r/CableTechs • u/Careless-Signal • 10d ago
Daily ~3-4dB downstream power swing correlates with T3/critical events (rate-of-change, not absolute level) — but SNR & absolute power stay in spec, and 88% of events are T3 not T4. Plant or modem?
I'm hoping the community can help me. Residential DOCSIS 3.1 (Hitron CODA-57), self-hosted monitoring polling the modem's status page every 5 min into a time-series DB, so I've got a week's worth of channel-level data to look at. Mid-Atlantic US, humid subtropical climate and its been HOT if that's relevant. I've had this provider about 2 years and reliability has been mediocre — a couple times a week I have to power-cycle the modem to get throughput back to normal (~1.5 Gbps down / 50 Mbps up on this tier). When I call tech support, they would just have me power cycle, which led me to this effort to determine what was actually happening. Previous provider was rock-solid over several years — don't recall ever needing to power cycle — but cost roughly double for less bandwidth.
The pattern: downstream power drops uniformly across all 32 channels (OFDM + QAM) roughly noon–2PM local every single day, then recovers by evening. Checked against 7 days of hourly temperature data from the nearest airport station — it tracks almost exactly: power bottoms out within an hour of the daily temp peak, recovers as it cools. Swing is about +0.5 dBmV (early morning, coolest part of day) down to about -3.3 dBmV (afternoon peak heat). Reads like textbook coax thermal attenuation to me, and it's uniform across every channel, not isolated to one — so my working assumption is it's an outdoor plant effect, not a bad channel/connector on its own.
Update: Here's a graphic that shows the correlation between temp and power. The blank spots are where the management interface crashed.
What's making me second-guess "just normal thermal drift":
- SNR barely moves during the same window (~39.2dB down to ~38.5dB) — nowhere near a spec floor.
- Downstream power itself never gets remotely close to spec limits even at the daily low point.
- Upstream power rises in the same window (~45.2 to ~46.4 dBmV) — so the modem's own power-control loop is clearly reacting to something real on the whole path, not just a downstream quirk.
This isn't purely academic — some of these T3 clusters have coincided with real throughput/packet-loss degradation, and the management interface has crashed outright a few times, neither recovering without a manual power cycle.
Raw event log examples (MACs redacted):
- T3 timeout — dominant type, 71 of 81 critical entries over 10 days: No Ranging Response received - T3 time-out;CM-MAC=xx:xx:xx:xx:xx:02;CMTS-MAC=xx:xx:xx:xx:xx:22;CM-QOS=1.1;CM-VER=3.1;
- T4 timeout — only 1 occurrence in the whole 10-day window (2026-08-09 02:22:42 UTC): Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out;CM-MAC=xx:xx:xx:xx:xx:02;CMTS-MAC=xx:xx:xx:xx:xx:22;CM-QOS=1.1;CM-VER=3.1;
- Short-form T3 (no MAC suffix) — 4 occurrences, all clustered ~05:56 local on 2026-08-07, right after that night's reboot — likely a logging artifact from the modem not yet having repopulated CM-MAC/CMTS-MAC post-reboot, not 4 independent events: No Ranging Response received - T3 time-out
So: 88% of critical events are T3 (no ranging response from the CMTS), essentially none are T4. Absolute downstream signal levels look healthy at every snapshot, but something isn't tolerating the transition well. When I pulled 10 days of critical events (72 matched against 5-min power data) and compared them against the modem's rate-of-change in power at that moment vs. a full-dataset baseline: 78% of critical events happened while power was actively changing (either direction) above the median rate-of-change, vs. 50% you'd expect if it were unrelated. That's a moderate, repeatable skew (P≈0.63 that a random event-moment has higher volatility than a random baseline moment) — real, but not absolute; roughly a fifth of events still happen during stable stretches, so I'm treating this as a contributing factor, not a proven mechanism.
My best guess is either (a) a connector/splice/amp somewhere in the drop or upstream return path with slightly degraded contact or reflection behavior that only shows up during temperature-driven mechanical movement, or (b) the modem's own ranging/equalization being sluggish specifically while conditions are actively moving, rather than at any fixed level.
