Does Compressing Files Improve Speed on Local Network Transfers?

Ciaran makes a good point in his comment; you have to know what the test is measuring, and this is not clear with a proprietary bandwidth tester.

Downloading a large file will give you a good idea of your TCP throughput, but this measurement will be very sensitive to your latency (Ping time) to the server, and may be affected by your TCP configuration settings (critically your TCP receive window).

Tools like IPerf and ixChariot are used by networking professionals , but these are not really suitable for casual use (IPerf does not work easily behind NAT for example). To gauge the ultimate capacity of your pipe you could flood your connection with a uni-directional stream of UDP datagrams using one of the above tools.

Online bandwidth tools use some tricks to try and push up the throughput beyond the performance of a single TCP connection. I find that speedtest.net generally gives you an average download speed ~30-40% higher than what you would measure by timing the download of a large file. I have confirmed this by timing downloads of the files used by speedtest itself.

Using Wireshark, I made the following observations;

1. The speedtest client downloads two files simaultaneously while measuring bandwidth - this gives a slightly higher aggregate throughput than a single file
2. The speedtest speedometer reports a high percentile (90-95%) of the per-second instantaneous throughputs - not the long term average

The speedtest GUI suggests that your download of an mp3 would proceed at the measured rate. This is slightly misleading as the measured rate is more representative of the peak throughput achievable on your link.

Edit: Sorry, I missed the Vista tag.

If you click the Performance tab of the Task Manager and choose Resource Monitor you then get more comprehensive stats including what you are asking for.

But Windows does actually have a much more advanced performance monitor that has been built in for a while and is extremely flexible.

• Choose Start->Run... and type perfmon, then press Enter
• In the Management Console window that appears, make sure that you select Performance Monitor in the left-hand pane
• Click the green + button in the toolbar
• In the Add Counters dialog find the Network Interface section
• Choose <all instances>
• Click the Add >> button
• Click OK
• You will get a graphs with loads of lines on it, thankfully you can delete all the stuff you don't want by shift+clicking the unwanted rows and pressing delete. It's made much more complicated by the fact that you may have virtual network adapters as well as physical ones. You'll have to work out which ones you actually need to monitor.