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File Transfer over TCP

Calculating how long a file transfer will take seems easy – divide the file size by the bandwidth and you’re done. But it turns out that FTP over TCP is very sensitive to the latency and packet loss rate of the link. Instead of using all of the bandwidth that’s available, TCP spends most of its time spinning its wheels waiting for acknowledgements to return from the receiver before sending more data. And if any packets are lost, it assumes the link is saturated and slows down. Over long distance links where the latency is high, or over wireless or satellite links where the loss rate is high, file transfers using FTP over TCP are only able to use a small fraction of the bandwidth.
Which brings up a couple of questions: first, how to know what throughput to expect, and second, how to do all of that bandwidth? The first is where we come in – use a WAN emulator like ours to simulate your link and test the system to determine the throughput. If the throughput isn’t good enough, then you need an acceleration solution, which can either be a general WAN optimization device or a file transfer application designed to handle latency and loss without slowing down.
Jscape has published a short article comparing throughput for FTP and their accelerated file transfer protocol for a variety of latency and loss conditions using a Linktropy WAN emulator for their testing. You can see the results at: With a latency of 200 ms and 0.5% packet loss, FTP took 842 sec to transfer a 100 MB file while their accelerated protocol was able to complete it in 27 sec, a difference of 30 times. Not 30% better, but 30x better.

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