CM500V Cable Modem Intermittently Loses Connection on Xfinity Network

[RAnPC]

Within the last few days of late May and early June 2020, my internet connection has randomly dropped and not recovered until the modem was power cycled.  Upon each instance of the connection failure, it was observed that the upstream indicator was always blinking while the downstream indicator remained solid green.  Reviewing NetGear's tips on checking power levels for the downstream connection (+7 to -7 dBmV)  and upstream connections (38-48 dBmV) it was confirmed that both levels for my network connection were indeed in range right after power cycling the modem, so nothing to post just yet.  I compared the power levels with and without an Xfinity-provided splitter as I have TVs at every coax connection.  What remains to be discovered is what the levels will be at the time of the next failure.  This is indeed not a router problem, so no need to power cycle it and interrupt the connected devices.  I will updated this post with screenshots of the power levels should they be out of specification.  

Xfinity customer

CM500V Modem

AC1750 Smart WiFi Router

[FURRYe38]

Have the ISP check the signal and line quality UP to the modem. 
Be sure there are no coax cable line splitters in the between the modem and ISP service box. 
Be sure your using good quality RG6 coax cable up to the modem. 
Be sure to power OFF the modem for 1 minute them back ON.

[RAnPC]

Hello Furrye38,

Thanks for your response.

Have the ISP check the signal and line quality UP to the modem. 

--This has been performed already with Xfinity.  Although some changes were made to the bandwidth and signaling, the problem continues to happen.

Be sure there are no coax cable line splitters in the between the modem and ISP service box. 

--A splitter is needed, as there are not extra drops available that can connect just the modem.

--Currently waiting for next failure to determine if power levels are out of spec for either up or downstream connections.

Be sure your using good quality RG6 coax cable up to the modem. 

--RG6 cables were provided by Xfinity.

Be sure to power OFF the modem for 1 minute them back ON.

--This has been performed already with Xfinity.

[RAnPC]

UPDATE - The Power Levels Have Fallen, Causing The Link To Fail

In the attached screenshots, it is evident that at random times, the power level had dropped to zero for one of the upstream channels, causing the link to fail and the uplink LED to blink.  When this happens, the modem must by power cycled to restore connectivity.  So, is this an Xfinity issue or a NetGear issue?

[FURRYe38]

I would let your ISP know about this and how them what your finding. 

You might get another modem and put online and see if that exhibits the same thing or not. IF it does, then the ISP needs to fix there signal on the line. 

[RAnPC]

After finally getting an Xfinity technician involved, it turns out that Xfinity may have been performing some maintenance on the line that my residence is connected to.  The tech indicated that the maintenance activity was noted in their "watchtower" system for the dates that I reported issues, and that the first level customer support should have seen this as well.  While the connection problems were solved by me each time by power cycling the modem, it was a major nuisance to experience this while working from home.  Thus, this was not a modem problem, however a fault tolerance feature that recognizes when a channel's power level reaches 0 and reboots the modem automatically would really be nice, given that Xfinity can cause such problems at any time without warning.  

My router logs (Nighthawk R6400v2) also revealed multiple DDOS attacks during the period in question.  The attacker seemed to have performed a network scan and located an open port on my router that I occaisionaly use for remote access to my system.  After closing this port, the scans continued for a few days, then subsequently stopped.  I failed to mention this to the Xfinity tech, as I wanted to him to reveal if Xfinity was truly the cause of this problem.  So, as it stands, the network attacks have stopped and Xfinity is finished maintaining lines in my area.  We will see if any of this happens again.