Denis, I missed the reference to 1DsMKII. That camera is not capable of tethering with USB. I remember reading about people having trouble with long firewire cables, but it depended on the computer. Laptops tended to have less powerful firewire ports, firewire specifications limit cable length to 4.5 metres (14.8 ft). Using a hub might allow you to extend it but I'm not sure.
Dave