sell it to you like it’s regular Internet and then your kids/ Family members get on and your datas gone because it’s hard to comprehend that it’s not regular Internet. I briefly had Hughes net GEN2or3. And that was after frontier which barely worked and our bill kept increasing as we cut services.
    We ended up with cellular which through AT&T managed to stay up throughout hurricane Sandy meanwhile a certain satellite provider didn’t and then after the waters receded told me that electrical currents from the dish would come in and fry everything in the house but that’s not a big issue you should have a tech come out and look at it but I’ll be out of your pocket. I even told the installer when they re-installed…. the dish that was now in a floodplain, oh it’ll be fine…. The dish got submerged.

    on That note the phone company should’ve been expanding their infrastructure for years and improving it then they would’ve been left with something like this

    Yes cable… “We have the largest fiber network” one of the big things they kept advertising for a while, that’s funny I looked out my window every time and I still saw copper.
    I am in a Comcast district, and I used to live in upstate New York and family tells me Time Warner cable got bought out or transferred to spectrum and didn’t fulfill their agreements.

    I would love to say as a public utility would require a guaranteed up time as the Internet is used for Waymore than a fad and now it’s being used for actual life-saving devices, security systems and emergency communications so it should require it to be consistent.

Leave a Reply

Your email address will not be published. Required fields are marked *