The maximum throughput of a TCP session is expressed as:
TCP-Window-Size-in-bits / Latency-in-seconds = Bits-per-second-throughput
Without window scaling, the maximum window size is 64KB. Given a 15ms RTT and default 64KB window, the maximum TCP throughput is:
Window size = 64KB = 65536 Bytes. 65536 * 8 = 524288 bits
15ms RTT = 0.015 seconds
524288 / 0.015 = 34.95 Mbps
So the maximum throughput on a single TCP session over WAN link with 15ms RTT will be less than 34.95 Mbps
The throughput drops significantly the higher the RTT gets.
Some more information on TCP performance at
http://en.wikipedia.org/wiki/TCP_tuningYou run iperf on both ends on the LAN, so on a Windows or Linux box. One side runs as the client while the other runs as the server. iperf is just a command line tool, and is available for Windows, Linux and OSX. Using iperf you can manually set the TCP windows size as well as number of threads to run, or you can use it to send UDP stream at a given rate. This will help to see what the theoretical maximum throughput of your WAN link is.
Edit: to emphasize window scaling, which most stacks support.