New Member
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1 Message
High Latency during active Upload
Hello,
For approximately the past month, I have been experiencing consistent latency issues specifically during upload activity.
Issue Details
Idle latency: ~20–25 ms (stable)
During upload activity: latency increases to 60–80+ ms with noticeable jitter
During download activity: latency remains stable with minimal increase
The issue is fully reproducible and occurs only under upload load.
Troubleshooting Performed
Fully wired connection (no Wi-Fi involved)
Tested directly connected to the modem (bridge mode, no router/switch)
→ issue persistsRebooted modem and network equipment multiple times
Continuous ping testing shows:
stable latency at idle
significant spikes during upload activity
Waveform bufferbloat testing shows:
Download latency: ~0–1 ms added
Upload latency: +40–50 ms added consistently
Additionally, Cloudflare speed testing shows extreme latency spikes during upload (into the hundreds to thousands of ms under load) while download remains stable.
Summary
Connection is stable at idle
Download performance is normal
Issue is isolated specifically to upload-induced latency and jitter
Issue persists even when bypassing all internal network equipment
Not sure what to do at this point. Support keeps telling me its fine.
Comcast_Airelle
Official Employee
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91 Messages
3 days ago
We really appreciate how detailed you laid this out. You’ve done more testing than most, and honestly, this is exactly the kind of info that helps us take it seriously.
What you’re describing lines up very closely with upload-side congestion (bufferbloat) or a signal issue on the upstream path. The key detail is that everything is stable until you push upload, then latency and jitter spike hard, even when you’re directly connected to the modem. That tells us this isn’t anything inside your home network.
A couple things we want to look at on our side:
First, upstream signal levels and noise. Even if download looks perfect, upload can struggle if there’s noise on the line or the modem is having to “shout” back to the network. That can cause exactly the latency spikes you’re seeing.
Second, node congestion. If the local node is busy, upload traffic is usually the first place you’ll feel it. That would also explain why download stays clean while upload latency jumps.
Third, provisioning or profile issues. Less common, but if something isn’t quite right with how your modem is configured on the network, it can impact upstream performance specifically.
You’ve already ruled out the big things on your end, so we don’t want to keep sending you through the same steps again. At this point, this needs a deeper look from our side.
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