Wired PC getting half the downstream speed as wireless laptop

I'm hoping someone has some insight into this problem, as it's got me stumped.

Comcast recently doubled my Blast! speeds from 30/6 (I'm guessing at the old speed tier) to 50+/10+ Mbps. After rebooting my less-than-a-month-old Motorola SB6121 modem and my Trendnet TEW-633GR router, I ran a speed test at speedtest.comcast.net from my laptop (connected wirelessly) and got around 53-54 Mbps down, and 10.3 Mbps up. As you might imagine, I was pretty psyched.

Yesterday I thought I'd test my wired PCs, one an old Pentium 4 running Win XP Pro with a D-Link gigabit Ethernet adapter, the other a modern Core i5 with 8 GB of RAM on an ASRock P67 Extreme6 motherboard, which includes two gigabit Ethernet ports (Realtek 8111E chip, according to ASRock's site). The XP Pro PC was pathetically slow (15/5 Mbps), but after running the latest version of the TCP Optimizer (Optimal settings, ISP speed set to 50 Mbps), and then rebooting it and the HP ProCurve switch, its speeds jumped to 38/9 Mbps, and those speeds are the same today. And given the outdated D-Link NIC in it, that's all good.

The Windows 7 Home Premium PC, which the Toshiba laptop also runs, and also runs the same Core i5 CPU, is a different story. Despite comparing the various TCP parameters to the laptop's via the netsh command, and despite running the latest TCP Optimizer on it, it never gets above 25 Mbps downstream. It consistently gets 10+ Mbps up. I've also tried updating the NICs' firmware, trying each NIC separately, as well as taking the switch out of the equation and connecting it directly to the router, but the results are the same.

Additionally, I tested an older duo core (Core Duo?) HP laptop, and it, like the newer Toshiba, gets the advertised 50/10 Mbps. I should note that both laptops and the router are connected via wireless "N," but that's rather beside the point, as my entire LAN is gigE and running jumbo packets of 9K.

To rule out the cable, I copied a 2 GB file from a Netgear ReadyNAS Ultra 4 to the PC at over 100 MBps (not Mbps).

The TCP parameters on the PC match those of the laptop, both in the netsh command and in the registry, with the exception of one registry parameter available to the PC's NICs that is not available on the laptop.

At this time, I can't connect directly to the modem, and I doubt that would make a difference.

The last thing I checked was the router's configuration, specifically for QOS settings. It's all pretty neutral. It is running the latest firmware.

So pretty clearly this is not a Comcast problem, not a modem problem, and I'm guessing not a router problem. It seems to me it's either the PC's NIC(s) or some network configuration issue I'm overlooking, but as noted, on the LAN I get blazing speed.

If anyone has any ideas, I'm all ears.

Thanks in advance.

I can't say why those machines are showing different wireless speeds as there are many factors involved, however there is something worth mentioning about the 54Mbps you are seeing now which is far from the full wireless N potential of 300Mbps.

In order to get true 802.11n speeds you need to go into your routers wireless settings and choose the 802.11n "Only" setting along with using WPA2 security and AES encryption which are both meant to work with wireless N. You won't be able to use the wireless N Only setting if you have wireless G or below clients that need to connect.

Perhaps I wasn't clear, but I'm talking about the speed test results from Comcast after they upgraded our speeds.

My wireless N laptops both get the full 50/10 Mbps down/up. My wired PC - connected through the same router - gets the full 10 Mbps up, but is throttled at 25 Mbps down.

This is not a wireless problem, nor is it a LAN throughput problem. As noted, I'm able to copy a file across the LAN at 125 MBps peak, which is 1000 Mbps, as reported by the Windows file copy dialog box. I should note that my NAS and the PC in question are connected via an HP ProCurve switch, everything running 9K jumbo frames.

Thus, it seems to be a question of some TCP setting on the wired PC, which is prohibiting it from seeing the full downstream bandwidth being delivered by Comcast.

Hope that clears things up.

Sancho12 said:

Perhaps I wasn't clear, but I'm talking about the speed test results from Comcast after they upgraded our speeds.

My wireless N laptops both get the full 50/10 Mbps down/up. My wired PC - connected through the same router - gets the full 10 Mbps up, but is throttled at 25 Mbps down.

This is not a wireless problem, nor is it a LAN throughput problem. As noted, I'm able to copy a file across the LAN at 125 MBps peak, which is 1000 Mbps, as reported by the Windows file copy dialog box. I should note that my NAS and the PC in question are connected via an HP ProCurve switch, everything running 9K jumbo frames.

Thus, it seems to be a question of some TCP setting on the wired PC, which is prohibiting it from seeing the full downstream bandwidth being delivered by Comcast.

Hope that clears things up.

Have you tried working with the network adaptors settings which you can find by going to Device Manager then right clicking the network adaptor, choose properties then open up the Advanced settings, there is no telling which one of those settings may have an effect so it will be a trial and error type of thing.

chev65 said:

Have you tried working with the network adaptors settings which you can find by going to Device Manager then right clicking the network adaptor, choose properties then open up the Advanced settings, there is no telling which one of those settings may have an effect so it will be a trial and error type of thing.

That's my next task, but as this is my HTPC, it's hard to find the time to tweak and reboot. There was one setting called "Green Ethernet" (Green something) that caught my eye. I'll disable that and see what happens. I guess I'll email Realtek and see if they have any ideas.

And as always, I'm all ears here

All right, I did quite a bit more testing. Here's the upshot:

The PC in question, when connected directly to the Motorola SB6121 modem, easily achieved the advertised 50/10 speeds.

I also checked my two laptops which use Realtek RTL81xx Ethernet chips, and they experience the same throttling when connected to the Trendnet TEW-633GR router. Recall that this same router has no problems delivering 50/10 speeds to the laptops when they are connected wirelessly.

I think you know where I'm going with this... it's the router. Yep, that's where I'm going. But wait, it's not quite that simple. My seven-year-old P-4 running an antiquated D-Link gigE adapter hits nearly 40 Mbps downstream cabled through the same devices as all PCs running the RTL81xx's.

Given that the WAN and all four LAN ports are 10/100/1000 on the Trendnet, it seems there's an incompatibility between whatever chipset it uses (still researching that) and the Realtek 81xx. The Realtek 81xx is not the bottleneck. The Trendnet's WiFi capabilities and its WAN port are not the bottlenecks.

It's certainly weird, at least to me, but perhaps the Trendnet's switch just isn't up to the task. I'm willing to replace, but now the question is, which router?

Zepher said:

I would download something from a fast site and see if it is just the speedtest app not running properly on that machine.

That's a good idea. Any suggestions for a "fast site"?

logicearth said:

Might try disabling or enabling TCP checksum offload. I have seen it cause a few problems with NICs.

Thanks for the specific suggestion.

I could try that, except for the fact that without changing any settings on the NIC it worked fine when I took the router out of the equation. But I guess I could try disabling various parameters while researching a new router.