Cable modem troubleshooting (DOCSIS Line faults & disconnections)

About the author

Michael Spalter

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Michael Spalter has been a networking technician for over 30 years and has been the CEO of DrayTek in the UK since the company’s formation in 1997. He has written and lectured extensively on networking topics. If you’ve an idea for a blog or a topic you’d like explored, please get in touch with us.

Cable Modem Troubleshooting & Disconnection Problems

"Cable Modem" or just "Cable" service is generally used to describe Internet or TV/Phone service provided over coaxial cable using DOCSIS technology.  This is provided mostly by Virgin Media (Liberty Global) in the UK and all carriers in North America.  Most cable companies do also provide DSL and pure fibre ('FTTP') service so always be sure about exactly what type of service you have.  

Cable modem service is generally reliable but like any other medium can have service problems.  These are most often due to service interruptions or maintenance by your provider or wiring or equipment faults somewhere between the cable company (ISP) and yourself.  If you don't have cable service, this article probably isn't much use to you, but may still have some interesting general principles.

This document is about diagnosing the cable service itself.  If you've lost 'local Wi-Fi' or Ethernet connectivity then it's a different problem so if your router and modem are combined, the cable service may be fine but it's an issue on the LAN-side.  This guide may still enable you to eliminate your cable service as the problem.  

If your cable service goes down, it may be due to one of the following:

• Scheduled or emergency maintenance by the cable company
• A cable fault in your street or further downstream
• A power issue or fault at the CMTS
• A mis-configuration of your line (at the CMTS)
• An IP layer connectivity issue (routing back to the Internet)
• Work going on in the street on the cablecos wiring
• A weak signal or interference on the line
• A problem with your in-home cabling (a break or disconnection)
• A fault with your cable modem

You, as the subscriber can only potentially solve some of those. For others, you're in the hands of your cable company (e.g. Virgin Media in the UK). Plain cable modems rarely fail - this has been verified by countless 'cable guys' and technicians over the years however there's a continuous battle between subscribers and the cableco customer service reps trying to avoid a costly call-out. Whereas DSL uses one dedicated line per user, DOCSIS uses a shared medium - one cable provides the feed to many homes/premises), splitting off into each home it passes. If they need to work on the street cable or equipment to repair or extend it, disconnecting it will disconnect multiple homes.

Check your Cable company's web site status page or social media feeds to see if there's any known service interruption but otherwise you may have to call them. Despite the cost, the advantage of renting or using their modem is that they can't blame 'your' hardware and then bill you for the call-out but $120/year rental is a lot more than the cost of buying a modem outright.

When you call your cable company, the initial advice, which is correct, is to check your connections and reboot the cable modem (turn it off for 10 seconds then back on).   This applies to set-top boxes for TV service too if they are on coax, not just your primary Internet cable modem.

Above: Your cable modem or router will have LEDs which give a basic status indication.

Check all of the physical connections (plugs/sockets) around your home and that check that no cables have been damaged or crushed.

Observe which LEDs light up once the modem has reinitiated - that can take up to 5 minutes after rebooting.  Modems LEDs will vary between models but will normally have an LED for US (Upstream) and DS (Downstream), then 'Online' (indicating registration) and LINK (indicating that the Ethernet port is connected or 1,2,3,4... if it's a router).  The US/DS LEDs may flash when establishing a connection and be steady on when stably connected but this does vary so consult your vendor's manual.   

Your cable modem or combined modem/router will have diagnostics screens in its GUI however not all cable companies allow access to those menus or lock them with an engineer-only password. The GUI is often reachable at 192.168.100.1 or 192.168.0.1 but check your specific model.  If you can get into them, check your signal strength:

The image above shows my own modem diagnostic readings before and after I tidied up my own wiring. Ideal signal will be between -7dBmV and 7dBmV. So -3dBmV signal would have been workable normally, but in my case I was getting 'mysterious' disconnections each morning at 8am, when presumably there was a local power fluctuation.

You can also check the 'signal to noise ratio' - that's how much of the signal is useful vs. noise and interference.  That will be shown in the SNR column or RXMer (Receive Modulation Error Ratio) on some modems.  An ideal SNR is around 36-40dB.

As in the image above, there should be multiple downstream channels active - up to 32 downstream channels on the fastest current DOCSIS 3.1 services, if your modem supports that. If your channel diagnostics shows only a signal channel then the modem has failed to detect or synchronise with any signal at all.

Modem Logs

Your cable modem (or router) should also have some error logs which may show the cause of the current or previous disconnections or other line problems.

