Upload speed under-provisioned

EDIT (for completeness. I was going to remove it but saw the reply right after I did, apologies)

Had a guess here that it seemed the modem was using DOCSIS 3.0 QAM256 channels as a reason for the limited bandwidth, as explained below it is actually using 3.1 OFDM for download and SC-QAM for upload, as expected.

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@EG wrote:

Channel 159 (marked "Other") is the DOCSIS 3.1 / OFDM downstream channel and it indicates that it is locked in.

 

Try calling 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.

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Thanks. I took a closer look at the table and then reread the troubleshooting guide (there is a section about this "other" channel being the 3.1 OFDM channel), realizing it right when you replied 🙂 I'll follow up with those steps.

Channel 159 (marked "Other") is the DOCSIS 3.1 / OFDM downstream channel and it indicates that it is locked in.

Try calling 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.

I ended up replacing my SB8200 with a Motorola MB8600 - the actual reason for that was to see if the LACP link aggregation would be more stable as the SB8200s are not with the currently pushed firmware. That's not too important, even if that doesn't work out; I'm just a curious software/systems engineer.

That aside, I was able to self-provision the brand new modem, and my account just shows the MB8600 and not the old SB8200. The upload speed (with and without LACP) still caps out at 24Mbps. The L1 agents I spoke with reported that the account config/signals looked good on their end (not sure if they can see the actual speeds, or just the tier) - I am definitely getting the gigabit *.cm file and seeing gigabit download speeds, but the upload remains a mystery.

extra: I walked around the neighborhood to look at the lines. I think we've got at least 2 (maybe 3) visible grey fiber nodes serving up to 120 homes, and I'm anywhere from 0.3-0.5mi away from one of them. Part of the neighborhood actually has FTTH that Verizon installed back in 2008, but they sold it off to FairPoint (now Consolidated), which does offer symmetric 1Gbps. My area has underground wires, so they didn't install.

@Crunchtime I have a working theory I just thought of. I think it is a signal issue with the upstream - checking the event log more closely, there's a bunch of T3 timeouts on all channels except on channel 4. The power levels seem OK (maybe a little high), interestingly it shows channel 1 as locked and 2-4 as not locked (of note,the SB8200 had all upstream channels locked and lower power, but had T3s). This makes sense, as the SC-QAM channels cap out at around 27Mbps each and I see 22-24, so I'm probably just using one channel rather than bonding over four. When I had Blast!, the upload was 10-12Mbps, so that wouldn't have been an issue then, but now multiple channels are needed. I'm going to test a direct connection to the tap (no splitters, right now I have just one -3.5dB using Comcast's own Antronix splitter that I've ensured are tight) - if I see T3s still, it's probably time to get a tech to check the lines.

Aside: on the LAG experiment, the MB8600 does go unstable after around 18 hours, though not as bad as the SB8200. In both modems, management (192.168.100.1) is unavailable, but the MB8600 doesn't drop the speed to 10Mbps - it drops it to 600Mbps, half of the 1200Mbps aggregation. Probably a hardware bug, since the two share the same Broadcom chipset. Unplugging and re-plugging a single link does restore management and full bandwidth without a reboot, which is nice, because I can handle it programatically on the host (router) side.

Were you able to solve this? I am in Indiana (46590) and just got the gigabit plan 9/8/20 and a brand new sb8200 ,I am also stuck at 20mbps instead of the 35mbps I am contracted and paying for to receive....

I have tried moving it to the tap and got the modem outside directly connected and a gigabit hardwired laptop setup.  Still the same and I am locked on 5 upstream channels , I can get 940 down which is correct but about 26.90 up is my maximum, I bet it's a configuration issue.  How were you able to view you config file name ?

let me know if you get this fixed I wasted hours with pointless tech support just rebooting things repeatedly,  xfinity sure has a roadblock in place that will allow you to reach anyone advanced.