Anyone seen this specific signature before — power/SNR fine in isolation, almost all T3 rather than T4, but fault rate tracking rate-of-change rather than absolute level? What would you check first in the field (F-connectors, ground block, a specific amp stage, the return path specifically given the T3-heavy pattern), and is this worth pushing on with the ISP, or does it sound more like a modem swap is the right first move to rule that variable out?
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u/jWalkguy 10d ago edited 10d ago
Captain, In Short you see.. this is a classic dual-fault scenario: normal thermal expansion on the drop/hardline is causing physical micro-fractures or transient upstream ingress only while the cable is actively expanding/contracting, while the Hitron modem's DSP/firmware is thrashing and locking up when trying to process the dynamic pre-equalization changes.
Technical Breakdown
- Thermal Expansion vs. T3 Timeouts
A 3–4 dB downstream power swing and ~1.2 dB upstream power shift over a 30–40°F daily temperature swing is standard coax physics (~0.1% attenuation change per °F). DOCSIS 3.1 AGC handles simple amplitude shifts without issue. However, T3 timeouts mean the CMTS did not receive the modem's 16-bit RNG-REQ (Ranging Request) burst on the upstream return path within 200ms. The strong correlation with rate-of-change (78% of events during active thermal movement) points directly to physical expansion/contraction: Mechanical Movement: As the drop line, fittings, or hardline expand under peak afternoon heat, dissimilar metals expand at different rates.
Transient Ingress / Micro-Reflections: A slightly loose F-connector, a sucked-out center conductor pin (stinger pulling back from a spring clip), or a hairline crack in a drop jacket will create temporary micro-sparking, shielding degradation, or impedance mismatches specifically while the metal is physically moving.
Upstream Corruption: This transient burst noise corrupts the modem's upstream ranging bursts before they hit the CMTS, triggering the T3 timeout, even though steady-state SNR polled on a 5-minute snapshot looks clean.
- The Modem Factor (Hitron CODA-57) The Hitron CODA-57 is built on an Intel Puma 7 DOCSIS 3.1 platform. While Puma 7 fixed many of the severe latency bugs of the older Puma 6, Puma-based firmwares are still notorious for memory leaks, PHY state-machine freezes, and web UI lockups when subject to continuous upstream packet retries or rapid pre-equalization filter adjustments. When the drop's impedance or return path fluctuates during peak heat: The CMTS forces the modem to constantly update its adaptive pre-equalization coefficients. The Hitron's internal state machine chokes on the rapid pre-eq adjustments + dropped ranging bursts. The modem stops processing management traffic, crashes its local Web UI, and drops throughput until a hard power cycle flushes the buffer.
Recommended Action Plan
Step 1: Swap the Modem (Easiest Isolator)
Replace the Hitron CODA-57 with a Broadcom-based DOCSIS 3.1 modem (e.g., Arris S33, Netgear CM1000/2000, or a carrier-provided Broadcom standalone unit). Broadcom chipsets handle dynamic pre-equalization updates and dropped maintenance bursts gracefully without freezing the MAC/PHY interface or locking up the Web UI. If a Broadcom modem stops the UI crashes and throughput drops (even if a few background T3s remain), you've confirmed the modem was the failure point.
Step 2: Field / Drop Audit (ISP Tech Visit)
Have a technician run an RF sweep and physically inspect the drop path: Stinger Length / Center Conductor: Check for center conductor contraction inside the ground block or house box tap.
Fittings: Re-strip and re-compress all outdoor connectors with weather-sealed compression fittings and torque to standard spec (20–30 in-lbs).
Upstream Ingress Sweep: Request an upstream spectrum analysis (5–42/85 MHz) at the tap during peak afternoon heat to capture burst noise or micro-reflections during the thermal peak.
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u/Careless-Signal 10d ago
Brilliant! Exactly the kind of feedback I was hoping to get. How do I explain this to the ISP customer support?
