MB8600 slower than expected connection; provisioned wrong?

I upgraded to the gigabit plan a few days ago while I still had my SB6141. The next day, my MB8600 was delivered.

With the SB6141, I consistently got around 280Mbps/12Mbps. After I upgraded to the Gigabit plan, I still got 280Mbps down with the SB6141, but I immediately began getting about 23-24Mbps up.

The next day, I installed and activated the MB8600 per Xfinity activation guide and app. A speed test then showed about 180Mbps/10Mbps. Obviously, a drastic speed decrease is not what I was expecting after upgrading my plan and my hardware.

I went through troubleshooting with Xfinity chat and was told that I needed to contact Motorola:
Xfinity rep: "Please don't get the wrong idea. I think it's not a problem on the modem because as I can see here everything is fully updated on the modem. I think we just need to contact the manufacturer so they can change some configuration on the modem.
ok!
I can actually do that to sync it to 1000mbps if that is Xfinity Modem. But since it's a motorola, we don't have a tools to change it. All you need to tell them is your current download speed is up to 1000mbps and they need to check if the bootfile is compatible to the speed.
And then that's it! They will know what to do there."

I contacted Motorola and the representative told me that he was confused as to why Xfinity referred me to Motorola because Motorola has nothing to do with sending boot files to modems. He was nice enough to look at the signal data and logs I sent from the modem and he said it was likely that Xfinity had sent the wrong file and that the modem was provisioned for the wrong speed, or that there was a signal problem. There were many T3 timeouts/ranging errors.

I contacted Xfinity again and the rep went through all the same troubleshooting steps, but ultimately was unable to do anything and had to send a tech out.

The tech came out today and when he conducted his test inside the house, he said there were a lot of dips and instability, but he also said that I should be getting every bit of 1000Mbps and his device just showed dips to around 600Mbps. He checked the exterior line and said it was very old, installed with interior, non-weatherproof fittings, and not running through a grounding block. He recommended replacing the entire line from the street to the house. So, I plan to have that done next week, and hopefully it will correct the issues.

Is it possible that Xfinity provisioned my modem incorrectly, though? I'm unclear as to why I'm not getting at least as much speed as I was getting BEFORE I changed to the newer modem and I'm unclear as to why his speed test inside the house showed that I should be getting 1000Mbps+.

If it was truly just an issue with the exterior line, then wouldn't I be getting at least the speed I was getting before the change, and wouldn't his speed test inside show drastically reduced speeds? Why is the newer modem only getting 10Mbps upstream when the older modem was getting 23-24 consistently?

Attached are the software, connection, and event log pages just pulled from the modem.

Responses

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Van4210

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7 Messages

2 years ago

Not sure why my post is flagged as private...? I redacted all the MAC addresses. Something else I need to do?

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Van4210

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7 Messages

2 years ago

Was just watching the gigabit troubleshooting guide and checking the "Device Info" page. Xfinity still has my SB6141 listed along with my MB8600. Shouldn't there only be one modem listed in my devices?

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EG

Expert

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110K Messages

2 years ago

They rate that modem only up to 949 Mbps: https://www.xfinity.com/support/devices#details

What happens is that if you use an incompatible modem for a faster speed tier, the modem will default down to using a slower-speed configuration file. A compatible device needs to be used.

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Van4210

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7 Messages

@EG Thank you for your reply! :)

So the modem has some default locally stored provision file that it uses instead of the file Xfinity sends it?

A couple different Xfinity reps have told me that the modem is correctly provisioned for the 1000Mbps plan I have, and none mentioned that the modem would use a slower speed file.

The Xfinity device info also does not mention that the MB8600 is incompatible with the 1000Mbps plan.

From the Motorola listing on Amazon where I bought the modem:

"Save Money - Eliminate up to 168 dollars per year in cable modem rental fees (Savings are shown for Comcast Xfinity and vary by cable service provider.) Approved by Comcast Xfinity and Xfinity X1, Cox, and CableOne for all speeds including Comcast Gigabit services and Cox Gigablast. Not compatible with RCN. No cable modem is compatible with fiber optic, DSL, or satellite services, available from Verizon, AT&T, CenturyLink, Frontier, and others.)"

So, do I need to buy an MB8611 or another modem that the Xfinity product page specifically lists as supporting over 1000Mbps then? It's a little frustrating if that is the case, because clicking "Show compatible devices" on that Device Info page *does* show the MB8600.

2 years ago

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EG

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110K Messages

@Van4210

@EG Thank you for your reply! :)

So the modem has some default locally stored provision file that it uses instead of the file Xfinity sends it?

Config / boot files get pushed to it every time it is rebooted.

A couple different Xfinity reps have told me that the modem is correctly provisioned for the 1000Mbps plan I have

I can ask an official corporate employee here to double-check on it for you if you wish.

and none mentioned that the modem would use a slower speed file.

