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Gigabit internet slowness (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems)

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Gigabit internet slowness (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems)

I am a recent subscriber to the gigabit plan and have a few questions (seeking an expert). I also am wondering if anyone else is having my issues.

 

Location: Woodbury NJ

 

Arris SB8200 with gigabit plan

 

Questions:

-What is up with channel ID 25 running at 675000000 Hz? Is this a data channel? It is the only channel with correctable codewords on my setup.

-Is this expected (high codeword errors on 1 channel)  if you have normal signal levels? I know these are not as bad as uncorrectable errors that involve the retransmit of data between the modem and CMTS but there must be some overhead.

 

Summary:

Otherwise this modem seems pretty stable. Most speed tests for me will cross 600Mbps down and every once in awhile I can get 900+. The upload seems to be cosistent at peaking around 40Mbps.

 

 

Netgear CM1000 with gigabit plan

 

Questions

-Now this modem has a lot more correctable codewords spread across different channels with the exact same wiring on my side. It is interesting that Netgear splits the channel ID 25 and frequency 675000000 Hz into another stat line seperated from the downstream channels (Downstream OFDM). This makes it look a little cleaner and hides the large correctable codewords sections. Seems to me that this was done on purpose. Does anyone know why this frequency is being handled outside of the downstream channels on the Netgear modem and not Arris?

 

Summary:

Has a lot more errors on downstream channels. Also has errors on downstream channel 25.  Also why is that frequency seperated on the stat page?

 

Notes:

I also included screenshots from 2x DOCSIS 3.1 modems I tested (see attachments). It is interesting to see the stat layout differ from SB8200 to CM1000.

 

 

 

 


Accepted Solutions
Frequent Visitor

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Ok so I perfected my test just now with a known good setup (not switching hardware or cable paths). I took my Windows 10 laptop and ran a speedtest. I received the normal random 500-700 down very inconsistent. I then rebooted into the latest Ubuntu distro with the exact same cabling and setup. I then launched Firefox and ran multiple speed tests. All speed tests exceeded 900Mbits a second consistently. If anyone wants to try it is simple (included steps below). I am convinced the incosistent speed results in Windows for various cable modems on the gigabit plan are all software related. I have to figure out what software tweaks are throttling the connection. It could be security, NIC drivers, or OS level settings. Stay tuned... I will also get my new CAT6 cable although I do not believe that will help solve this issue.

 

Sample test (laptop with same cabling preferred.. No Wireless!)

-Run test in Windows (7\8\10) and record at least 3 results (http://www.speedtest.net/, http://speedtest.xfinity.com/, https://www.speakeasy.net/speedtest/)

-Download latest Ubuntu LTS and make a bootable USB drive using Rufus (https://tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-windows#0) 

-Reboot and set BIOS or hotkey to boot from USB drive. (## No need to install just run off the USB ###)

-Run same tests as 1st line above. You will notice you get max speed (or close to it... there will be some overhead)


All Replies
Expert

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Channel 25 0n the 8200 modem is the DOCSIS 3.1 OFDM / PLC channel for the gigabit speed tier. Some models of  modems list it as "OFDM" and some as "Other". Seems that all of the data is traversing that channel therefore you see all of the bit error being listed there.

 

There are no "uncorrectable" errors on either modem, so there is nothing to worry about.

Expert

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems



It’s normal for the OFDM channel to have correctables and uncorrectables, it is not a cause for concern. The easiest way to identify this channel is that the modulation type will be displayed as “other”, but it's nice if they break the OFDM out of the downstream channels, since it's not really a downstream channel. The status pages are created by their respective manufacturers w/o any specific standards, so it's normal to have different interfaces. 

 

In my experience, the Arris is a bit of a stinker - Are you seeing the same speeds with the Netgear and the Arris? 

"Keepin' it clean since '09" --I am not a Comcast employee.
Frequent Visitor

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Thanks for the reply. I honestly have a mixed feeling about both modems. I have been running a lot of tests and notice the best performance on Linux hosts when using the sb8200. It could be that the NIC is better (hardware\driver)  on my Linux box in combination with not running local AV for Linux. I also was using SNORT on my firewall that I built and notcied it can only handle ~450Mbps down when enabled. There seems to be a lot of things to consider for gigabit that could be bottlenecks or issues. I will have to do a lot more testing but it is nice to see what others are experiencing with the new service. When you say "stinker" for the Arris SB8200 can you give an example of something that was negative experience with the modem?

