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Gigabit upgrade results in intermittent speed drops, usually 33Mbps

New Poster

Gigabit upgrade results in intermittent speed drops, usually 33Mbps

I've had 250 Mbps service to my house for several years, and never had any issue, always getting 250-350 Mbps at all times.  Gigabit just became available in my area, so I paid for the upgrade.  Since then, my connection is normally 33 Mbps, with it going up to 380 Mbps about 20% of the time.  (It'll get faster then slower again randomly, for no apparent reason.)

 

The only time I've seen >380 Mbps is right after the cable modem boots -- I'll get that speed for maybe 30-60 seconds before it dumps to 33 again.

 

However, my signals look fine:

 

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159
 
Lock Status
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
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Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Locked
Frequency
663 MHz
483 MHz
489 MHz
495 MHz
507 MHz
513 MHz
519 MHz
525 MHz
531 MHz
543 MHz
549 MHz
555 MHz
561 MHz
567 MHz
573 MHz
579 MHz
585 MHz
591 MHz
597 MHz
603 MHz
609 MHz
615 MHz
621 MHz
627 MHz
633 MHz
639 MHz
645 MHz
651 MHz
657 MHz
669 MHz
675 MHz
681 MHz
722 MHz
629600000
SNR
34.0 dB
37.7 dB
37.4 dB
37.3 dB
36.8 dB
36.6 dB
36.5 dB
36.5 dB
36.5 dB
36.5 dB
36.5 dB
36.4 dB
36.3 dB
36.4 dB
36.2 dB
35.9 dB
36.1 dB
35.8 dB
35.7 dB
35.6 dB
35.6 dB
35.6 dB
35.5 dB
35.5 dB
35.3 dB
35.1 dB
35.0 dB
34.7 dB
34.5 dB
33.7 dB
32.9 dB
33.7 dB
12.7 dB
NA
Power Level
-10.1 dBmV
-5.7 dBmV
-6.1 dBmV
-6.5 dBmV
-7.1 dBmV
-7.5 dBmV
-7.8 dBmV
-7.8 dBmV
-7.8 dBmV
-7.6 dBmV
-7.6 dBmV
-7.8 dBmV
-7.8 dBmV
-8.0 dBmV
-8.1 dBmV
-8.4 dBmV
-8.4 dBmV
-8.7 dBmV
-8.7 dBmV
-8.7 dBmV
-8.6 dBmV
-8.7 dBmV
-8.5 dBmV
-8.7 dBmV
-8.7 dBmV
-8.9 dBmV
-9.1 dBmV
-9.2 dBmV
-9.5 dBmV
-10.3 dBmV
-10.4 dBmV
-10.3 dBmV
-9.5 dBmV
NA
Modulation
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
256 QAM
OFDM
OFDM

 

Upstream
Channel Bonding Value
Index
1
2
3
4
Lock Status
Locked
Locked
Locked
Locked
Frequency
30 MHz
36 MHz
17 MHz
23 MHz
Symbol Rate
5120
5120
5120
5120
Power Level
42.0 dBmV
41.5 dBmV
41.0 dBmV
41.5 dBmV
Modulation
QAM
QAM
QAM
QAM
Channel Type
ATDMA
ATDMA
ATDMA
ATDMA

 

If you don't want to peruse that, my worst SNR is 32.9 dB, and my worst power level is -10.4 dBmv.  

 

My modem is an Arris SB8200, but as part of troubleshooting I am currently using the new Xfinity-provided modem instead.  All speed tests are done via wired Gigabit ethernet from a desktop PC.  If I run a speed test from the same PC to a NAS on the same router as the cable modem, I get 980 Mbps, so the internal Ethernet network is fine.  (I've also tried wiring a PC directly to the cable modem, to the same effect.)

 

The Xfinity cable modem system logs show nothing (literally, nothing but my own admin logins.)

 

Given the signal strengths there, is there any possibility this is caused by the cable (coax) wiring inside my house, or does it have to be something upstream or on the cable modem?

Diamond Problem Solver

Re: Gigabit upgrade results in intermittent speed drops, usually 33Mbps

It can be either, you didn’t say how many splitters are in your house to determine one way or another. Hook up your modem before any splitter for a more accurate comparison

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Expert

Re: Gigabit upgrade results in intermittent speed drops, usually 33Mbps

FWIW, you commented that the signals are "fine". They are not. They are mostly weak


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Highlighted
New Poster

Re: Gigabit upgrade results in intermittent speed drops, usually 33Mbps

There aren't any splitters.  The wire from Comcast goes to the outside of the house, where there's a barrel coupler to the cable that runs directly to my cable modem.

 

This is kind of why I'm trying to figure out if this could be caused by the interior wiring -- the only remaining troubleshooting step I've thought of is ripping out the cable inside my house and replacing it, which is not a trivial undertaking as it goes through walls, attic, etc.  But it's one cable, there's no splitters or splices or couplers in it.

Diamond Problem Solver

Re: Gigabit upgrade results in intermittent speed drops, usually 33Mbps

Take a measurement at the ground block if you can, you need something to compare before you start running new cables or just have a tech out.

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New Poster

Re: Gigabit upgrade results in intermittent speed drops, usually 33Mbps

Well, had a technician out (for the 3rd time) to run a new cable jack inside, replacing the interior wiring.  Unfortunately, they couldn't figure out how to do it (my house is a tri-level and does not have either crawlspace or attic to the room where the cable modem, router, switch, and other network gear is), and won't do any other troubleshooting until I hire somebody else to run a new line.  (Of course, that leaves me wondering if I do that, if they'll still consider that line to be the problem since Comcast didn't run it themselves.)

