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Downstream issues
I recently switched plans (from 175Mbps to 600Mbps) and then a couple of weeks later switched modems from Motorola SB6121 (4x4) Motorola MB7621 (24x8). Since the modem switch, I've had all sorts of trouble, most pragmatically extremely fluctuating download speeds (between 1 and 90Mbps, which is slower than the 135Mbps that I used to get with the older modem), along with short disconnections throughout the day.
Looking into this more, the downstream channels aren't bonding, many of the channels are not locking, power levels are out of spec and some channels' SNR are too low. I also have hundreds of thousands/millions of uncorrected errors. There's also a wide range on the power and SNR levels.
Upstream looks fine, if a little on the higher end on the power levels. The upstream connectivity is quite steady and locked right around 17Mbps regularly, no matter what the downstream is like.
I've read through the long and very helpful Internet Troubleshooting Tips thread along with https://pickmymodem.com/how-to-fix-your-docsis-3-0-3-1-signal-issues and https://pickmymodem.com/signal-levels-docsis-3-03-1-cable-modem and https://www.motorolacable.com/documents/CableConnection.pdf
I also connected the modem directly to the cable outside my house (to the grounded connector) and that did not help much. I basically have a home run from there to the modem's usual place inside, with no splitters at all, just a couple of 1:1 connectors and the wallplate connector.
I've included numbers below of what it looked like when connected directly outside. Though the power levels are above -15dBmV, when inside they drop to between -15 and -17. This is after about 7 minutes of uptime, note the large number of uncorrected errors, some of which are more than the corrected ones. Usually channels 19-21 are not locked, but sometimes others are not locked too.
(You may need to scroll right to see all the columns)
| Channel | Lock Status | Modulation | Channel ID | Freq. (MHz) | Pwr (dBmV) | SNR (dB) | Corrected | Uncorrected | Uncorrected/Corrected |
| 1 | Locked | QAM256 | 21 | 615 | -10.9 | 40.8 | 0 | 0 | |
| 2 | Locked | QAM256 | 2 | 489 | -13.1 | 0 | 4703 | 23237 | 494% |
| 3 | Locked | QAM256 | 3 | 495 | -13.1 | 38.9 | 0 | 0 | |
| 4 | Locked | QAM256 | 4 | 507 | -13.3 | 34.7 | 0 | 0 | |
| 5 | Locked | QAM256 | 5 | 513 | -13.3 | 38.4 | 0 | 0 | |
| 6 | Locked | QAM256 | 6 | 519 | -13.6 | 29.9 | 1654688 | 107419 | 6% |
| 7 | Locked | QAM256 | 7 | 525 | -13.4 | 0 | 0 | 0 | |
| 8 | Locked | QAM256 | 8 | 531 | -13.1 | 0 | 0 | 0 | |
| 9 | Locked | QAM256 | 9 | 543 | -12.7 | 31.8 | 357319 | 21259 | 6% |
| 10 | Locked | QAM256 | 10 | 549 | -12.9 | 39.2 | 0 | 0 | |
| 11 | Locked | QAM256 | 11 | 555 | -12.4 | 30.8 | 29 | 0 | 0% |
| 12 | Locked | QAM256 | 12 | 561 | -12.2 | 38.5 | 0 | 0 | |
| 13 | Locked | QAM256 | 13 | 567 | -12.1 | 0 | 79116 | 179177 | 226% |
| 14 | Locked | QAM256 | 14 | 573 | -11.6 | 0 | 156451 | 720202 | 460% |
| 15 | Locked | QAM256 | 15 | 579 | -12 | 33.3 | 14933 | 104 | 1% |
| 16 | Locked | QAM256 | 16 | 585 | -11.5 | 36.4 | 0 | 0 | |
| 17 | Locked | QAM256 | 17 | 591 | -11.6 | 36.7 | 0 | 0 | |
| 18 | Locked | QAM256 | 18 | 597 | -11.2 | 38.5 | 0 | 0 | |
| 19 | Not Locked | Unknown | 0 | 603 | 0 | 0 | 0 | 0 | |
| 20 | Not Locked | Unknown | 0 | 609 | 0 | 0 | 2240159 | 478860 | 21% |
| 21 | Not Locked | Unknown | 0 | 483 | 0 | 0 | 0 | 0 | |
| 22 | Locked | QAM256 | 22 | 621 | -11.3 | 40.3 | 0 | 0 | |
| 23 | Locked | QAM256 | 23 | 627 | -10.6 | 40.9 | 0 | 0 | |
| 24 | Locked | QAM256 | 24 | 633 | -10.7 | 41.9 | 0 | 0 | |
| Min | -13.6 | 29.9 | |||||||
| Max | -10.7 | 41.9 | |||||||
| Range | 2.9 | 12 | |||||||
| Average | -12.2 | 36.9 |
| Channel | Lock Status | Channel Type | Channel ID | Symb. Rate (Ksym/sec) | Freq. (MHz) | Pwr (dBmV) |
| 1 | Locked | ATDMA | 2 | 5120 | 32.4 | 47 |
| 2 | Locked | ATDMA | 1 | 5120 | 38.8 | 48.1 |
| 3 | Locked | ATDMA | 3 | 5120 | 26 | 47.2 |
| 4 | Locked | ATDMA | 4 | 5120 | 19.6 | 45.8 |
| 5 | Not Locked | Unknown | 0 | 0 | 0 | 0 |
| 6 | Not Locked | Unknown | 0 | 0 | 0 | 0 |
| 7 | Not Locked | Unknown | 0 | 0 | 0 | 0 |
| 8 | Not Locked | Unknown | 0 | 0 | 0 | 0 |
I should mention that on the Comcast Status Centre, it says that it can't connect to my modem, although it's able to successfully do a remote restart, though this doesn't help.
