Have you got 75 ohm terminators on every open F connector port in your "cable plant?" All the comast wall outlets, if not in use, should have a terminator, and any unused splitter port as well. THis probably isn't your issue, but it's considered a "best practice" to terminate all lines in their characteristic impedance to prevent reflections. But I bet it's a Comcast line problem. Som corroded connctor someplace. Or a line amp that has drifted below spec. I would not be too concerned about stuff in the log, the logs can be quite verbose- as long as you are seeing good quality IP communications. No packet loss, no loss of connectivity, modem constantly rebooting itself, extreme latency, excess jitter, etc. Do a bunch of line quality tests ( https://packetlosstest.com/ etc) and see if there is a pattern - in my case, it seems that during "peak" hours - right after 5 or 6 O'Clock - the service isn't quite as good. But I think that is to be expected. In the middle of the night it is fine. Once you have some data on line quality, then make your decision on whether or not to call in Comcast.
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The upstream power levels that your SB200 is reporting are the levels of electronic power that the SB200 needs to push out in order for Comcast to "hear" it clearly. The way this works is the central system at Comcast waits for the SB200 to send data to it, and when it gets the data it ackowledges receipt back to the SB200. So the SB200 starts at low output power levels, basically shouting "Hi can you hear me?" and if Comcast's central system does not reply then the SB200 tries again, at a higher power level. This iterates until the SB200 reaches it's maximum power level capacity which is someting like +65 dBmv. According to what I read something between 35 and 49 is considered GOOD, and 51 dBmv for 4 bonded channels is considered the maximum acceptable level. If your SB200 is having to push +55 dBmv of signal in order for Comcast to "hear" it, then there is likely something wrong. ( See https://pickmymodem.com/signal-levels-docsis-3-03-1-cable-modem/ ) It IS possible that your SB200 has developed an issue of some kind. I have seen cable modems fail in funky ways over the years, and there seems to be a certain amount of "wear" over time; I suspect this is from various voltage surges induced into the cable line by lightning, switching of heavy loads on nearby power lines, and so forth. I have no proof that this is what causes cable modems to degrade and fail over time; it's just my suspicion. But whatever the cause I have seen cable modems degrade such that they perform poorly and I've also seen them fail outright. Depending on the age of your SB200, you might consider getting a new cable modem. I've had an SB200 for about 4 years and it's still OK, but it would not surprise me if I found myself replaing it in the next year or two. But before replaing the cable modem, THE FIRST THING I would look at is the coax cable in your house or apartment. Does your feed from Comcast come directly to your SB200? Or does it go through splitters and hundreds of feet of loose cable coiled up behind your tv.... If your "cable plant" isn't fairly direct, and TIGHT, it isn't going to allow transmission of signals from your SB200 back to Comcast. So, design your cabling with as few splitters as possible, and with your SB200 with as few connections between you and Comcast's system up on the pole or buried underground. Every splitter that sits between your SB200 and Comcast robs the signal. And using low-quality splitters is even worse. Also, each connector (in cable these are known as "F" connectors) ecah F connector has a potential to cause problems, so as few F connectors along the line as possible, and no male tofemal-to-male "splices" along the cable run between your SB200 and Comcast. And use GOOD QUALITY cables. Don't have to be super high end stuff, but have to be solidly made, no corroded F connectors, no F connectors that are loose on the cable ready to be pulled off, no home-made cabling unless you know how to properly build cabling using high quality connectors like Snap-N-Seal and a pro-quality crimping jig. And the F connectors have to be TIGHT. Not strip-the-threads-with-a-12-inch-vice-grip tight, but maybe just a bit tighter than finger tight using a pliers or open end wrench. If you have more than one Comcast wall socket in your home or apartment, make sure that whatever is connected to ALL of them is tight. If you have unused, empty comcast F connector wall sockets, or empty unused ports on any splitters, make sure each of these has a terminator installed in to them. Empty ports on splitters and open Comcast cable wall sockets cause impedance kinks in the line that can reflect signals backwards, this can cause some trouble, so get terminators such as these from Amazon, you get 25 for $7 https://www.amazon.com/dp/B00JJYDU4Q/ref=cm_sw_em_r_mt_dp_Y9qjFbHXK08HP For testing purposes you might want to connect your SB200 direct to the Cmcast wall outlet, and disconnect EVERYTHING else - TVs, cable boxes, etc - from Comcast so that the ONLY thing connected is your SB200 wired DIRECTLY with a single length of coax and NO SPLITTERS right to the wall socket. OK, so if you are sure the wiring between your SB200 and the Comcast wall socket is good, next make sure that the cabling that runs through your house or apartment is good. Look in wiring closets etc to make sure all the F-connectors in there are tight, that there arent 5 splitters splitting and splitting and splitting the signal... AND if ALL of that is for sure good, then the problem is with the Comcast cable that runs from your home to their system- Comcast might have a bad connecor or the local loading of the line might not be balanced. Only Comcast can do anything about this. So if FOR SURE you are confident that all the wiring on your permises is clean, tight and direct, and you are still having problems and STILL seeing upstream power levels above 51 dBmv, have Comcast send a tech out to fix it.
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