wired PC getting half the downstream speed it should

[thurstonx]

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 And given the outdated D-Link NIC in it, that's all good.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.

[thurstonx]

Addendum
--------
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?

[thurstonx]

Just wanted to update this thread to say that the limitation was indeed the switch built into the Trendnet TEW-633R router. I bought an amped wireless R10000G, since they offer a 30-day money back guarantee when buying directly from them. No problems getting 50 Mbps down, 10 Mbps up on PCs wired through an HP ProCurve switch, which in turn is wired to the router.

I've no idea if other Trendnet routers might have this problem, but using smallnetbuilder.com one can see the internals:

Internal Details
---------------
Internally, the TEW-633GR contains the same main components as the DIR-655—namely a Vitesse VSC7385 switch, a Ubicom 5160 processor, and an Atheros 5416 baseband/MAC chip and AR2133 3x3 MIMO 2.4 GHz radio (AR5008 series).


Ubicom and Vitesse were new to me, but since I had no problem achieving full speed wirelessly, all signs point to the Vitesse switch.

Hope this all proves useful to someone in some way.

Cheers.

[Philip]

Thanks for sharing and posting back the issue for the benefit of others.

[drbaltazar]

wasnt ms having issue if we had more then one device to connect on the web, i am rusted here but if i recall ms used to split the internet in 2 so if you had a internal internet thing (like most of us have)and use some router it would split speed in 2 so instead of 50 mb down you would end up with 25 mb down!