Skip to main content
user_7lz69r
1st Topic
1st Reply
1st Like

8 Messages

Sunday, May 18th, 2025

only half of download channels in use and slow download speeds with gigabit service and new docsis 3.1 modem

I have been having problems with only getting 200-400 Mbps download speeds for about a year, even though I have 1200 Mbps service.  I recently upgraded from a CAX30 to a G36 to get me into Docsis 3.1, but nothing has changed.  Still slow (recently 200-240 Mbps speedtests from Wi-Fi connected and wired connection to the gateway.  I am seeing this, though (which I did not see with the CAX30): half of the downstream QAM channels show no traffic at all.

Question

 • 

Updated 

10 days ago

45

4

0

Oldest First
Selected Oldest First

Accepted Solution

EG

Expert

 • 

112.8K Messages

10 days ago

There may be a firmware glitch with the correctable / uncorrectable error table. What magnifies this is that there are no unerrored codewords being shown, which is of course impossible. Don't go by that. The downstream channel table shows that there are the typical 32 carriers locked in, and the upstream channel table shows that the typical 5 carriers are locked in. There may be an incorrect speed provisioning issue.

I'm going to escalate your issue to the Comcast corporate employees (The Digital Care Team) who are available to these boards. They will be able to check things from their end. Good luck !

0

EG

Expert

 • 

112.8K Messages

3 months ago

Please copy all of the text in its entirety of the *Downstream Power Levels*, the *SNR's* (Signal to Noise Ratios), and the *Upstream Power Level* numbers and paste them into your next post.

2

0

user_7lz69r

8 Messages

My apologies for not getting back to this sooner...its not my fulltime job, but it is starting to affect my fulltime job (work from home cybersecurity analyst).  Here's the data you requested:

Downstream QAM

Index Frequency (MHz) Power Level (dBmV) SNR (dB) Modulation Lock Status
43 675 -6.299999 40.366287 QAM256 Locked
20 531 -3.900002 40.946209 QAM256 Locked
28 585 -5.799999 40.366287 QAM256 Locked
36 633 -5.799999 40.946209 QAM256 Locked
44 681 -6.299999 40.946209 QAM256 Locked
13 483 -2.900002 40.946209 QAM256 Locked
14 489 -3.000000 40.366287 QAM256 Locked
15 495 -3.200001 40.366287 QAM256 Locked
16 507 -3.299999 40.946209 QAM256 Locked
17 513 -3.299999 40.946209 QAM256 Locked

18 519 -3.599998 40.946209 QAM256 Locked
19 525 -3.799999 40.366287 QAM256 Locked
21 537 -4.200001 40.366287 QAM256 Locked
22 543 -4.200001 40.946209 QAM256 Locked
23 555 -4.799999 40.366287 QAM256 Locked
24 561 -5.099998 40.366287 QAM256 Locked
25 567 -5.400002 40.366287 QAM256 Locked
26 573 -5.700001 40.366287 QAM256 Locked
27 579 -5.799999 40.946209 QAM256 Locked
29 591 -5.799999 40.366287 QAM256 Locked

30 597 -5.900002 40.366287 QAM256 Locked
31 603 -5.900002 40.366287 QAM256 Locked
32 609 -6.000000 40.366287 QAM256 Locked
33 615 -6.000000 40.366287 QAM256 Locked
34 621 -5.700001 40.366287 QAM256 Locked
35 627 -5.799999 40.366287 QAM256 Locked
37 639 -5.799999 40.366287 QAM256 Locked
38 645 -5.900002 40.366287 QAM256 Locked
39 651 -5.799999 40.366287 QAM256 Locked
40 657 -5.799999 40.366287 QAM256 Locked

41 663 -6.000000 40.946209 QAM256 Locked
42 669 -6.200001 40.366287 QAM256 Locked