They very well may not even know about that...

The Xfinity device info also does not mention that the MB8600 is incompatible with the 1000Mbps plan.

It says 949 so you put two and two together..

From the Motorola listing on Amazon where I bought the modem:

Only what Comcast lists it for matters. Retailers can claim anything that they like..

So, do I need to buy an MB8611

I wouldn't buy that model !!! It's been very problematic on the Comcast system. Just search around this board and you will see !!!

or another modem that the Xfinity product page specifically lists as supporting over 1000Mbps then?

Yep.

It's a little frustrating if that is the case, because clicking "Show compatible devices" on that Device Info page *does* show the MB8600.

Always click on the "Details" button and see what the speed rating says there !

I am not a Comcast Employee.
I am a Customer Expert volunteering my time to help other customers here in the Forums.
We ask that you post publicly so people with similar questions may benefit from the conversation.

Was your question answered? Please mark an Accepted Answer! tick

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We ask that you post publicly so people with similar questions may benefit from the conversation.

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2 years ago

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Van4210

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7 Messages

@EG

Thanks for replying! :)

I just had another chat session with another rep who again confirmed the modem is correctly provisioned. Would the official corporate employee know or be able to see anything which the reps in the chat from Xfinity Assistant don't know or can't see? For instance, be able to actually confirm that the modem is being provisioned with a slower speed file?

The part about that which doesn't make sense, though, is that I was still running my DOCSIS 3.0 SB6141 when I upgraded to 1000Mbps plan. The down speed stayed capped around 280Mbps on that modem, but the up speed increased from 12Mbps up to 23-24 Mbps. So, I guess I don't get why the old modem at least showed some increase and the new modem is even slower than the old one was BEFORE I upgraded plans.

"The Xfinity device info also does not mention that the MB8600 is incompatible with the 1000Mbps plan.

It says 949 so you put two and two together.."

I would expect to get the 949 Mbps that the modem is rated for, not have the speeds drop to 180/10. I was on the 400 Mbps plan with the SB6141 which Xfinity shows rated at 284 Mbps, but that modem didn't drop down to incredibly slow speeds, it performed at its rated maximum. So, it seems it's clearly possible for a modem to be provisioned for a speed higher than the modem is rated for, especially since I was getting the up speeds of the Gigabit plan with the SB6141, which means it must have been provisioned for the Gigabit plan, I would assume.

Is there a model you would recommend? I've searched these forums for a few of the most highly rated cable modems, but they all seem to have people posting at least some issues with them. The last Xfinity rep offered to temporarily send me an Xfinity modem to troubleshoot. I have to admit that I will be very suspicious if the Xfinity modem miraculously works when there are issues with the other modems. I have no interest in renting a cable modem. I think I've had the SB6141 for 10+ years now, so I would have paid almost $2000 in rental fees by now for what was maybe a $150 modem at the time. lol

2 years ago

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Van4210

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7 Messages

2 years ago

The technician was able to come out yesterday to replace the line from the street to the house. He said it was in pretty bad shape. There was a supervisor here as well, and he mentioned that he had to check the provisioning and my modem had the 800 Mbps file instead of the 1000 Mbps file.

After he replaced everything, he said his meter showed the connection normal to the street, then we tested it with my MB8600 and it was getting right around 940 Mbps/24 Mbps! Very exciting progress!

So, we tested the connection behind the router and it only showed around 300 Mbps. So, it seems my router was failing which was compounding the also failing cable run from the street.

I decided to try out a Netgear Nighthawk CAX80 since it's approved by Xfinity for well beyond the 1000 Mbps plan and it would replace the lower speed modem and the failing router.

Unfortunately, now it is giving me a "Downstream Power Level" error and getting extremely poor downstream speeds, though the upstream speeds are now consistently where they should be. I was going to try changing back to my MB8600, but the Xfinity app says "Sorry, we're having some trouble" when I try to use it to activate, so I'm just using the CAX80 while waiting.

Is there any reason the MB8600 would be getting 940 Mbps/24 Mbps, but the CAX80 (a decidedly superior modem) would not perform well with the same cable connection and only get <200Mbps down?

All the reps and techs from Xfinity I've interacted with so far have been very nice and helpful. The service visits have been scheduled very quickly as well! I think it does pay to be nice and patient with customer service folks, since they deal with angry people taking things out on them all day! Keep that in mind, people, when you call or chat with them!

Connection diagnostic data:

Cable Diagnostic

Status: Poor

Action:
[Downstream Power Level]
Try these actions (Click the Refresh button after each step):
1) Make sure the coaxial cable is tightly connected.
2) Remove any unnecessary splitters.
3) Replace any required splitters.
4) Contact your service provider for troubleshooting help.

Internet Access: Good

Downstream Status: Poor

Downstream Power Level: Poor
Downstream power is poor (12.1dBmV). The recommended level is between -10dBmV to 10dBmV.

