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

Responses

EG

Expert

•

111.4K Messages

12 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.

user_mahgs1

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)

11 hours ago

EG

Expert

•

111.4K Messages

9 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 !

Your Home Network

user_mahgs1

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

EG

user_mahgs1

EG

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