Suddenly slower speed with Arris SB8200

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.

The upstream power is out of spec, period. And it may be intermittently fluctuating even farther out of spec. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.

In a self troubleshooting effort to try to obtain better connectivity / more wiggle room, check to see if there are there any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage types like GE, RadioShack, RCA, Philips, Leviton, Magnavox, and Rocketfish from big box stores like Home Depot, Lowes, Target, Wal-Mart etc. Splitters should be swapped with known to be good / new ones to test

If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two way splitter connected directly off of the drop from the street/pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed, and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.

Thanks, both Bill and EG for your answers to my issue. I get only internet service from Xfinity, so the cable comes in directly from the street and ends at my cable modem with no stops or splits along the way. There have been no changes at my house, other than the aging of the cable, installed about eight years ago, and the cable modem, about three. I certainly recognize that that is sufficient for weaknesses to be exposed. I have checked the upstream power levels this morning, and everything is reporting the same as yesterday evening. However, when I run the same speedtest as before, I am now getting the normal and expected speeds I had been getting in the past - 119 Mbps. So my tendency would be to leave things as they are, until and unless the problem recurs. At that point, I think I would have no choice but to follow EG's advice and have Xfinity come for a visit.

I'd get a tech out if there is nothing more that you can do.

Bear in mind that if the premises facing techs can not find or fix a problem at your home, it is they who are responsible for escalating it to their line / network / maintenance dept. techs. The problem may lie beyond your home in the local neighborhood infrastructure somewhere but it is their S.O.P. to start at the home.

Too high / out of spec. A problem just waiting to happen if it's not happening already....... The worse part that's next is a complete loss of connectivity. Good luck.

Thanks, EG. I really appreciate your help and advice. I know you have been providing similar help on these forums for a long time. 

Following your advice, I called Xfinity and reported the numbers for the upstream channels. The agent said he created a ticket for review of the issue and said he believed they would send someone to review the signal in my area, but not specifically to my house. He invited me to call back if I had an issue with the download speed in the meantime. That was as far as he was willing to take it. I don't know if I should have or could have insisted that they send someone specifically to my house to investigate. He also tried to blame my SB8200, and suggested I should buy a new modem, but I pushed back on that. In any case, what I don't know, and can't know at this point, is how long those upstream bonded channels were too high. I don't know if that is recent or if they have been that way for a long time. I am only aware of what they are now.  I will monitor my speed more frequently and make sure to report back to Xfinity if it slows down again. That seems to be all I can do at this point, unless you are recommending that I escalate this further with them and insist that they send someone to my house based on those channels being out of range.

I would. That will get the ball rolling because as stated, the only way that the outside maintenance tech department will get involved is if the premises techs write a ticket for maintenance. And it may take several complaints....

Good luck with it !