Tool to test file download times from our server using various speeds

[pmeenan]

The download times for files that large are a straight math calculation - connections usually do very well once they warm up and assuming there is suitable bandwidth in the middle and on the server they can approach the theoretical speed:

(<file size in bytes> * 8) / <Bandwidth in megabits> = # of seconds to download

The *8 is just to go from bytes to bits.

That said, you have to watch out a bit for some TCP-level window issues when you get into high latency (50-100ms+) high bandwidth situations (10Mbps+). SCP or FTP in particular are notorious for using small buffers and they tend to get limited by the TCP window size and not the actual bandwidth.

For a full answer you need to know the OS on both sides, the software being used for the transfer, the bandwidth and latency and then use the Bandwidth Delay Product calculation to get accurate results (there are online calculators that will take the inputs and give you a result).

For example, a 10Mbps connection with 100ms latency and 32KB TCP window will actually be limited to 2.5Mbps for the transfer.