Cox Ultimate Broadband - 50Mbps

I dumped my old ISP and signed up for Cox's "Ultimate" broadband package, with speeds up to 55Mbps downstream and 5Mbps upstream. This is the bleeding edge, and getting it working at full speed is tricky.

Amazingly, Cox was able to get the DOCSIS 3.0 modem installed and running on the first try. Given that this technology is new for Cox, I was quite surprised.

So I fired up Speedtest.net and obtained:
22 Mbps download
9 Mbps upstream

Astute readers will notice that this is half of what I was expecting downstream and almost twice what I was expecting upstream. Strange.

I'll save you the details of four hours of sleuthing and simply present my results:

• The Router.The problem is almost entirely caused by my WRT54G router running DD-WRT v24-sp1. The router maxes out at 22 Mbps. If one IP connection is running at that speed, the WRT54G won't even accept wired http connections to the status page. I tried installing TOMATO, and that maxed out at 28.5Mbps. A respectable improvement, but far short of what I needed. Discussions on dslreports.com indicate that my results are typical and that the WRT54G is simply too slow to meet my needs.
  
• Buffer management.You might think a fast Broadband connection would be just like a 100MB local Ethernet connection, but it's not. Local Ethernet connections typically have submillisecond latencies. Broadband connections can easily have 200ms latencies, which means that over a megabyte of data can be transmitted before an ACK is received. This is a major change in how buffers are managed in the Ethernet stack. I found that tuning for 50Mbps broadband required parameters very similar for max throughput for Gigabit Ethernet.
  
• RWIN and MTU.Windows Vista and Windows 7 automatically tune IP parameters, so I didn't need to adjust the RWIN, MTU, or other parameters. However, Windows XP and earlier users will almost certainly need to hand tune parameters to make things work properly. Verizon has a Speed Optimizer tool that does this automatically, but Cox does not. Don't use Verizon's tool - Verizon uses an MTU of 1492 and Cox uses 1500.
  
• Wireless Connections.If you are using 802.11g (as most people do), your connection maxes out at about 20Mbps. (Maybe 25Mbps if your wireless connection is perfect.) Buying 28Mbps or 50Mbps service is simply a waste of money. You need to upgrade to 802.11n to run full speed.
  
• Testing. 50 Mbps is fairly slow for a LAN, but it's the bleeding edge for consumer WAN technology. Most web sites simply can't service data that fast. Speedtest.net won't go that fast if it's busy. I can only download at about 10Mbps from my corporate server, which has a 100Mbps connection to a backbone. This is probably a TCP tuning problem on the RedHat server, but this demonstrates that both Windows and Linux have default connections that are not well suited to this configuration.
  

[Update 2/26/2010] Here is the speed test result from the iMac in the office. This was performed without any tuning of OS X: