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

7 hours 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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6 hours ago

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EG

Expert

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120K Messages

5 hours 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

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

EG

smallxmac

EG

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