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Question Diagnosing Possible Modem/Router Issue

tailes151

Senior member
Mar 3, 2006
867
9
81
Searched the forums for a similar issue, and couldn't find the same scenario.

First, the hardware:

- Motorola Surfboard SB6121 modem
- TP-Link Archer C9 Router
- Window 10 PC
- Various connected WiFi devices: 2 cell phones, 3 alexa's/dots, Sony Smart TV, Laptop, WiFi printer, Chromecast, PS4, Switch

Here's what's going on.

I live in my home with my wife and no kids. We stream a lot of videos but not very rarely simultaneously. Our internet is through Comcast/Xfinity on a 275Mbps dl connection. I've been with them for ~8 years and have had no previous issues. I purchased this modem/router combo 2 or 3 years back and they had been working great. My initial wired speed tests (Modem -> Router -> PC) showed DL speeds around 250Mbps. I don't recall performing tests with any of our WiFi devices, but I had no troubles with any of our usual activities, which again, is mostly watching Netflix/Hulu, or downloading a game on Steam or my PS4.

Lately I'd been noticing some significant slowdowns on mobile devices, so I ran a speed test. Was averaging ~12-15 Mbps on one test, and ~55Mbps on another. My upload speed was at 15Mbps on both, which is normal. I checked on the wired computer and it was much faster, although it still showed a significant slowdown ~120-130Mbps.

Things I've tried:

-Swapped between numerous ethernet cables, all cat 5e or 6. Have not noticed any differences.
-Connected the modem directly to the PC. No effect to the wired speed of ~120-130 Mbps.
-Updated the firmware on my router. No changes to either wired or WiFi speeds.
-Hard resets on both the modem and the router. Nothing
-Refreshed the signal to the modem through Comcast's site. Nada

Since my modem/router are owned, Comcast don't appear willing to be of much help. Does anyone have any guesses as to whether this is this more likely a service/Xfinity issue, a modem issue, a router issue, or some combination between these?
 

SamirD

Golden Member
Jun 12, 2019
1,052
156
66
www.huntsvillecarscene.com
Sounds like the straight modem to pc test is showing there's either an issue with the modem (doubt it) or the service, which is more likely.

The problem is the finger pointing issue you get with your own equipment. Unless you have a second modem to prove that it's not your modem, you would risk being assessed a fee for a service call if it is not your modem.

The good thing is that almost any old modem can hit 300Mbps, so if you can find someone selling a used one locally for cheap, that might be your best bet.
 

ch33zw1z

Lifer
Nov 4, 2004
30,186
9,171
136
IIRC, the SB6121 maxes out around 200mbps.

Since you bring your own modem, you should keep a spare handy to verify with, that will require a phone call with Comcast. May as well drop $50-$100 on the upgrade. Verify what speeds you should be getting, because they change their plans every so often to align with their marketing.

However, the modems age shouldnt interfere with the wireless speeds really, but stranger things have happened.

You should use iperf to test wlan speeds, not want speeds
 

SamirD

Golden Member
Jun 12, 2019
1,052
156
66
www.huntsvillecarscene.com
IIRC, the SB6121 maxes out around 200mbps.
Very good point! I missed this.

So this is the culprit--you should have actually never gotten the same speeds with your own modem as with the provided one because your own modem doesn't support speeds fast enough.

Almost any of the models after the sb6121 are capable of speeds up to 343Mbps, and the sb6183 and sb6190 really shine (don't worry about the 'puma' scare with the sb6190--I have 2x of these and they work perfect). Swap out to any of these modems and I think you'll be back to normal.
 

tailes151

Senior member
Mar 3, 2006
867
9
81
You guys seem to have nailed it down - it looks like Comcast also EOL'd their support of the SB6121 sometime in the last year or so. Still doesn't explain the issue of my wireless devices getting approximately 10% of the wired speeds, so I'm going to pick up a new router as well and see if that is the culprit.

Thanks for the help!
 

SamirD

Golden Member
Jun 12, 2019
1,052
156
66
www.huntsvillecarscene.com
You guys seem to have nailed it down - it looks like Comcast also EOL'd their support of the SB6121 sometime in the last year or so. Still doesn't explain the issue of my wireless devices getting approximately 10% of the wired speeds, so I'm going to pick up a new router as well and see if that is the culprit.

Thanks for the help!
That eol would explain it as they would stop pushing out a supported firmware and only a generic one that would limit the speed.

However, I wouldn't touch your router at this time. One component at a time or you'll still be scratching your head if something doesn't work.
 

ch33zw1z

Lifer
Nov 4, 2004
30,186
9,171
136
Very good point! I missed this.

So this is the culprit--you should have actually never gotten the same speeds with your own modem as with the provided one because your own modem doesn't support speeds fast enough.

