DOCSIS 3 Cable Modems

The internet is being relied upon for many different functions. One thing that I am 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 user name and password should be on the bottom of the modem. I also Googled my modem manufacture 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 more 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 which 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 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 weather proof 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.

Air conditioning? We don’t need no stinking air conditioning!

It has been hot out around here the last week or so. Somebody’s office server needed a little extra help:

Office server fan
Office server fan

I am not a fan (pun intended) of this type of thing.  Too often, we make do with things that are simply substandard.  In an emergency, I get it; you do what you have to to get things going again.  However, after the system is recovered comes the remedial phase, which includes making permanent repairs, replacing outdated equipment, installing things properly, making sure that wiring meets electrical code, documentation, labeling, etc.

The remedial phase is often neglected or forgotten altogether.   There are two reasons for this; the “saving money” reason, or the too busy to deal with it reason.  However, later on, we or the person that follow us, will have to deal with this again after some sort of catastrophic failure.  Then there will be the questions:  How did this happen?  How long has it been like that? and so on.

As far as saving money goes; you are not.  Cutting corners may save a few pennies in the short term, but long term, it only creates bigger problems which will have to be dealt with at some point.  Doing things the right way will shift the engineering effort from a reactive (e.g. fire fighting) to a proactive stance and everyone will be much happier.

Network Security, part II

With the spate of ransomware and crypto virus attacks on automation systems, perhaps a quick review of network security is in order:

  1. Isolate the automation system on a separate network from the general office network and do not allow internet access on the automation system’s work stations or servers.
  2. Use a separate switch for all automation network connections.
  3. install a small router between the automation network and the office network.  On the router, the WAN port faces outward toward the office network, make the WAN port non-pingable.  Grant access from the office network for certain users; e.g. traffic, music director, etc via access lists.  Open up a few ports for VNC or RDP on the router so technicians can remotely access machines to do maintenance and troubleshooting.
  4. Use supported and up to date operating systems.
  5. Use separate admin and user accounts, make sure that admin rights are removed from user accounts and keep machines logged in as users.  This ensures that some errant DJ or other person does not install any unauthorized programs.
  6. Install and keep up to date a good antivirus program.
  7. Back up the data and test the backups.

The office network is more vulnerable because of the human element.  Internet access is require, of course.  Click on a pop up, sure!  Hey, that photograph has a funny file extension, lets open it and see what it is.  I never heard of this person before, but look, they sent me an executable!

Much of the office network security will rely on the quality of the router connected to the internet and the antivirus software installed.  Of course, the network users have a good deal of responsibility also.

Cable Porn

On occasion, the company I currently work for does installation work. Thus, I am always keeping my eyes open for new equipment and tools to make that job easier. The cable comb seems like it is just such a thing:

ACOM tools cable comb
ACOM tools cable comb

Instructional video from youtube:

Then there is this:

Which is simply amazing. It is described as “1320 Category 6 cables, dressed and terminated.”

Incidentally, there is an entire sub-reddit: for all those cable geeks that like to look at neat cabling work.