We're having trouble finding a signal
I've power cycled the modem numerous times and have talked to Comcast support on chat several times, who were not able to make any progress.
I'm thinking that at this point, I need to schedule a technician to come and take a look at the external line - is that right, and if so, what's the best way to accomplish that?
Thank you!
CCAndrew
Gold Problem Solver
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25.9K Messages
5 years ago
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akselk
Frequent Visitor
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9 Messages
5 years ago
Thank you Andrew, for the confirmation! It's not just me then 🙂
Yeah, I was also thinking that the new modem is tring to coordinate 6x as much as the old one.
Thank you for escalating! I guess at this point, I should just sit tight and wait for the escalation process to run its course, right? Given that this repros entirely outside the house, I'm hopeful that they would be able to look at it without having to come inside.
Have a great weekend and take care,
Dennis
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CCAmir
Gold Problem Solver
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7.2K Messages
5 years ago
Hello akselk, I'd be happy to help you with your signal issues. I'll need to run some tests and look at your signal levels from my end to determine if a on site tech is needed. To get the ball rolling on this please send me a private message with your full name by clicking on my name (ComcastAmir) and then click "Send a message".
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akselk
Frequent Visitor
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9 Messages
5 years ago
Thanks Amir! Sent you a message 🙂
Dennis
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CCAmir
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7.2K Messages
5 years ago
Thank you for this information Dennis, and thank you for being a valued customer with us since 2003. We appreciate your loyalty and I hope that you continue to choose us as your service provider for many years to come. After looking into this I see that there's two modems still listed on the system, could you please confirm your MAC address with me here?
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akselk
Frequent Visitor
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9 Messages
5 years ago
Sure thing -
Old 4x4 modem: Motorola SB6121 (A4:7A:A4:78:D8:FB)
New 24x8 modem: Motorola MB7621 (00:40:36:65:83:33)
Let me know if there's any other info I could provide!
Dennis
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akselk
Frequent Visitor
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9 Messages
5 years ago
Hi Amir, just wanted to check if you had had a chance to check into signal levels on your end, and also if there was anything else I could provide or you'd like me to try from here.
Thanks so much for looking into this!
Dennis
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akselk
Frequent Visitor
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9 Messages
5 years ago
Hi,
Just wanted to check on this -- I haven't heard back since my last message, so I wanted to make sure everything was on track to get this figured out 🙂
Thanks!
Dennis
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akselk
Frequent Visitor
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9 Messages
5 years ago
I've had problematic connectivity and downstream speed issues for about a month and have diagnosed it to be a problem outside of my house. I have a forum post about it here: https://forums.xfinity.com/t5/Your-Home-Network/Downstream-issues/m-p/3330119
I did a bunch of troubleshooting and CCAndrew confirmed my findings too. He escalated to Comcast and a Comcast employee got in touch initially, but I haven't been able to hear anything back since the initial contact.
I'm looking for some advice on if there's a way to move this forward, or how to schedule a technician to take a look at the node or the drop. They wouldn't need to enter my home at all, since the issue is entirely reproducible outside at the ground block.
Thanks!
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EG
Expert
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110K Messages
5 years ago
I'm going to attempt to re-escalate this issue. Good luck !
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XfinityJoeTru
Official Employee
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7K Messages
5 years ago
Hello akselk. Thanks for reaching out for help here on our Forum. Apologies your internet issues continue. We thank you for your patience during these difficult times. In many cases, our process to escalate any line or node / plant issues must come from an in-home facing technician's referral. As you may already know, due to Coronavirus, our technicians are limited. I can assist with further troubleshooting, to determine what more can be done for you. Please send me a private message and include your full name so I can assist you.
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akselk
Frequent Visitor
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9 Messages
5 years ago
Thank you @EG !
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akselk
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9 Messages
5 years ago
Thank you @ComcastJoeTru , will do!
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EG
Expert
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110K Messages
5 years ago
Quite welcome ! Good luck with it !
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