U

Visitor

 • 

2 Messages

Monday, June 30th, 2025 3:16 PM

Severe DOCSIS 3.1 OFDM Errors, Low Throughput after switching to 2Gbps plan, Maple Valley, WA

I recently upgraded from a 1.2 Gbps to 2 Gbps Xfinity plan. Since the change, my internet performance has dropped drastically getting 700 Mbps download and 5 Mbps upload. My line has been error-free for years before switching my plans. QAM256 channels are great, ruling out in-home issues (and nothing has changed here). 

Symptoms:

  • Download speed averages 600-700 Mbps, upload rarely exceeds 10 Mbps (plan is 2 Gbps down and 250 Mbps up)
  • Modem (MB8611) OFDM PLC (DOCSIS 3.1) channels show millions of errors within minutes of reboot (1.5M in under 10 minutes, over 300M in 7 hours)
  • Downstream QAM256 channels are perfect: 0 corrected/uncorrected errors, SNR and power levels ideal
  • No issues with home wiring: direct modem connection, no splitters, all new cables

What's been tried:

  • Multiple remote reboots with Xfinity support, modem resets, and re-provisioning. Errors always return and climb quickly.
  • Xfinity advanced support says "all looks fine", but OFDM error counters and speed tests from Xfinity (speedtest.xfinity.com), OOKLA, and Cloudflare all report the same: <700 Mbps down, <10 Mbps up
  • A tech is scheduled to come out in two more days to investigate further

Requests:

  • Has anyone experienced a similar issue after a node or POP reassignment or plan upgrade?
  • Looking for advice on getting this escalated to Xfinity's network engineering team, or any way to restore service to the previous, stable configuration.
  • Would appreciate confirmation from an Xfinity engineer/mod that this issue is recognized and being addressed

Screenshots:

OFDM error rate 8 minutes and 37 seconds after reboot of 1.3M on one channel, 153K on another.

Xfinity speed test 6/30 8:44 AM

Expert

 • 

111.4K Messages

19 hours ago

@user_mahgs1 

Whatever else may be going on, the 8611 is not compatible with that speed tier. Please see this topic from yesterday: https://forums.xfinity.com/conversations/your-home-network/my-plan-has-much-lower-upload-speed-that-it-should/68615698d2b4741a720cc422?commentId=68617303d2b4741a720f8f43 

Also. OFDM channels are very wide bandwidth channels that pack a lot of data within. Due to that fact, seeing a large number of corrected errors is normal. The FEC (Forward Error Correction) circuitry / system is doing its job. All of those errors are *corrected* ones. There are zero *uncorrected* errors. 

That said, the downstream power is too high. It may be over-driving the front-end receiver circuit of the modem. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.


Is there a drop amplifier on the coax cable line leading to the modem ? If so, try removing / bypassing it and see. If not, you can try installing a -6 dB forward path-only attenuator pad such as this one on the coax port on the rear of the modem to knock that power level down and see;

https://www.amazon.com/line-Signal-Forward-Attenuator-FPA6-54/dp/B07882H96R  

For areas that have been upgraded to the new Next Gen (mid-split) upload speeds;

https://www.multicominc.com/product/multicom-mul-fpa85-forward-path-attenuator  

These won't affect the upload power, which is within specs.

Visitor

 • 

2 Messages

Thank you for the detailed reply and the links.

  • No drop amp is present, and the modem is direct to strike. All new cabling, no splitters.
  • Downstream power is on the higher side of DOCSIS spec, consistently +9 to +12 mBmV. I can try at -6 dB forward-path attenuator to bring it closer to +3-6 dBmV
  • I understand the MB8611 has compatibility issues with the upstream and aware of other reports from users with this modem. I have a replacement modem on the list you sent inbound to replace
  • My OFDM channels are showing hundreds of millions of corrected errors per hour, but uncorrected errors are zero (see below)

To be precise: I understand that some corrected errors are normal, but this rate is far higher than what I've seen for years at the same address. The problem started exactly when I changed service to 2 Gbps from 1.2 Gbps. My QAM256 channels remain with zero corrected or uncorrected errors. I suspect the line is "on the edge" and FEC is making a real impairment.

I'll try an attenuator and post the results, but would appreciate input from others as well.

Updated screen shot 2 hours 19 minutes uptime, 311M corrected errors on one OFDM channel and 3.4M on the other. This is not expected on a healthy DOCSIS 3.1 segment?

(edited)

Expert

 • 

111.4K Messages

16 hours ago

Try that attenuator and a compatible modem. If problems remain, you can go from there. Please post back with an update. Good luck !

forum icon

New to the Community?

Start Here