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smallxmac
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Wednesday, August 12th, 2026 7:58 AM

Very Noticeable Latency Spikes at first Xfinity hop under downstream load even with clean RF/FEC

I’m seeing reproducible severe downstream latency under load. My LAN gateway remains around 0.3 ms and my Hitron CODA56 management interface remains around 1–2 ms, while Comcast’s first routed hop rises from roughly 13 ms into the 200–450 ms range. External destinations such as 1.1.1.1 rise by essentially the same amount.

The issue is directly correlated with downstream load and improves when I reduce download throughput. I reproduced it off-peak as well.

I reset the modem’s FEC counters and tested several minutes of sustained >90 MiB/s downstream traffic. The DOCSIS 3.1 OFDM channel remained locked with ~38–39 dB SNR and 0 uncorrectable.  Downstream channels are approximately +7 to +9 dBmV with ~39 dB SNR and essentially no uncorrectables. Upstream channels are roughly 36.8 - 39.2 dBmV.

Is there any way to get insights into the downstream CMTS/node or connect with a network engineer/tech that could check? 

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EG

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120K Messages

2 days ago

Please also post the upstream power level values.

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smallxmac

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Ops, didn't realize I left that part out. Edited the original message with it. Though for the sake of data here's the current stats which look almost identical to when I'm experiencing latency spikes.

Downstream Overview
Port ID Frequency (Hz) Modulation Signal strength (dBmV) Signal noise ratio (dB) Octets Correcteds Uncorrectables Channel ID
1 507000000 256QAM 7.400 38.605 3 * 2e32 + 2926028949 6 0 20
3 405000000 256QAM 7.600 38.605 3 * 2e32 + 2924766977 0 0 4
4 411000000 256QAM 7.400 38.605 3 * 2e32 + 2924767209 0 0 5
5 417000000 256QAM 7.200 38.605 3 * 2e32 + 2924771824 0 0 6
6 423000000 256QAM 7.300 38.983 3 * 2e32 + 2924776534 0 0 7
7 429000000 256QAM 7.600 38.983 3 * 2e32 + 2924781182 0 0 8
8 435000000 256QAM 7.600 38.605 3 * 2e32 + 2924758577 0 0 9
9 441000000 256QAM 7.200 38.983 3 * 2e32 + 2924785943 0 0 10
10 447000000 256QAM 7.000 38.605 3 * 2e32 + 2924795154 0 0 11
11 453000000 256QAM 7.300 38.983 3 * 2e32 + 2924799799 0 0 12
12 459000000 256QAM 7.700 38.983 3 * 2e32 + 2924804458 0 0 13
13 465000000 256QAM 7.800 38.983 3 * 2e32 + 2924809165 0 0 14
14 471000000 256QAM 7.600 38.605 3 * 2e32 + 2924791135 0 0 15
15 477000000 256QAM 7.500 38.605 3 * 2e32 + 2924813969 2 0 16
16 483000000 256QAM 7.400 38.605 3 * 2e32 + 2924755000 0 0 17
17 489000000 256QAM 7.900 38.605 3 * 2e32 + 2924818734 9 0 18
18 495000000 256QAM 8.200 38.983 3 * 2e32 + 2924823242 0 0 19
19 513000000 256QAM 7.600 38.605 3 * 2e32 + 2924828095 0 0 21
20 519000000 256QAM 8.000 38.605 3 * 2e32 + 2924832769 0 0 22
21 525000000 256QAM 8.200 38.605 3 * 2e32 + 2924837401 0 0 23
22 531000000 256QAM 8.200 38.605 3 * 2e32 + 2924842149 0 0 24
23 537000000 256QAM 7.700 38.983 3 * 2e32 + 2924846748 0 0 25
24 543000000 256QAM 7.400 38.983 3 * 2e32 + 2924855929 1 0 26
25 549000000 256QAM 7.800 38.605 3 * 2e32 + 2924860593 0 0 27
26 555000000 256QAM 8.300 38.983 3 * 2e32 + 2924851608 0 0 28
27 561000000 256QAM 8.400 38.605 3 * 2e32 + 2924865357 0 0 29
28 567000000 256QAM 8.300 38.605 3 * 2e32 + 2924870049 0 0 30
29 573000000 256QAM 7.900 38.605 3 * 2e32 + 2924765638 0 0 31
30 579000000 256QAM 8.000 38.605 3 * 2e32 + 2924874814 2 0 32
31 585000000 256QAM 8.000 38.605 3 * 2e32 + 2924879481 0 0 33
32 591000000 256QAM 8.000 38.605 3 * 2e32 + 2924752430 0 0 34
OFDM Downstream Overview
Receiver FFT type Subcarr 0 Frequency(Hz) PLC locked NCP locked MDC1 locked PLC power(dBmv) SNR(dB) Octets Correcteds Uncorrectables
0 NA NA NO NO NO NA NA NA NA NA
1 4K 713600000 YES YES YES 5.300003 38 830436762 815833857 1203
Upstream Overview
Port ID Frequency (Hz) BandWidth (Hz) Modulation Type DOCSIS Mode Signal Strength (dBmV) Channel ID
1 16400000 6400000 64QAM ATDMA 38.271 2
2 40400000 3200000 64QAM ATDMA 36.760 11
3 35600000 6400000 64QAM ATDMA 37.021 5
4 29200000 6400000 64QAM ATDMA 38.271 4
5 22800000 6400000 64QAM ATDMA 38.521 3
6 10400000 3200000 16QAM ATDMA 39.260 1
OFDMA Upstream Overview
Channel Index State Subcarr 0 Frequency(Hz) lin Digital Att Digital Att BW (sc's*fft) Report Power Report Power1_6 FFT Size
0 DISABLED 0 0.0000 0.0000 0.0000 0.0000 0.0000 2K
1 DISABLED 0 0.0000 0.0000 0.0000 0.0000 0.0000 2K

 

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2 days ago

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EG

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120K Messages

2 days ago

The upstream is in spec.


This may or may not be the root cause of the problem (YMMV), but it should be addressed regardless.


The downstream power is on the high side. It may be overdriving 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://store.orbitalconnect.com/ppc-belden-forward-path-attenuator-1800-mhz-series/ 


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

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smallxmac

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Thanks for advice; I think I agree with you that it's likely not the root cause, but the filter is cheap enough to give it a shot.

> Is there a drop amplifier on the coax cable line leading to the modem

As far as I can tell it just 2 barrel connectors from the ground line to the modem.

I don't _think_ I'm in a mid split area, but I think I'm on the bounds of it. Just two streets over they have higher upload offering.

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2 days ago

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XfinityBenjaminM

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3.2K Messages

13 hours ago

 

smallxmac Hello! Thank you for reaching out to us here on our Community Forum. We are sorry to hear about the issues you're having with your connection, and we'll be happy to help. We also appreciate you working with our Expert as actively as you have been. We'll be happy to take a look at your signal more closely and go from there. Please send a Direct Message with your full name and address.

 Click "Sign In" if necessary
  • Click the "Direct Message" icon (upper right corner of this page)
  • Click the "Start a New Conversation" (pencil and paper) icon
  • Type "Xfinity Support" in the to line and select "Xfinity Support" from the drop-down list
  • Type your message in the text area near the bottom of the window
  • Press Enter to send your message

 

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EG

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120K Messages

5 hours ago

@smallxmac @XfinityBenjaminM 

Please circle back here and post any possible solutions for the issue here in these open public forums so that all readers here may benefit from the exchange / info. This is in keeping with the spirit for which these public help forums were originally intended. Thank you.

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