Visitor
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2 Messages
Intermittent upstream noise: upstream transmit power pegging at 54.5 dBmV, OFDMA disabling, recurring drops
Hi Xfinity team,
I’m looking for help diagnosing an intermittent upstream issue that has been ongoing for several days and keeps resurfacing after temporary improvements.
Symptoms
Internet becomes unstable intermittently (latency spikes, packet loss, brief dropouts)
Sometimes improves on its own for hours, then degrades again
Happens regardless of router (gateway is in bridge mode)
Environment
Gateway: XB8 (CGM4981COM)
Mode: Bridge mode
Customer-owned router behind gateway
Wired testing (Ethernet)
No recent changes to in-home wiring or equipment
What I’m seeing in the gateway diagnostics
When the issue occurs, the modem consistently shows:
Upstream transmit power pegged at 54.5 dBmV on multiple ATDMA channels
OFDMA upstream disables and falls back to TDMA
Large and rapidly increasing correctable and uncorrectable codewords
Downstream levels and SNR remain excellent (≈43–44 dB, ~0–1 dBmV)
When things temporarily improve, upstream power drops back into the low–mid 40s dBmV and OFDMA returns — but the problem eventually comes back.
Why I believe this is a plant-side issue
Sustained upstream levels at 54.5 dBmV indicate the modem is maxed out trying to reach the CMTS
OFDMA upstream disabling itself strongly suggests return-path noise
Clean downstream rules out in-home coax issues
Behavior is intermittent and time-dependent, which matches ingress/noise in the plant rather than a constant wiring fault
What I’m hoping for
I’m looking for help reviewing:
Upstream SNR and flap history over the last 24–48 hours
Any return-path noise/ingress seen at the node or tap
Whether this warrants a line/maintenance tech to inspect the tap, drop, or upstream path
I’m happy to provide screenshots of signal levels or error counters if helpful.
Thanks in advance, I appreciate any guidance from someone familiar with upstream/plant diagnostics.
BruceW
Gold Problem Solver
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26.9K Messages
1 day ago
High upstream power and loss of OFDMA do suggest a problem with the return path from the modem, but only "somewhere" between the modem and the CMTS. The symptoms can vary with vibration and temperature changes. Most often, the problem is due to poor coax connections or damaged coax cable, usually in or near your home. Running the cable through a surge protector, a defective splitter, or too many splitters can cause signal problems as well. If there is an amplifier in the line make sure it's getting power.
If you can't find the cause of the high upstream levels or you'd rather have Comcast take care of it and an employee does not respond to your message here, call them at the phone number on your bill or 1-800-Comcast, or use one of the options on https://www.xfinity.com/support/contact-us/. It's not likely they can fix the problem remotely. If not, insist they send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit (approx $100) unless you have their Service Protection Plan ( https://www.xfinity.com/support/articles/service-protection-plan, closed to customers that don't already have it). If the trouble is due to a faulty Comcast rental device or anything outside your home you shouldn't be charged.
Please be aware that there are 2 kinds of responses in this Forum: Replies and Comments. When you Comment on a post by scrolling down to "Comment on this post here...", I am notified of your response. But if you select Reply, I am NOT notified and may not be aware of your response.
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user_z19jso
Visitor
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2 Messages
19 hours ago
Thanks Bruce, I appreciate the detailed response.
I agree that high upstream power and OFDMA loss point to a return-path issue somewhere between the modem and the CMTS, and that vibration/temperature sensitivity can suggest a physical impairment. One extra detail that may matter, I’m in an apartment complex, so there’s building wiring and shared infrastructure between the demarc/tap and my unit that I can’t fully inspect. In my unit, the gateway is on a single direct coax run to the wall plate. No surge protector, no amplifier, and no active splitters in the path. Downstream power and SNR stay very strong (around 43–44 dB SNR and roughly 0 to +1 dBmV) even when the upstream transmit power pegs at 54.5 dBmV and the DOCSIS 3.1 upstream/OFDMA drops. After a restart, upstream usually returns to the mid-40s dBmV and OFDMA comes back, but then it degrades again hours later. This cycle has repeated multiple times over the last several days.
I did check behind the wall plate and saw an unused splitter (the modem feed appears to be on a separate run). I’m going to add 75-ohm terminators to any unused ports to rule out ingress from that angle. If a Community Specialist can review historical upstream SNR/flap history and advise whether this looks like intermittent return-path noise that needs a maintenance/line escalation (potentially at the building demarc/tap), I’d really appreciate it. I can provide additional screenshots/stats if needed.
Thanks again.
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