Turns out that there is a TCP parameter in the registry called the Receive Window (RWIN) that can really limit Internet download speeds. It's supposed to be in the range of 29200 and 75920 but my machines were set at 4288, 17520 and 64240 respectively. I downloaded a tool to tweak this value so that they all were set to the fast machine's value of 64240 and Voila! Three computers all running at true broadband speeds.
Now, here's the secret of how I found this out. BroadbandReports.com offers Internet Speed Tests from major providers including Speakeasy, MegaPath and LinkLine. I ran a couple of these and confirmed my suspicions that the computers were performing at different download speeds on the same high speed line. Next I ran the "Tweak Test" that suggested the Receive Window was set too low and offered FAQs and the opportunity to download a tool (DrTCP) to change it. I did that on the two slower machines. It took less than an hour all told and results are like night and day.
In a larger sense, this is the sort of thing that can happen whenever you start making upgrades. Equipment that worked just fine before either doesn't seem to benefit from the improvements or starts acting flaky. Some companies are experiencing disappointments with VoIP deployment. It's not because the technology is inherently flawed. It's just that there are all sorts of characteristics, like latency, bandwidth, QoS priorities, jitter and so on that can rear their ugly heads to ruin a perfectly good signal. You'll have better results when you know what sort of gremlins to look out for in advance. It also makes a BIG difference to have the right tests to run when you suspect all is not well.