Some messages include error codes 'T1 to T5' which are DOCSIS timeout codes and indicate that no response was received from the far end (the CMTS or cable company). Your modem may also classify error codes from 1-6. The most common categories are 3 'Critical', 4 'Error', 5 'Warning' and 6 'Notice'. Your error log will rarely be blank. Even a service which is working well and providing no apparent issues will have occasional errors or company maintenance. These will usually pass without you noticing, though errors shouldn't occur many times a day. Unfortunately, the most common messages in the logs indicate issues which you yourself can't do anything about, other than checking that your modem and cabling is all working and intact as they may be issues for the cable company to resolve.

Spectrum Analyser

Many Cable Modems have a built-in Spectrum Analyser; this shows a graph of the RF spectrum available for the cable modem and which parts of it are active.  In theory, if you have, say 8 downstream bonded channels and 4 upstream, you would see spikes at each of those 12 frequencies on the graph when they are actively transmitting (uploading or downloading).  In reality, DOCSIS works hard to make use of all available channel bandwidth but not every part of every channel is usable to the same degree and in DOCSIS 3.1 with OFDMA, frequency usage is even more granular.  

In the example image above, we've set the frequency range of the analyser to 635Mhz to 655Mhz which corresponds to 3 of the 16 downstream bonded channels. Between each channel is a dip, which is the interchannel spacing.  You can see the peaks representing the width of each channel - 6Mhz each (or 8Mhz in Europe).  Note that these are consumer grade analysers - not scientifically accurate or calibrated tools, but still give a useful approximation.

Cable modem spectrum analysers are usually found on custom HTTP ports, not within the normal web interface. These are the common ones: http://192.168.100.1:8080 (Arris/Surfboard), 192.168.100.1:49200 (Netgear), 192.168.100.1:22267 (Zoom). On Ubee modems look under the main diagnostic tab.  Arris are the world's largest producer of cable modems and make many of the models which are badged under cable company's brands, including Virgin Media.

If you have your own cable modem then having a spare one enables you to check it on your line to identify whether it's a hardware fault.  In the UK, Virgin media do not allow you to use your own modem but your neighbour may allow you to borrow theirs - but also check if they've lost service too.  Theoretically, in the UK, even though Virgin won't authenticate your own modem, if you have one, it should still get DOCSIS sync. but won't connect to Internet service.

Cable, sockets and plugs Integrity

Check that your cables have not been damaged and are connected well. F-Connectors should be snugly tight (but not too tight - there should be no movement). If you have many devices around the home on the coax, and thus multiple splitters, try the modem on the primary coax feed (where it first enters the house) and remove any splitters/extensions temporarily.
Splitters and extensions will weaken your signal and if the connection is marginal, it may fail each time there's a power fluctuation. I had this myself when I'd lose Internet connectivity at 8am every morning - presumably other local power draws caused a voltage drop around that time which made the cable signal strength just drop below the usable threshold.  Removing my unnecessary extensions and splits around the house solved the problem (signal attenuation went from -3dBmV to 7.87dBmV which is quite an extreme improvement. It's been fine ever since.

Above: In the basement, the cable signal was split multiple times to provide many outlets around the home. The resulting low signal strength led to regular disconnections. Couplers/joins also add loss (reduce signal strength) so try to have complete unbroken runs rather than multiple cable segments joined.

Cable Company and Street Equipment

If your outside cable is accessible or visible check it as it leaves your home for any damage. Check any junction boxes for damage or water ingress.  I've seen coax cable run into homes in some quite shocking ways leaving them prone to damage or interference such as through hedges and if cables are run at surface (not overhead or underground) then can be easily damaged. Builders, gardeners or other workers can sometimes accidentally cut or remove cables when working. If you have overhead cables, check if the pole has fallen or been damaged and report that to your cable company.  Do not interfere with any equipment or junctions in the street or on a pole.

Left:  A coax service running down the street, providing two drops into homes as it passes.  Right: The cable as it enters a house. You should not interfere with either of these locations, but you can check them for any damage or any obvious problems and report those to your cable company.

Above : There may also be splitters outside of your building which are in a position where they might be damaged, connectors work loose or corrode if they are not protected from water.  All outdoor connections should be housed - splitters should not be explosed.   For safety, you should also check that cables are not protruding or providing a tripping or choking hazard.

Beyond checking all of the above, if you can't find a local cause, you're then in the hands of your cable company - you will need to contact them, but armed with the information you may have collected in some of the checks above, they may escalate your issue more quickly.  Note : The first thing that they will always ask you to do is reboot your modem so do that before you call, or while you're on hold.

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