USB external modem (CDC) & selective suspend support

Hello,

I'm looking for the requirements necessary to let the user enable the check
box "Allow the computer to turn off the device to save power" inside the
"PowerManagement" Tab of the device driver GUI. The modem itself is fully USB
CDC ACM compatible and uses usbser.sys as lower filter driver. Microsoft
points out that usbser.sys supports Selective Suspend there. The modem itself
uses no own device driver. It refers to the MS one (usbser.sys). All works
fine but it is not possible to enable the above named check box. Other
devices and their drivers used on the same USB hub port (IRDA/USB bridge
etc.) as the modem is allow the enabling of this feature via the GUI.

Are there any inf file or other properties/requirements necessary to enable
this selective suspend feature for such modems?
PS: RemoteWakeup is working too.

Thank you in advance,
Oliver Lindner