Visitor
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3 Messages
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?
EG
Expert
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120K Messages
15 hours ago
Please also post the upstream power level values.
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EG
Expert
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120K Messages
13 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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