Almost any of the models after the sb6121 are capable of speeds up to 343Mbps, and the sb6183 and sb6190 really shine (don't worry about the 'puma' scare with the sb6190--I have 2x of these and they work perfect). Swap out to any of these modems and I think you'll be back to normal.
yup, i use a 6183, flawless.
 

mindless1

Diamond Member
Aug 11, 2001
5,330
435
126
Combination, the ISP is probably provisioning less bandwidth per channel now, so you need at least an 8 channel modem (better to get 16 or 32, why face obsolescence again sooner?), plus there's something up with your wifi, whether the router is failing (typically from bad capacitors in it or the AC/DC adapter) or you have wifi congestion and should do a site survey and try a different channel.
 

WaCableTech

Junior Member
Jul 8, 2019
5
1
41
Good wired speed and bad wifi speed implies router or wireless environment, but let's try a few easy commonly seen issue. Wireless printers are often cheap wireless G tech, and like a single lane highway the slowest car sets the max speed, wireless G caps out around 15Mbps real world, I don't care how many times they claim 54 Mbps on the packaging. Unplug power from the printer, reboot the router and retest? Better? If so, I recommend kicking that printer off the wifi and running an Ethernet cable to it until you replace it and since ink and toner are more expensive than the printers anymore that should be soon. If not, you need to one by one kick wifi devices off the network by unplugging them from power, it is not enough to just remove the wifi SSID from them they may be broadcasting erroneously and making interference, so de-power them one by one and test after each one until it seems better, if this never happens, its router shopping time and you may want to consider a mesh network if you have a large or oddly shaped house. In the last week I have been to several wifi service calls one guy had a bad wifi light system [you know the ones you can tell goolge and alexa to turn on and off, etc...], one a wireless G printer, and one guy a my best one was the wireless B router guy. Sir you need a new router it says Wireless B on it they made wireless G, N, AC and now another one is coming out, its old sir [not to mention 10Mb Ethernet ports anyway].

If you added any wireless devices around when this seems to started happening, I'd start by de powering them first. wireless cameras and door bells and lights and whatever else, see if they are the issue if they were introduced near the time you noticed the issue.
 

VirtualLarry

Lifer
Aug 25, 2001
48,864
5,338
126
and one guy a my best one was the wireless B router guy. Sir you need a new router it says Wireless B on it
LOL! I had a wireless 'B' router... back in the 90s/early 2000s. Surprised it's still working.

802.11b, was like the dial-up of wifi. Good luck getting a decent connection speed of over 1-2 Mbit/sec with it. I think theoretical max was 11Mbit/sec?
 

ch33zw1z

Lifer
Nov 4, 2004
30,186
9,171
136
In practice that is really about 120 - 130 Mbps. Had to replace with a SB6141 when Spectrum bumped the base rate to 200/10
Yea, it's not uncommon to see that. There's overhead involved. Same way Ethernet will say it's 100mbps or 1000mbps but actual data moving speeds are 80-90% of that.

For modems, it comes down to ISP provisioning / channels. The sb6121 is docsis 3.0, IIRC can get to 1gbps downstream (don't remember the upstream), but by the time our U.S. cable ISP's even got there, docsis 3.1 and then 4 had been released....and the ISP's aren't supporting a legacy piece of hardware like the sb6121 anymore.
 

SamirD

Golden Member
Jun 12, 2019
1,052
156
66
www.huntsvillecarscene.com
Yea, it's not uncommon to see that. There's overhead involved. Same way Ethernet will say it's 100mbps or 1000mbps but actual data moving speeds are 80-90% of that.

For modems, it comes down to ISP provisioning / channels. The sb6121 is docsis 3.0, IIRC can get to 1gbps downstream (don't remember the upstream), but by the time our U.S. cable ISP's even got there, docsis 3.1 and then 4 had been released....and the ISP's aren't supporting a legacy piece of hardware like the sb6121 anymore.
sb6121 is spec'd at 343Mbps down. The sb6190 is spec'd at 1.4Gbps, but I don't think any provider is provisioning it at those speeds.

You'd be surprised where an sb6121 is supported. Legacy and 'senior' plans that are only 25Mbps easily work with these modems.
 

ch33zw1z

Lifer
Nov 4, 2004
30,186
9,171
136
sb6121 is spec'd at 343Mbps down. The sb6190 is spec'd at 1.4Gbps, but I don't think any provider is provisioning it at those speeds.

You'd be surprised where an sb6121 is supported. Legacy and 'senior' plans that are only 25Mbps easily work with these modems.
Sure, but the SB6121 is still technically a DOCSIS 3.0 device. Whether or not the modem can push that connection speed is not really my point. The internal hardware of the SB6121 is the limitation of the device. And with newer models, the ISP is the limitation.

My point is that ISP's drop support for legacy modems (SB6121) and are very slow to progress to new technologies. This allows for modem makers to generate much more revenue as well, releasing modems with just enough hardware to meet, or exceed, current ISP supported speeds, even tho the technology allows for much higher speeds.
 
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