Problem of frequent connection dropping out and packet loss, and tried fixes

Problem:

I had good internet connection by Comcast for many years. But starting from this Tue, my internet started dropping out frequently.

My setup is:

• The 400Mbps/10Mbps plan
• An XB7 modem in bridge mode
• The modem connects to a Unifi USG-3P router (--> Netgear POE switch --> Unifi APs for WiFi)

The symptoms are:

• From clients:
  • Everything (including webpage, streaming, etc.) paused every a few seconds. Phones complained no internet connection or limited connection on WiFi.
  • When running "ping 8.8.8.8", the first one (icmp_seq=0) was always fine but the rest pings were all request timeout.
  • "traceroute 8.8.8.8" worked well for most of time and didn't show packet loss.
• From the XB7 modem:
  • The logs from the modem repeated "No offer or ack received X times" (X=1-5) every 10 seconds.
  • All the Diagnostic Tools from the modem worked well and didn't show any problems.
  • The downstream SNR seemed normal (between 40 to 43), but the power level seemed high (QAM256 channels at 12-14dbmV, the OFDM channel 193 at 18dbmV).
  • The upstream power level seemed normal (around 40).

Fix try #1 - restart the modem:

I connected my laptop directly to the modem (bypass the router) and restarted the modem. In the first 5 minutes after the restart, pings were normal without packet loss but then the internet quickly started dropping out.

Fix try #2 - install a 6db attenuator pad:

According to some posts, I suspected the issue was the high downstream power level. So I added a 6db attenuator pad to the end of the cable yesterday afternoon. Here was what I got:

• It seemed to work on the downstream power level.
  • It was brought down to around 7 (QAM256) and 11 (OFDM), about 6db less.
  • The upstream power level was increased to around 47, still in the range.
• But the good internet didn't last for an hour and then still kept dropping out after.
  • E.g., "No offer or ack received" was gone from the logs but was back again after an hour.

The interesting thing is that I saw the downstream power level was actually increasing slowly to around 10 (QAM256) and 13 (OFDM) in a few hours. After I removed the attenuator, the increase was retained to around 15 (QAM256) and 19 (OFDM). The issue was the same.

Fix try #3 - switch the modem from bridge mode to router mode [FIXED]:

I noticed the weirdest thing was that although the modem logs showed "No offer or ack received" and the laptop directly connected to the modem had broken internet and packet loss, but the tests by modem's Diagnostic Tools always passed. I felt the modem seemed able to handle something better internally. So I did a last try last night to switch the modem from bridge mode to router mode. The only difference was the WAN port address of my Unifi router changed from a public IP to a local IP (10.0.0.x), still good for my use. But it seemed to fix the issue now.

No dropping out anymore.

I also tested adding and removing the attenuator. The downstream power levels were still higher than the original ones, but there was no difference at client side: steady internet connection at the right speed. The only difference I noticed was at the ping latency (ping 8.8.8.8):

• Without attenuator: around 30ms
• With attenuator: around 11ms

So I decided to leave the attenuator there.

Also, in this morning, the downstream power levels (with attenuator) suddenly dropped to 8 (QAM256) and 10 (OFDM). Not sure what happened at the Comcast end.

Questions:

Without any understanding of the issue, the root cause, or the fix, my internet connection seems to be back to normal now. The downstream power level looks like a mystery to me. I'm not sure if anything may change in the future just like this issue suddenly happened from nowhere. I appreciate if anyone could explain it and confirm the fix (switch from bridge mode to router mode), so that this may help others solve similar issues. Thanks!