 

Screenshot of speedtest from Linux host with SB8200

speedtest_SB8200.JPG

 

Screenshot of Win7 on SB8200 with AV\Firewall disabled

speedtest_SB8200_win7.JPG

Expert

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Just do a search on the SB8200 and you'll see a lot of complaints about it not achieving expected speeds.  I see the same types of posts in other help forums such as Reddit. It's interesting that you are seeing full speeds with Linux. And yes, there are so many variables it's often difficult to help users. Thanks so much for posting some comparison details! I'd love to hear more about your experience with these two modems.   

"Keepin' it clean since '09" --I am not a Comcast employee.
Expert

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems


@The_Janitor wrote:


It’s normal for the OFDM channel to have correctables and uncorrectables, it is not a cause for concern. The easiest way to identify this channel is that the modulation type will be displayed as “other”, but it's nice if they break the OFDM out of the downstream channels, since it's not really a downstream channel. The status pages are created by their respective manufacturers w/o any specific standards, so it's normal to have different interfaces.  


Darn echoes........................ rolleyes.gif

Frequent Visitor

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Thanks guys. I will poke around on reddit also. The funny thing is I can only get the speed from the Linux host while using the speakeasy.net test and pointing to the Washington location. It makes you think that some of these random speed testers may not be qualified gigabit testers and everyones roundtrip path will be different. You can easily hit an issue upstream and can make it look like it is an issue on your end. It is almost better if they made test sites that were qualified gigabit end to end or someway to analyse that before running the test. Do you agree?

Frequent Visitor

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Since I am a big nerd I found an advanced Intel diagnostic tool that you can run against a cable connection. This would be interesting if it is actually my problem.... Will try replacing with before\after screenshots. The general healthy of the cable and negotiated speed is 1Gb but the low level cable diagnostic failed. I swear it was a good deal on Amazon! Smiley Happy My Intel NUC that ran the previous benchmark for Linux is using a different cable.

 

intel_cable_test.JPG

Frequent Visitor

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Ok so I perfected my test just now with a known good setup (not switching hardware or cable paths). I took my Windows 10 laptop and ran a speedtest. I received the normal random 500-700 down very inconsistent. I then rebooted into the latest Ubuntu distro with the exact same cabling and setup. I then launched Firefox and ran multiple speed tests. All speed tests exceeded 900Mbits a second consistently. If anyone wants to try it is simple (included steps below). I am convinced the incosistent speed results in Windows for various cable modems on the gigabit plan are all software related. I have to figure out what software tweaks are throttling the connection. It could be security, NIC drivers, or OS level settings. Stay tuned... I will also get my new CAT6 cable although I do not believe that will help solve this issue.

 

Sample test (laptop with same cabling preferred.. No Wireless!)

-Run test in Windows (7\8\10) and record at least 3 results (http://www.speedtest.net/, http://speedtest.xfinity.com/, https://www.speakeasy.net/speedtest/)

-Download latest Ubuntu LTS and make a bootable USB drive using Rufus (https://tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-windows#0) 

-Reboot and set BIOS or hotkey to boot from USB drive. (## No need to install just run off the USB ###)

-Run same tests as 1st line above. You will notice you get max speed (or close to it... there will be some overhead)

Expert

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

Nice! Keep us posted! 

"Keepin' it clean since '09" --I am not a Comcast employee.
Frequent Visitor

Re: Gigabit internet (Cable modems, signals, and codeword errors) using DOCSIS 3.1 modems

This setting appears to have helped a lot for me. I am not sure if this will help everyone but setting the below tcp autotuning setting fixed it for me. There are a few settings to play with but mine was disabled so I was limiting my received tcp window to 64KB. I am linking to other articles about adjusting this value. I am now getting the expected 900+ Mbps download speed in Windows.

 

Documentation:

https://blogs.technet.microsoft.com/networking/2016/08/11/an-update-on-windows-tcp-autotuninglevel/

https://www.sevenforums.com/network-sharing/74556-enable-disable-tcp-auto-tuning.html

 

Fix: (run CMD prompt as admin because elevated access required)

netsh int tcp set heuristics disabled

netsh interface tcp set global autotuninglevel=highlyrestricted