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Cable Modem Channel "Suckout"
Version 1.2b (updated September 17, 2021)

The problem: Is your cable modem Internet connection working most of the time, but dropping sporadically and periodically (like only during the middle of every day)? If so, check to see if cable modem channel 'suckout' is the problem -- something you can check yourself.
The technique described below works best with DOCSIS 3.1 modems with 32 download channels.
"Suckout" Defined: A "Notch in the frequency response affecting one or more channels, caused by loose modules, module covers, printed circuit boards, poor grounding, and similar problems inside of active or passive device housings." See source for this definition and second source.
Some online comments also suggest that temperature variations, causing loose outdoor cables to expand/contract, can also be a root cause of suckout.
YouTube video showing "suckout"
My Internet was dropping daily: I was seeing sporadic and periodic Internet outages (often for just seconds, sometimes for minutes to hours) almost every single day for years. After numerous tech visits (over years), with each tech stating that the Internet was working just fine, finally the last tech said that the problem was "suckout", caused by a cable company device (a 'tap') up the telephone pole.

Modem Diagnostics Webpage: When you look at your modem's diagnostics web page (often at, look for a "Downstream Bonded Channels" table with "Power" and "SNR" numbers per channel. All channels should have 'similar' "Power" and "SNR" values. If not, this 'imbalance' may indicate a problem (a 'minor' imbalance is OK; a 'major' imbalance is not). Here is a 'good' example (we expect the power level for all channels to be 'similar'):
Suckout Example: For my modem, I was getting a major power imbalance between channels. Graphing the "Power" level for each cable modem downstream channel (by MHz) showed a significant 'notch' or 'suckout' (a big dip or valley) for the first channels:
Subtract the minimum power level seen (for any channel) from the maximum power level seen. From the above graph, this value is 8 - (-9) = 17. This is too large. Instead, the result should have been much closer to zero (example below).

After the fix: After the cable company fixed the 'suckout' issue, power levels (and SNR) looked a LOT more normal (consistent/level/flat). Here is a graph of the "Power" level for each downstream channel (by MHz) after the fix:
Subtracting the minimum from the maximum yields '2' -- a very good result. The modem at this location was a Netgear CM1000V2 using 32 bonded downstream channels.
Notice that spotting "suckout" becomes much easier with lots of bonded channels (each channel is 6 MHz wide). With 8 bonded channels covering only 48 MHz of spectrum you are not very likely to notice 'suckout'. But with 32 bonded channels covering at least 192 MHz of spectrum, spotting 'suckout' becomes much easier.
Are you seeing suckout? Eliminate/Confirm house wiring as the problem: The cable company loves to blame "house wiring" as the problem. So find out for yourself (don't take the word of the cable tech)! Spend $15 and get a high quality 100-foot RG6 cable and connect your modem directly to the cable line feeding your house.

The goal is to disconnect your entire house from the cable company, and run a NEW line to only your modem (and nothing else) and see if the 'suckout' problem remains.
Do not use any splitters or other RG6 cable. You want a direct connection from your cable modem to the RG6 feed provided by the cable company using only the NEW line. Look for a gray cable box on the outside of your house where utilities (electrical/phone/cable/gas/etc) enter your house (the 'demarc' location).
If the 'suckout' problem goes away, then you know the problem is somewhere inside your house. Slowly add splitters/cable back in until the problem is found.
This may be very tricky, because the problem could be a bad cable or splitter anywhere in the house -- the problem may not be on the one run feeding the modem. A bad connection on a cable in a bedroom could negatively affect the modem running in a room on the other side of the house.
However, if the 'suckout' problem remains (after disconnecting your entire house), then you KNOW the problem is 'upstream' and with the cable company (or with a bad cable modem).
It was only after I provided this power imbalance evidence to the cable company that they finally admitted that the "suckout" problem was their problem to fix!

Extreme Broadband IPA1008D-RSVF
TIP: How to improve CATV signal quality / SNR: The CATV industry now makes very affordable 'lossless' 8-port splitters/amps for home use.

The key advantage these new amps have (over amps made years ago ago) is that these new devices are 'smart' and have NO power loss in both the downstream (52-1002 MHz) and upstream (5-42 MHz) paths, which is critical for DOCSIS devices (and VoD set top boxes) to work.
Many older amps (still sold today) only amplified the downstream, and left the upstream with full (lots of) power loss, which could negatively impact VoD cable boxes. These old style amps have "passive return" and must be avoided.
One example is the "Extreme Broadband IPA1008D-RSVF - 8 Way Splitter" (seen above right) for around $45 on Amazon. This is the exact brand/model that my cable company (Charter/Spectrum) in North Carolina installed.
So if the IPA1008D-RSV is 'good enough' for the cable company itself to provide and use, I feel confident enough to buy/use elsewhere (and I have).
The IPA1008D is remotely powered via a 'power injector' added to one of the cable runs in the house. By design, ONE passive port of the amp remains active after a power outage and all other active ports 'go down'. This one (and only) passive port trades -3.5 dB of loss for this feature/functionality -- so that modems with VoIP remain online and can still place phone calls during power outages.
It is strongly recommended that you connect your cable modem (regardless of having VoIP, or not) to the passive port on the amp.

While cable modems WILL continue work on any port (and this is something that I have tested) the noise on the un-amplified return path on the passive port 'should' be slightly better (less) than the noise on the amplified ports -- as this avoids the "11 dB loss + 15 dB gain" return path (see below). I say 'should' because the critical SNR value that would definitively answer this question (how much of a noise difference) is in the CMTS and is not accessible to us. In fact, in Maryland, a Comcast tech installed a PPC EVO1-9-U/U and connected the cable modem to one of the amplified ports (not the passive port) -- so yes, it works.
How these new amps work: A block diagram representing (generally) how these new amps work (source):
Notice that these new amps are just an unamplified 2-way splitter followed by a bi-directional amplified 8-way splitter.
The bi-directional amp amplifies only "H" (high) frequencies (52-1002 MHz) in the downstream direction (cable modem download) and only "L" (low) frequencies (5-42 MHz) in the upstream/return direction (cable modem upload).
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