Slow network speed over WAN

When copying files between two Windows 2003 SP2 Servers across our WAN, I
can't get more than about 3Mbps for a single copy operation. When running
multiple copies simultaneously, each one gets about 3Mbps up to the maximum
bandwidth of the connection (on both a 1GB and a 100MB connection).

My servers are in a data center on each coast of the US (one on the West
Coast, the other on the East Coast), with about 100ms of latency. I have
observed the performance limits with both a drag-and-drop from Windows
Explorer and from Robocopy. When using iPerf with the default settings I see
the same performance, but when running multiple simultaneous tests or
increasing the TCP Window size I can get nearly my full speed.

When I attempt to make any changes to the registry (Tcp1323Opts=3 and
GlobalMaxTcpWindowSize={various values tried} in
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Tcpip\Parameters) I get
no performance increase.

I've also tried adding DefaultSendWindow to
[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Servic es\AFD\Parameters] with no
luck.


David