I opened the "Control Panel" - "System" - "Device Manager".
Then under "Network Adapters" i opened the properties for the "Intel Centrino Advanced-N 6205".
In the register card "Advanced" i set the setting "preferred band" to "3. prefer 5,2-GHz band".
It then immediately re-initialized the network connection and connected to the 5,2 GHz WLAN.
For my card (Austria / Europe) I had to set my router to "N Only". The setting "A/N Mixed" did NOT work.
Security is AES - higher data rates only work with AES and not with TKIP.
Now the data throuput is optimal and since the 5,2 GHz band is completely free in my area I don't have to share it with other users.