One of the "lessons learned" from our testing of higher speed (>10Mbps) network adapters is that you don't always get what you think you're getting, i.e. higher network speed. Things that didn't matter at lower network speeds come into play when using products such as 100Mbps Ethernet adapters and even the newer 802.11a wireless network adapters.
This Need To Know will show you some of the throughput-stealers that you may not know you have, and suggest some ways to get the most out of your Fast Network Adapters.
Our standard test platform for wireless PC card adapters has been a Compaq Presario 1650. "Ol' Faithful" has a 266MHz Pentium II processor with 96MB (the max, unfortunately) of RAM. It now runs WinXP Home (surprisingly well, actually), but is set up for dual-booting to Win98SE. It doesn't have a built-in Ethernet adapter, but through the courtesy of Linksys, we have a PCM200 10/100 Ethernet CardBus card at our disposal. The PCM200 has a 32bit CardBus interface, which is essential if you're going to come anywhere near utilizing all of the bandwidth in a 100BaseT connection.
The other essential piece of test gear is NetIQ's Chariot network peformance evaluation program. Although it has waaay more bells and whistles than we need, it has the ability to run tests for extended periods of time, allows control over how much data is sent and how often, and has a very nice comparison tool that allows comparison of test results in both graphical and numerical formats...complete with some simple statistical analysis, too.
The "partner" for all the tests that were run for this article is a Compaq Deskpro (733MHz Pentium III, 319MB RAM, Win98SE) with an Accton 1207D PCI adapter. Both adapters are set to auto-negotiate connection speed and are set to factory defaults. The computers are connected through the 10/100 switch in an SMC7008 Barricade router.
Transmit? Receive? What's the difference?
You'd expect your LAN to work the same whether your computer is sending or receiving data, right? While that may have been true when LANs ran at 10Mbps speeds, it turns out to often not be the case on a 100BaseT LAN. Take a look at the first Chariot plot below.
Plot 1: Compaq laptop - Receive vs Send
Plot 1 shows a one minute test between the Compaq laptop and Deskpro desktop. It clearly shows that the average receive throughput for the laptop is almost twice that of its transmit throughput. On the other hand,however, the laptop performance when sending data has a wider distribution, with the max speed of 84Mbps more than 300% higher than the minimum!
In researching possible causes for this problem, we found numerous complaints (example) of lopsided Windows networking performance. The most often mentioned suspected culprits were the various speed-tweak programs that people use to speed up their cable modem or DSL connections. Some people had success with removing the tweaks (such as in this thread from Google Groups) others found that going back to the Windows defaults didn't help their situation.
Chariot's help section suggests changing the TCP Receive Window (via a Registry tweak) to tune performance. But since we want to test products under as-equal-as-possible conditions, we'll leave the registry tweaking to someone else. If you want to experiment, however, the information over at SpeedGuide.net is about the most comprehensive that you'll find on the subject. Just remember to proceed at your own risk!