gigabit ethernet connection seemingly limited to 100 Mbps

[Special K]

Here is my setup:

- ethernet cable connected from rented Arris DG1670A cable modem/router to gigabit ethernet port on PC running Vista Business x64
- 100 Mbps down 10 Mbps up internet connection from Time Warner Cable

Last week after installing and activating my own cable modem I noticed my download speeds, as measured per speedtest.net, had dropped from 100 Mbps to 70 Mbps. Previously I was consistently getting 100 Mbps on the same speedtest. The upload speed was still at 10 Mbps.

I reconnected and reactivated the Arris DG1670A modem/router, yet my download speed remained stuck at 70 Mbps. This limit was very consistent: every time I would run the test, the download speed would be right at 70-71 Mbps.

I called TWC, thinking they had mistakenly reduced my speed limit, yet when the tech came out, he was able to get 100 Mbps down when plugged into my modem/router. I even tried my PC plugged into the port he was using with the same ethernet cable and was still stuck at 70 Mbps.

If my ethernet port were limited to 100 Mbps, the 70 Mbps speedtest result would make sense after factoring in the overhead of the ethernet interface. Yet if I go to the networking tab of Task Manager, my "Local Area Connection" shows a link speed of 1 Gbps. The modem/router's interface also shows a speed of "1000(Full)".

It's as though the ethernet interface is operating at 100 Mbps even though Windows thinks it's operating at 1 Gbps.

What could be the problem here?

The only other thing I noticed is that when I go to Task Manager->Networking, I see a "VirtualBox Host-Only Network" running at 100 Mbps. Could that be causing my 1 Gbps ethernet port to be running at 100 Mbps? FWIW, I've been using that virtualbox connection for as long as I've had my 100/10 internet speeds and have never had this problem until now.

 

[dave_the_nerd]

Vitualbox isn't doing that.

Probably just line overhead - I'm betting the cable guy is using internal testing and not speedtest.net. Try a different speedtest server?

What does your modem/router report as its connection speed?

 

[Special K]

Vitualbox isn't doing that.

Probably just line overhead - I'm betting the cable guy is using internal testing and not speedtest.net. Try a different speedtest server?

What does your modem/router report as its connection speed?

The cable guy used the exact same speed test I was using.

The modem/router's interface also shows a speed of "1000(Full)".

 

[VirtualLarry]

The cable guy used the exact same speed test I was using.

The modem/router's interface also shows a speed of "1000(Full)".

Not the LAN jack. What does the modem report as your WAN speed? (US / DS)

 

[JackMDS]

Try it at 5 A.M

 

[Special K]

Not the LAN jack. What does the modem report as your WAN speed? (US / DS)

OK, I found the following buried in one of the modem's config pages:

UpstreamServiceFlow =
    SfReference = 0
#   SfId = 337313823  -- assigned by CMTS, correlates to PcSfId
    SfClassName = "us_hsd_res"
    SfQosSetType = 7
    SfTrafficPriority = 0
    SfMaxTrafficRate = 11264000
    SfMaxTrafficBurst = 20480
    SfMinReservedRate = 0
    SfMinReservedRatePktsize = 0
    SfActiveQosTimeout = 0
    SfAdmittedQosTimeout = 200
    SfMaxConcatBurst = 20480
    SfSchedulingType  = 2

DownstreamServiceFlow =
    SfReference = 0
#   SfId = 337322016  -- assigned by CMTS, correlates to PcSfId
    SfClassName = "ds_hsd_res"
    SfQosSetType = 7
    SfTrafficPriority = 0
    SfMaxTrafficRate = 112640000
    SfMaxTrafficBurst = 20480
    SfMinReservedRate = 0
    SfMinReservedRatePktsize = 0
    SfActiveQosTimeout = 0
    SfAdmittedQosTimeout = 200

Based on some quick searching, us_hsd_res is the upstream speed, and ds_hsd_res is the downstream speed. Doing some quick math:

ds_hsd_res:    112640000/(1024^2) = 107.42 Mbps
us_hsd_res:    11264000/(1024^2) = 10.742 Mbps

So it appears the modem is provisioned correctly. The cable tech's speed test results (and my own before this whole problem started) match up almost exactly with the speeds listed above.

 

[Special K]

Try it at 5 A.M

Did you read my post? Here is what happened:

1. cable tech plugs his laptop into my modem/router and runs speed test; gets rated speed
2. cable tech unplugs ethernet cable from his laptop and into my desktop
3. speed test is run on my computer; I get 70 Mbps down / 10 Mbps up consistently

 

[JackMDS]

I even tried my PC plugged into the port he was using with the same ethernet cable and was still stuck at 70 Mbps.

Well if you plug another computer directly and it is the same then what?

Unfortunately many ISPs behavior does not project them as very trustworthy, it is hard to know what the border between reality and make believe reality is.

My cable service is on the spot at 5 A.M when almost every one is Not online.

Later in the morning it starts to fall and keep slowly falling until late at night.

That is in nature of Node based Cable service. More user, less bandwidth to each one on the Node.

That said very few people are really hurting if their Speed is 70 instead of 100.

 

[dave_the_nerd]

Did you read my post? Here is what happened:

1. cable tech plugs his laptop into my modem/router and runs speed test; gets rated speed
2. cable tech unplugs ethernet cable from his laptop and into my desktop
3. speed test is run on my computer; I get 70 Mbps down / 10 Mbps up consistently

If that's true then there's something wrong with your computer.

Boot from a linux livecd and try the speedtest. If it works at full speed, then your hardware's fine and you csn nuke'n'pave.

 

[Special K]

If that's true then there's something wrong with your computer.

Boot from a linux livecd and try the speedtest. If it works at full speed, then your hardware's fine and you csn nuke'n'pave.

Thanks for the suggestion. I booted a Ubuntu liveCD and ran the speedtest from within Chromium and got my advertised 100/10 speed. That rules out the hardware as the culprit and pretty much places the blame squarely on Windows IMO.

At this point I have no idea what obscure Windows setting is causing my speed to be limited. I'm thinking the amount of time it would take me to debug it would be greater than or equal to the amount of time it would take to just do a clean install.

 

[dave_the_nerd]

I'm thinking the amount of time it would take me to debug it would be greater than or equal to the amount of time it would take to just do a clean install.

That is usually correct.

You are now a believing member of the Church of the Reimage-Fixes-Everything. Please pick up your lanyard and thumb drive at the door, and don't forget our motto: "It doesn't matter what caused the problem, it works now."

 

[Special K]

OK, after doing some more searches that included phrases like "linux internet faster than windows", I was able to find a number of articles that reference automatic "optimizations" that windows can perform that actually reduce the performance of your internet:

https://www.duckware.com/blog/how-windows-is-killing-internet-download-speeds/index.html
http://superuser.com/questions/151414/windows-7-internet-speed-significantly-slower-than-ubuntu
http://blogs.msdn.com/b/wndp/archive/2007/07/05/receive-window-auto-tuning-on-vista.aspx
http://www.speedguide.net/faq/how-to-disable-windows-vista-tcpip-auto-tuning-247

These articles list a number of commands you can run from an elevated command prompt that are supposed to undo any damage caused by Windows automatically tweaking TCP settings.

I ran the commands and they had no result, but then stumbled on the TCP Optimizer program:

http://www.speedguide.net/downloads.php

All I did was set my connection speed and tell the program to set the relevant TCP registry settings to their "optimal" values. After rebooting my computer I'm now once again seeing the full rated speed. It appears that windows at some point changed some TCP setting without notifying me that had a major impact on my bandwidth.

In any case it's good to know what caused the problem, approximately at least.

I never would have thought that those TCP tweaks would still have such a large impact on performance. I remember using them back in the late 90's/early 2000's to squeeze every last drop out of my 1.5 Mbit DSL line.

 

[Binky]

A long time ago I used one of those game utilities that are supposed to improve gaming lag by adjusting TCP settings in windows. This caused speed issues on downloads, not terribly different than what you are seeing.

 

[VirtualLarry]

Make sure that you check this out too:
http://forums.anandtech.com/showthread.php?t=2398779