Yes many of the same types of errors in the logs , i dont think its significant and the issue a config im really leaning on , have you been able to get the config file and download a copy to see the actual speed provisioning inside it?

i am on this config , d11_m_sb8200_gigabit_c01.cm

here is my log.

To get the config file name to appear, add main.html to the URL: http://192.168.100.1/main.html

It works on the SB8200, but not the MB8600, I'm assuming I have the equivalent d11_m_mb8600_gigabit_c01.cm.

I was able to test on the tap directly this morning and confirmed that I still see T3 timeouts on channel 1-3 with 24Mbps bandwidth. To add, the DOCSIS spec indicates that that upstream bandwidth is n channels * 30.72Mbps, but n * 27Mbps after overhead. These are the signals, indicating upstreams are in the 30s (no higher than 43). Going to try to schedule an appointment soon - yeah, it's a grind at L1. Are you seeing anything similar in your logs?

Thanks, good to know that the symptoms/signal environment are similar at least. AFAIK there's no way for a customer to trivially obtain (and decipher, as it's a binary) the config file contents from the modem itself, it's purposely locked down from a management perspective. We'd need to be an employee with access to the backend :-). I do wish these modems could at least display the speed  they're set at; I've never seen one do so, having owned around 8 starting with the SB5101.

I kind of want to test my theory by visiting my friend's house a couple miles out and plugging in there. I know he's seeing 40 (maybe 60 somehow?) upload on gigabit service, unsure of the modem though. I just moved home and had symmetric 1Gb FiOS before and really miss that now. Kind of want to see if I can get the Gigabit Pro FTTH service, but it's several months' lead time (and the cost justification).

Still working on getting an appointment set up. @EG - wondering if you have any thoughts based on the latest info?

update: I've contacted them today; been busy, as I only have certain windows of time to cutover my family internet to my failover T-Mobile LTE link (ironically, the upload is faster there, but it is soft-capped and more variable). Essentially, I asked them to run through the diagnostics and then schedule an appointment if there are issues.

She said that she was unable to run the diagnostics on her end - it was blocked, IIRC she said that there is already a note for a tech to check the lines in the neighborhood. She then scheduled an appointment for the earliest at November 13. I got a text but it is not in the account yet, assuming it shows up, I'll see if I can push it forward. In the meantime, will continue to look into this.

Update part 2: Just called in to the technical services (L2, I guess?) Essentially, the modem activation phone #. It is definitely a step up from mainline L1 support and online chat.

I had him remove and add the MB8600 as suggested, and had him confirm the account info was correct. He said he can only see the bootfile name is gigabit and not its contents, as I figured. The backend should be provisioning the same speed parameters to all subscribers of a tier and modem - same file, same SHA1, so to speak. The speed remained the same: 940 down as expected, 22-24 up.

More importantly, he did clarify the existing tech visit - they're actually servicing the poles and performing line upgrades on/around Nov. 13. I believe it was the first agent that said that the earliest available visit to my house was Nov 3, but I think she scheduled it the 13th so that it would be when the changes are done. He did suggest that the line upgrades would increase the upload speed (hinting it was bringing fiber closer to subscribers, i.e. N+0). I'd agree in that if they are adding fiber nodes and/or if they move to OFDMA in the near future, then yes, aggregate bandwidth is increased. But I'm curious if there is a such thing as a "local limit" between the CMTS or the fiber node - it seems unusual to manage the very limited upstream capacity like this even during off-peak hours.

Pretty amazing they deleted my post that is critical of their comical and horrible customer/technical service. I'm rather shocked

Any update ?

Unfortunately, not for at least another month - I contacted them on Twitter and they confirmed that there is some pole work on 11/13 and wouldn't schedule anything earlier, so I've decided to wait for that. In the meantime I bought an active return amp (PCT-MA-B1015-2A) and tried it. As expected, upstream signals are around 26-29dBmV but just the one channel is locked (and same speeds), still.

The temporary solution for now has been to use the 30Mbps LTE connection when two people are using a lot of upload bandwidth, and aggregate them via OpenMPTCProuter to get 50Mbps if one link is needed.