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Not getting full download speed with xfinity with sbg8300

  • 2 January 2024
  • 13 replies
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I just purchased a SBG8300 and I have xfinity 1GB plan. I previously was using an sbg6950ac2. We are direct connecting via ethernet.

With the old modem, we were getting speeds of around 400 MBS using comcast speed test so we decided to upgrade. However, with the new modem we are only getting around 150MBS. We tried multiple devices and different cables. Interestingly I am getting around 350MBS using my galaxy phone using wifi…

Comcast of course says they are not the problem and that I should contact the manufacturer of the modem. 

Any suggestions?

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Best answer by tsandat8 21 January 2024, 11:53

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plemans
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Thats a hardwired speedtest? 

What app are you using? Or website? 

What device are you testing with (computer/tablet/pc, etc)

Did you check with xfinity? Maybe there’s a line issue

Current setup: Netgear CM3000-->Netgear RBE973S-->2.5gig tp-link switch, Xfinity 1.4gig/240mbps
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3 different laptops running windows 10.

Hard wired.

Used xfinity speed test and another site I found online.

Yes, talked to xfinity - they say it’s not their fault. And the fact that I am getting way faster speeds from the old arris modem than the new one supports that.

plemans
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got a screensnip of the cable connections page? Xfinity’s first response is always “its not our line”

Current setup: Netgear CM3000-->Netgear RBE973S-->2.5gig tp-link switch, Xfinity 1.4gig/240mbps
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Which cable connections page? And I am not surprised they said it’s not us, but I still find it odd that the old modem is performing so much better than the new modem.

plemans
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It wouldn’t be the first time that a line was loosened while changing modems. 

the connections page shows the upstream/downstream data for the modem side. Thats what we’re curious about. 

Current setup: Netgear CM3000-->Netgear RBE973S-->2.5gig tp-link switch, Xfinity 1.4gig/240mbps
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Is this what you are looking for?

Downstream QAM

Index Frequency (Hz) Power Level (dBmV) SNR (dB) Modulation Lock Status
20 531000000 -8.099998 38.605377 QAM256 Locked
28 579000000 -8.599998 38.983261 QAM256 Locked
36 627000000 -9.799999 37.636276 QAM256 Locked
44 675000000 -10.400002 38.605377 QAM256 Locked
13 489000000 -6.700001 38.983261 QAM256 Locked
14 495000000 -9.299999 38.983261 QAM256 Locked
15 501000000 -7.900002 38.605377 QAM256 Locked
16 507000000 -7.700001 38.605377 QAM256 Locked
17 513000000 -9.000000 38.605377 QAM256 Locked
18 519000000 -7.200001 40.366287 QAM256 Locked
  • <<
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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 192 3800 811 1000 38 28 32 -21.4 QAM2048 Locked

Upstream QAM

Index Frequency (Hz) Power Level (dBmV) Channel Type Symbol Rate (KSym/sec) Modulation Lock Status
18 22800000 49.520599 US_TYPE_STDMA 5120 KSym/sec 64QAM ACTIVE
20 35600000 49.520599 US_TYPE_STDMA 5120 KSym/sec 64QAM ACTIVE
19 29200000 48.020599 US_TYPE_STDMA 5120 KSym/sec 64QAM ACTIVE
17 16400000 47.270599 US_TYPE_STDMA 5120 KSym/sec 64QAM ACTIVE
S
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You have mentioned the speed plan is 1GB, but this modem SBG8300 is compatible only up to 800mbps for Xfinity.

I suggest you to degrade your speed plan to fix the slow speed internet connection.

SURFboard Community Moderator Team
plemans
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Your power levels are also out of spec. They should be fairly consistent but yours bounce around quite a bit. Plus they’re not in optimal range. I’d check your line for issues, loose connectors, kinks, corrosion, or other issues with the line

Current setup: Netgear CM3000-->Netgear RBE973S-->2.5gig tp-link switch, Xfinity 1.4gig/240mbps
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I used to get full 800 but never 900mbps with this sb3600 modem when I first got it 2 years ago on the Xfinity 1.2 gigabit plan. Every 6 months or so my speed drops and consistently stays at lower level.

I just had the Xfinity service come out and told me there was a problem with my line on outside of house and it was affecting other in area. They rewired the whole house for me (free), from the pole to the inside of my house to my modem. Before they did this I was at 400mbps now right after I cant even get 400  no matter the time I test speed. I average around 350-370mbps now.

I use “speedtest.net” and” Xfinity” own speed testing site. Never tested wifi speeds because I dont care about them. But I do transmit them for my wifi devices my kids have.

Using the same home built Windows 11 PC, 10ft of Ethernet wired into sb3600 modem. I even Installed a new network card in PC, a 2.5 gigabit PCIe adapter and no change in speed.

What am I missing to have such a significant degrade in speed?

T
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Here is an update. I have personally restarted my modem many times in past. the Power to whole house has been off many of times over the past year. Some times for days and even once for a week(tornadoes) I’m in central IL. I have never seen any change when I personally do the modem reset.

I just called Xfinity and had them do a reset electronically  and guess what. I am currently Speed testing at 960mbps. How strange is that? After all this time. I just hate calling them all you get is a run around with their automated system. I just wonder how long it will last at this speed now.

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The SBG8300 is supported by Xfinity and has a speed plan of only up to 800Mbps. It looks like the speed plan that you have currently with Xfinity supports 1Gb so we suggest to degrade the speed plan to 800 Mbps and checking the hardwired speed directly connecting the device to the ethernet port of the modem without any splitters.

 

 

 

SURFboard Community Moderator Team
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I used to get full 800 but never 900mbps with this sb3600 modem when I first got it 2 years ago on the Xfinity 1.2 gigabit plan. Every 6 months or so my speed drops and consistently stays at lower level.

I just had the Xfinity service come out and told me there was a problem with my line on outside of house and it was affecting other in area. They rewired the whole house for me (free), from the pole to the inside of my house to my modem. Before they did this I was at 400mbps now right after I cant even get 400  no matter the time I test speed. I average around 350-370mbps now.

I use “speedtest.net” and” Xfinity” own speed testing site. Never tested wifi speeds because I dont care about them. But I do transmit them for my wifi devices my kids have.

Using the same home built Windows 11 PC, 10ft of Ethernet wired into sb3600 modem. I even Installed a new network card in PC, a 2.5 gigabit PCIe adapter and no change in speed.

What am I missing to have such a significant degrade in speed?

Hi 

Thanks for sharing you’re experience on the modem  “SB3600” Please check and let us know if is this an Arris device. Thanks 

SURFboard Community Moderator Team
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The modem I’m using is the Arris SBG8300. Yes I Know now its rated at 900mbps down. But after  Xfinity did a rewire from pole in alley to house I was getting less than 400 mpbs. Couple days later went though the long unnecessarily complicate automated trouble shooting by calling Xfinity. I can now get download speeds of 960mbps and upload at 42mbps. So my problem was solved not by resetting the modem my self in the home. It was solved by having Xfintiy remotely reset my connection to me.

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