Downstream SNR: Good

Upstream Status: Good

Upstream Power Level: Good

Current time: Sun Feb 26 11:42:03 2023

Startup Procedure
Acquire Downstream Channel: 447 MHz Locked
Connectivity State: OK Operational
Boot State: OK Operational
Security: Enabled BPI+

Downstream Bonded Channels
Channel LockedStatus Modulation ChannelID Frequency Power SNR Correctables Uncorrectables
1 Locked 256 QAM 42 447000000 Hz 9.2 dBmV 41.2 dB 0 0
2 Locked 256 QAM 1 495000000 Hz 9.7 dBmV 41.4 dB 0 0
3 Locked 256 QAM 2 501000000 Hz 9.9 dBmV 41.4 dB 0 0
4 Locked 256 QAM 3 507000000 Hz 9.9 dBmV 41.3 dB 0 0
5 Locked 256 QAM 4 513000000 Hz 10 dBmV 41.4 dB 0 0
6 Locked 256 QAM 5 519000000 Hz 10.2 dBmV 41.5 dB 0 0
7 Locked 256 QAM 6 525000000 Hz 10.2 dBmV 41.5 dB 0 0
8 Locked 256 QAM 7 531000000 Hz 10.4 dBmV 41.5 dB 0 0
9 Locked 256 QAM 8 537000000 Hz 10.5 dBmV 41.5 dB 0 0
10 Locked 256 QAM 9 543000000 Hz 10.4 dBmV 41.3 dB 0 0
11 Locked 256 QAM 10 555000000 Hz 10.7 dBmV 41.5 dB 0 0
12 Locked 256 QAM 11 561000000 Hz 11 dBmV 41.5 dB 0 0
13 Locked 256 QAM 12 567000000 Hz 11 dBmV 41.6 dB 0 0
14 Locked 256 QAM 13 573000000 Hz 11 dBmV 41.6 dB 0 0
15 Locked 256 QAM 14 579000000 Hz 11.2 dBmV 41.7 dB 0 0
16 Locked 256 QAM 15 585000000 Hz 11.3 dBmV 41.7 dB 0 0
17 Locked 256 QAM 16 591000000 Hz 11.3 dBmV 41.6 dB 0 0
18 Locked 256 QAM 17 597000000 Hz 11.5 dBmV 41.7 dB 0 0
19 Locked 256 QAM 18 603000000 Hz 11.5 dBmV 41.6 dB 0 0
20 Locked 256 QAM 19 609000000 Hz 11.4 dBmV 41.6 dB 0 0
21 Locked 256 QAM 20 615000000 Hz 11.5 dBmV 41.6 dB 0 0
22 Locked 256 QAM 21 621000000 Hz 11.8 dBmV 41.6 dB 0 0
23 Locked 256 QAM 22 627000000 Hz 12 dBmV 41.7 dB 0 0
24 Locked 256 QAM 23 633000000 Hz 12.1 dBmV 41.8 dB 0 0
25 Locked 256 QAM 38 471000000 Hz 9.4 dBmV 41.2 dB 0 0
26 Locked 256 QAM 39 477000000 Hz 9.4 dBmV 41.2 dB 0 0
27 Locked 256 QAM 40 483000000 Hz 9.7 dBmV 41.4 dB 0 0
28 Locked 256 QAM 41 489000000 Hz 9.7 dBmV 41.3 dB 0 0
29 Locked 256 QAM 43 453000000 Hz 9.3 dBmV 41.3 dB 0 0
30 Locked 256 QAM 44 459000000 Hz 9.3 dBmV 41.2 dB 0 0
31 Locked 256 QAM 45 465000000 Hz 9.4 dBmV 41.2 dB 0 0
32 Not Locked Unknown 0 0 Hz 0.0 0.0 0 0

Upstream Bonded Channels
Channel LockedStatus ChannelType ChannelID SymbolRate Frequency Power
1 Locked ATDMA 9 2560 Ksym/sec 40400000 Hz 40.8 dBmV
2 Locked ATDMA 1 5120 Ksym/sec 35600000 Hz 40.5 dBmV
3 Locked ATDMA 2 5120 Ksym/sec 29200000 Hz 40.5 dBmV
4 Locked ATDMA 3 5120 Ksym/sec 22800000 Hz 40.3 dBmV
5 Locked ATDMA 4 5120 Ksym/sec 16400000 Hz 40.3 dBmV
6 Not Locked Unknown 0 0 0 0.0
7 Not Locked Unknown 0 0 0 0.0
8 Not Locked Unknown 0 0 0 0.0