Downstream OFDM

Index FFT Type Channel Width (dB) # of Active Subcarriers First Active Subcarrier (MHz) Last Active Subcarrier (MHz) Avg RxMER Pilot (dB) Avg RxMER PLC (dB) Avg RxMER Data (dB) Power Level (dBmV) Modulation Lock Status
193 4K 90 1721 688 775 47 41 41 -6.4 QAM4096 Locked

Upstream QAM

Index Frequency (MHz) Power Level (dBmV) Channel Type Symbol Rate (KSym/sec) Modulation Lock Status
4 16.4 45.520599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
5 39.6 42.750000 US_TYPE_ATDMA 1280 KSym/sec 64QAM Locked
1 35.6 45.520599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
3 22.8 45.520599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
2 29.2 45.270599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked

CM Error Codewords

Index Unerrored Codewords Correctable Codewords Uncorrectable Codewords
43 0 1156004465 0
20 0 0 0
28 0 1155955656 0
36 0 0 0
44 0 1155956279 0
13 0 0 0
14 0 1155961471 0
15 0 0 0
16 0 1155961550 0
17 8 0 0

Index Unerrored Codewords Correctable Codewords Uncorrectable Codewords
18 0 1155961655 0
19 0 0 0
21 0 1155961749 0
22 0 0 0
23 0 1155961379 0
24 0 0 0
25 0 1155950383 0
26 2 0 0
27 0 1155967659 0
29 0 0 0

Index Unerrored Codewords Correctable Codewords Uncorrectable Codewords
30 0 1155968111 0
31 0 0 0
32 0 1155961201 0
33 0 0 0
34 0 1155972673 0
35 0 0 0
37 0 1155977218 0
38 0 0 0
39 0 1155961308 0
40 0 0 0

Index Unerrored Codewords Correctable Codewords Uncorrectable Codewords
41 0 1155955403 0
42 0 0 0

(edited)

Like

Reply

12 days ago

0

XfinityAirelle

Official Employee

 • 

2.8K Messages

@user_7lz69r

Thanks for sharing your modem logs — they’re really helpful. What we’re seeing is that several of your downstream channels are showing heavy error correction, and some aren’t passing traffic cleanly. Even though your modem is DOCSIS 3.1 capable and the OFDM channel is locked, the signal levels and error counts are preventing it from delivering the gigabit speeds your plan supports.

 

This points to a signal issue on the line rather than the modem itself. We would like to lok into it further as suggested by @EG. Can you initiate a direct message and start with your full name and address? To start a direct message, you must be signed in. Please click the "Direct Messaging" icon in the top right of the page next to the bell, and send your message to "Xfinity Support". Talk to you there!

I am an Official Xfinity Employee.
Official Employees are from multiple teams within Xfinity: CARE, Product, Leadership.
We ask that you post publicly so people with similar questions may benefit from the conversation.
Was your question answered? Please, mark a reply as the Accepted Answer.tick

Like

Reply

10 days ago

0

user_7lz69r

8 Messages

12 days ago

Data posted.  Any help you can provide is greatly appreciated.  Thanks!

0

0

EG

Expert

 • 

112.8K Messages

11 days ago

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


The downstream power is on the low / weak side and it may be intermittently fluctuating even lower to out of spec levels. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.

In an effort to try to obtain better connectivity / more wiggle room, check to see if there are any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/reconfigured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage from Home Depot, Target, Wal-Mart, etc. Splitters should be swapped with known to be good / new ones to test.

Also, check the coax cable for any damage such as cuts, nicks, abrasions, kinks, sharp bends, animal chews.

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.


Good luck with it !

2

0

user_7lz69r

8 Messages

We originally had just cable on this line, so there is a 4-way splitter.  We no longer have cable and use just one-in, one-out on the splitter that is a home run to the modem.  I have capped the other 3 outputs, but I understand there is some attenuation at the outputs with splitters.  Since I no longer need the other ports, I will try using an F coupler as a start to at least narrow that down.  Thanks for your advice!

Like

Reply

11 days ago

user_7lz69r

8 Messages

@EG​ switched out the 4 port splitter for a coupler and downstream power seems to have to improved for QAM and OFDM.  SNR is about the same and uniform except two channels at 483 and 519 MHz where it is up quite a bit.  Upstream power has dropped some.

