By ""only one can connect to RDP at a time", do you mean that their RDP connections are just very slow when two connect, or is one of them being refused an RDP connection?
What are they RDPing to at the office? Their desktops? A server?
Here's one article giving a recommended minimum bandwidth for RDP:
"Minimum bandwidth for a useable RDP Session = 26.4Kbps
800x600 @16 bit color - 26.4Kbps per session
1024x768 @256 colors - 24.6Kbps per session"
The useability of a VPN/RDP connection depends on the slowest side of the connection. Each side has an upload and a download speed and the slowest of those four speeds will be the limiting factor.
The Ping time of the connection can also greatly affect the useability. If you click on a button and nothing happens for several seconds, it can be pretty annoying.
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Last edited by RebateMonger; 10-14-2010 at 10:55 AM.