Downstream OFDM Channels
Channel LockedStatus ProfileID ChannelID Frequency Power SNR/MER ActiveSubcarrier Unerror Correctable Uncorrectable
1 Locked 0 ,1 ,2 ,3 37 762000000 Hz 14.48 dBmV 42.2 dB 1108 ~ 2987 72658569 54726829 0
2 Not Locked 0 0 0 Hz 0 dBmV 0 dB 0 ~ 4095 0 0 0

Upstream OFDMA Channels
Channel LockedStatus ProfileID ChannelID Frequency Power
1 Not Locked 0 0 0 Hz 0 dBmV
2 Not Locked 0 0 0 Hz 0 dBmV

Event Log
Time Priority Description
Sun Feb 26 10:39:10 2023 Notice DS profile assignment change. DS Chan ID: 32; Previous Profile: ; New Profile: 1 2 3.;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Sun Feb 26 10:38:49 2023 Notice TLV-11 - unrecognized OID;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:00:52 1970 Notice Honoring MDD; IP provisioning mode = IPv6
Thu Jan 1 00:00:44 1970 Critical SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:00:48 1970 Critical No Ranging Response received - T3 time-out;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:00:43 1970 Critical SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:00:50 1970 Critical No Ranging Response received - T3 time-out;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:00:43 1970 Critical SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Sat Feb 25 23:46:48 2023 Critical SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:03:08 1970 Critical No Ranging Response received - T3 time-out;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:03:02 1970 Critical SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:02:39 1970 Critical SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;
Thu Jan 1 00:00:47 1970 Critical SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.1;

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EG

Expert

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110K Messages

2 years ago

This may or may not be the root cause of the problem (YMMV) but it should be addressed regardless.

The downstream power is too high on many of the channels, especially on the OFDM channel. It may be over-driving the front-end receiver circuit of the modem. That can cause random disconnects, spontaneous re-booting of the modem, speed problems, packet loss, latency problems, and the un-bonding of channels.

Did they install a drop amplifier on the coax cable line leading to the modem ? If so, try removing / bypassing it and see. If not, you can try installing a -6 dB forward path-only attenuator pad such as this one on the coax port on the rear of the modem to knock that power level down and see;

https://www.amazon.com/FPA6-54-Forward-Attenuator-DOCSIS-Internet/dp/B08KTRC3XZ

It won't affect the upstream power level which is within spec. Good luck !

(edited)

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Van4210

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7 Messages

@EG Thanks for replying again. I appreciate you volunteering your time.

There is no amplifier inside, and the only thing in the house box outside is a -3.5dB splitter coupling the line from the street to the line that goes into the house.

I've been occasionally speed testing, and now, without having changed anything and the downstream power error still persisting, I am getting 940 Mbps down/24 Mbps up to all my wired devices on the network, which is what I expect since it's all still Cat 5e. I have no idea why it's seemingly corrected, as I have not power cycled anything and have not had any service interruptions. lol it's a mystery...

I guess I'll leave things alone for a few days to see if it's stable like this or if the downstream power changes, and then I'll look into possibly adding the attenuator. You can see from my earlier post that the signal power was all within spec before the cable from the street was replaced, so I don't know if something changed there. I did not think to record signal power data with the MB8600 right after he replaced the line to see if there was a difference.

Thanks again for your help!

2 years ago

CCAnna

Problem Solver

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874 Messages

Thank you for the update, @Van4210! It sounds like you have certainly put in the work to get this fixed, we appreciate that. With the downstream power levels out of sync, we might need to get out another technician to have a look at the line they replaced and make sure they don't need to add/subtract an amplifier or just have another look and inspection.

You could wait a few days, or if you don't notice any problems with service leave it be. Our team would be happy to help if/when you are ready. We will need to look at your account details. You can send us a direct message with your full name, name of account holder (if different) and service address? To send a direct message, please click the chat icon next to the bell in the upper right corner. Click the “notepad and pen” button. Send the full message to our shared support handle “Xfinity Support”.

Thank you, @EG as usual your knowledge and help is very much appreciated. Our team would not know what to do without you!!

I no longer work for Comcast.

2 years ago

EG

Expert

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110K Messages

@Van4210

Quite welcome ! Thanks for the update !

I am not a Comcast Employee.
I am a Customer Expert volunteering my time to help other customers here in the Forums.
We ask that you post publicly so people with similar questions may benefit from the conversation.

Was your question answered? Please mark an Accepted Answer! tick

2 years ago

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EG

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110K Messages

2 years ago

@XfinityAnna

Awww shucks. Thanks for your kind words ! It's truly my pleasure Anna !! 😊

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nene503

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7 Messages

2 years ago

@Van4210 It seems that the root of the problem as to why Xfinity doesn't work well with the MB8600 for services above 1 GBps is that Xfinity doesn't provision the modem with the correct bootfile. So Xfinity is limiting the performance of the MB8600 (that's what Motorola tech support stated to me).

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Van4210

MB8600 slower than expected connection; provisioned wrong?

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