Unerrored codeword counts hanging around zero, mostly all correctables and no uncorrectables which is about the same as before.  Still not sure I understand why only half the channels are in use...is that expected?


Downstream QAM

Index Frequency (MHz) Power Level (dBmV) SNR (dB) Modulation Lock Status
20 531 3.299999 40.946209 QAM256 Locked
28 585 1.400002 40.946209 QAM256 Locked
36 633 1.500000 40.946209 QAM256 Locked
44 681 1.000000 40.946209 QAM256 Locked
13 483 3.900002 43.376591 QAM256 Locked
14 489 4.000000 40.946209 QAM256 Locked
15 495 3.599998 40.946209 QAM256 Locked
16 507 3.599998 40.366287 QAM256 Locked
17 513 3.799999 40.946209 QAM256 Locked
18 519 3.500000 43.376591 QAM256 Locked
19 525 3.200001 40.946209 QAM256 Locked
21 537 2.799999 40.946209 QAM256 Locked
22 543 3.000000 40.946209 QAM256 Locked
23 555 2.299999 40.946209 QAM256 Locked
24 561 2.000000 40.946209 QAM256 Locked
25 567 1.799999 40.946209 QAM256 Locked
26 573 1.500000 40.946209 QAM256 Locked
27 579 1.400002 40.946209 QAM256 Locked
29 591 1.400002 40.946209 QAM256 Locked
30 597 1.299999 40.946209 QAM256 Locked
31 603 1.200001 40.366287 QAM256 Locked
32 609 1.299999 40.366287 QAM256 Locked
33 615 1.200001 40.366287 QAM256 Locked
34 621 1.500000 40.366287 QAM256 Locked
35 627 1.500000 40.366287 QAM256 Locked
37 639 1.500000 40.366287 QAM256 Locked
38 645 1.400002 40.946209 QAM256 Locked
39 651 1.500000 40.366287 QAM256 Locked
40 657 1.500000 40.946209 QAM256 Locked
41 663 1.400002 40.946209 QAM256 Locked
42 669 1.200001 40.946209 QAM256 Locked
43 675 1.099998 40.946209 QAM256 Locked

Downstream OFDM

Index FFT Type Channel Width (dB) # of Active Subcarriers First Active Subcarrier (MHz) Last Active Subcarrier (MHz) Avg RxMER Pilot (dB) Avg RxMER PLC (dB) Avg RxMER Data (dB) Power Level (dBmV) Modulation Lock Status
193 4K 90 1721 688 775 49 43 43 1.0 QAM4096 Locked

Upstream QAM

Index Frequency (MHz) Power Level (dBmV) Channel Type Symbol Rate (KSym/sec) Modulation Lock Status
4 16.4 39.270599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
5 39.6 36.500000 US_TYPE_ATDMA 1280 KSym/sec 64QAM Locked
1 35.6 39.270599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
3 22.8 39.270599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
2 29.2 39.020599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked

CM Error Codewords

Index Unerrored Codewords Correctable Codewords Uncorrectable Codewords
20 0 76796392 0
28 0 0 0
36 0 70294370 0
44 0 0 0
13 0 70289759 0
14 0 0 0
15 0 70298959 0
16 6 0 0
17 0 70276317 0
18 0 0 0
19 0 70289878 0
21 0 0 0
22 0 70308078 0
23 0 0 0
24 0 70312601 0
25 0 0 0
26 0 70303133 0
27 0 0 0
29 0 70326290 0
30 0 0 0
31 0 70317156 0
32 0 0 0
33 0 70330732 0
34 0 0 0
35 0 70279959 0
37 0 0 0
38 0 70339885 0
39 0 0 0
40 0 70321747 0
41 0 0 0
42 0 70340337 0
43 0 0 0

Like

Reply

10 days ago

0

forum icon

New to the Community?

Start Here