Downstream issues

[akselk]

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)

Startup Step	Status	Comment
Acquire Downstream Channel	543000000 Hz	Locked
Upstream Connection	OK	Operational
Boot State	OK	Operational
Configuration File	OK	d11_m_mb7621_extremeproplus_c01.cm
Security	Enabled	BPI+

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

Make sure your device is connected to a power source. If you're still experiencing issues, try restarting your device.

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]

There’s no easy way to put it but your downstream levels are garbage. . Remember, this modem is accessing channels, the old modem didn’t. If it’s bad at the ground block, more than likely a bad drop but because you have service, because of the pandemic they may or may not run a tech for it. I’ll escalate it up to an Official Employee.

[akselk]

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

[ComcastAmir]

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".

[akselk]

Thanks Amir! Sent you a message 🙂

Dennis

[ComcastAmir]

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? 

[akselk]

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

[akselk]

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

[akselk]

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

[akselk]

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!

[EG]

I'm going to attempt to re-escalate this issue. Good luck !

[ComcastJoeTru]

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. 

[akselk]

Thank you @EG !

[akselk]

Thank you @ComcastJoeTru , will do!

[EG]

Quite welcome ! Good luck with it !