who can help me to slove this confusion problem!!

I use KLowerDevice to open a com port(which is a comport created by a USB modem),it created successfully,but when I Send DeviceIOControl command(such as IRP_MJ_SET_BAUDRATE) to it,it returns unsuccessful.how to make it return successfully? I suppose that the com port device is not start,How can I start it?Do I need to Use KPnpLowerDevice to open this port?The bonus for high hand is 20 cent!!!