 

I tried running an extension cord outside around the house and testing a cable modem plugged into the line into the house.  I did get gigabit speed there, though I only tested it for 5 minutes and I've managed to get gigabit speed briefly inside the house, too, it just never lasts more than a few minutes.  The SNR and power levels were a bit better out there (-6 or -7 dBmV, rather than the -8 or -10 of inside.)

 

One other oddity: on every cable modem I've tried, it shows DOCSIS 3.1 downstream but 3.0 upstream (i.e. one blue light and one green light.)  This is true even if I hook it up outside the house.  However, that may be perfectly normal on Comcast's network, I have no idea.

 

In any case, the result is they downgraded my service to 400 Mbps because they can't actually deliver 1 Gbps in my house.  On the bright side that's still better than I had before, and $10 less.  Unfortunate side effect, while trying to figure out how to wire it they left three holes in my ceiling while looking around in the attic.  Honestly, if I have to have someone else do the interior wiring, I'm not going to do coax at all -- I'll run Cat-7 ethernet right to the exterior wall where the cable drop comes in and have Comcast put the cable jack there.  That way if any issues persist there will be no interior wiring to consider at all.

 

Despite nothing being changed as far as I know, my power levels seem to have improved.

 

Channel ID Lock Status Modulation Frequency Power SNR/MER Corrected Uncorrectables
29 Locked QAM256 663000000 Hz -8.2 dBmV 35.5 dB 0 0
1 Locked QAM256 483000000 Hz -3.2 dBmV 38.6 dB 0 0
2 Locked QAM256 489000000 Hz -3.5 dBmV 38.4 dB 0 0
3 Locked QAM256 495000000 Hz -3.9 dBmV 38.4 dB 0 0
4 Locked QAM256 507000000 Hz -4.5 dBmV 38.1 dB 0 0
5 Locked QAM256 513000000 Hz -5.0 dBmV 37.9 dB 0 0
6 Locked QAM256 519000000 Hz -5.4 dBmV 37.7 dB 0 0
7 Locked QAM256 525000000 Hz -5.4 dBmV 37.6 dB 0 0
8 Locked QAM256 531000000 Hz -5.5 dBmV 37.6 dB 0 0
9 Locked QAM256 543000000 Hz -5.5 dBmV 37.7 dB 0 0
10 Locked QAM256 549000000 Hz -5.6 dBmV 37.7 dB 0 0
11 Locked QAM256 555000000 Hz -5.9 dBmV 37.4 dB 0 0
12 Locked QAM256 561000000 Hz -6.0 dBmV 37.5 dB 0 0
13 Locked QAM256 567000000 Hz -6.2 dBmV 37.5 dB 0 0
14 Locked QAM256 573000000 Hz -6.4 dBmV 37.3 dB 0 0
15 Locked QAM256 579000000 Hz -6.8 dBmV 37.2 dB 1 0
16 Locked QAM256 585000000 Hz -6.8 dBmV 37.1 dB 0 0
17 Locked QAM256 591000000 Hz -7.1 dBmV 36.9 dB 0 0
18 Locked QAM256 597000000 Hz -7.1 dBmV 36.9 dB 0 0
19 Locked QAM256 603000000 Hz -7.1 dBmV 36.9 dB 0 0
20 Locked QAM256 609000000 Hz -7.0 dBmV 37.0 dB 0 0
21 Locked QAM256 615000000 Hz -7.0 dBmV 36.8 dB 0 0
22 Locked QAM256 621000000 Hz -6.8 dBmV 36.8 dB 1 0
23 Locked QAM256 627000000 Hz -7.1 dBmV 36.6 dB 0 0
24 Locked QAM256 633000000 Hz -7.0 dBmV 36.5 dB 1 0
25 Locked QAM256 639000000 Hz -7.2 dBmV 36.4 dB 2 0
26 Locked QAM256 645000000 Hz -7.2 dBmV 36.4 dB 1 0
27 Locked QAM256 651000000 Hz -7.4 dBmV 36.1 dB 4 0
28 Locked QAM256 657000000 Hz -7.7 dBmV 36.0 dB 1 0
30 Locked QAM256 669000000 Hz -8.5 dBmV 35.4 dB 0 0
31 Locked QAM256 675000000 Hz -8.4 dBmV 34.6 dB 2 0
32 Locked QAM256 681000000 Hz -8.4 dBmV 35.3 dB 1 0
159 Locked Other 722000000 Hz -7.4 dBmV 33.5 dB 286664378 0

 

Channel Channel ID Lock Status US Channel Type Frequency Width Power
1 1 Locked SC-QAM Upstream 30100000 Hz 6400000 Hz 39.0 dBmV
2 2 Locked SC-QAM Upstream 36500000 Hz 6400000 Hz 39.0 dBmV
3 3 Locked SC-QAM Upstream 17300000 Hz 6400000 Hz 39.0 dBmV
4 4 Locked SC-QAM Upstream 23700000 Hz 6400000 Hz 39.0 dBmV
Expert

Re: Gigabit upgrade results in intermittent speed drops, usually 33Mbps


@Fishsupreme wrote:

 

One other oddity: on every cable modem I've tried, it shows DOCSIS 3.1 downstream but 3.0 upstream (i.e. one blue light and one green light.)  This is true even if I hook it up outside the house.  However, that may be perfectly normal on Comcast's network, I have no idea.

 


FWIW, that is normal. Comcast does not yet offer DOCSIS 3.1 service on the upstream, so the light will never be blue.



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