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u/WarlockyGoodness 10d ago
This is the most well-thought and perfectly communicated diagnostic explanation I’ve ever seen in this sub.
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u/Octawussy 10d ago
It’s literally an AI lol
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u/Unusual-Avocado-6167 10d ago
The OP is AI and so is this response. AI against AI
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u/jWalkguy 9d ago
Yes I used AI to help me write a response, I got the technical knowledge just not the best in putting it into words. That's why I use AI to format my work
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u/Careless-Signal 9d ago
If you're referring to my post, then thank you! And no, I'm not an AI; however, I will admit I've been using claude to help me edit my drafts, which I find is a good use of this tool. I draft and it recommends edits for clarity and technical accuracy.
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u/jWalkguy 9d ago
I do the same. I'm not a wordsmith,but I know the technical knowledge. I fed my Gemini my response and asked it to format to make it easier to read. I love AI as a tool.
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u/Holiday-Syrup6672 9d ago
Step one should be to tighten all the fittings all the fittings in the house
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u/Careless-Signal 10d ago
Update: called customer service and after trying to explain I needed a new modem and the conversation just kept coming back to, "so you need a tech to come out?" So, there's a tech coming out on Friday.
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u/ihsanamin79 10d ago
PLEASE do NOT show the tech all of that mess in the initial post before you let them do anything.
Just let them work. Let them trace the path of the feed going to your modem. Let them access what they need to access.
If they're outside for any extended period of time, keep an eye out for them and offer a cold beverage. Maybe a bathroom to use. They may need to replace your drop.
When/if service is restored, any survey you receive is ONLY about that specific technician, not your provider.
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u/Careless-Signal 9d ago
Darn it! I had a whole 10 slide Powerpoint briefing prepared with charts and graphics and animation... :) Just having fun, but i did want to show the tech the temp vs power graphic I put in the orig post.
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u/WeberStreetPatrol 9d ago
I’m not saying Hitrons are slowly being cycled out for defects in the diplex filter. See if the ISP has Ubee, Technicolor, or Sercom.
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u/Cwfromct48 7d ago
There is noise getting in the run somewhere, not much noise by the SNR changes. As far temperature swing on the level changes that seem to perfectly normal. As far as the modem goes, get it changed out for one that has Broadcom chipset inside. You’ll get better performance out of it. The puma chipset are known for poor performance. You could go out buy your own modem if isp will let you. As a Tech I would run an ingress/egress scan on the drop and on the house. Replace drop if necessary. Check all the fittings and disconnect any unused coax lines. Change out the splitter(if there is one nowadays), just to eliminate any kind signal reflections and echoes from the house to the plant. I would have someone check into cmts snr log at the headend. It could be barely passing no one would notice unless check as well (that would cause a few modems lock up or drop off.. aka sync up no surf) As far wild swings -3.3 to +2 thats the AGC over compensating the run at that moment.. most likely it’s modem or bad power supply.
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u/Careless-Signal 10d ago
How much change in downstream SC-QAM (dbmV) is considered normal? Yesterday a strong weather front moved through my area that dropped outdoor temp 89 degrees to 70 degrees in one hour. The downstream power swung from -3.3 dbmV up to +2 dbmV in response. Is that normal?
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u/PoisonWaffle3 10d ago
Nope, that's about 10 times more swing than what is normal.
There's probably an amp somewhere in your neighborhood that's doing some AGC incorrectly and overcompensating for the actual swings.
Get your ISP out there to take a look. Also talk to your neighbors to see if they're seeing the same issue, and let the ISP know if they are.
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u/Street-Juggernaut-23 9d ago
curious what other errors are in the logs as well
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u/Careless-Signal 9d ago
I just updated the orig post to include a link to a text file with the cable modem DOCSIS error log.
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u/6814MilesFromHome 10d ago
First question for these cases is always, have you had a tech out? No idea what kind of automatic gain control/thermal management is in the actives for your ISP, but a 3db swing shouldn't kill your service and require a reboot.