Sub-carriers mask. A bitmap of 512 bits that allows masking up to 512 downstream sub-carriers, depending on NSCds. If bit i (0 <= i < NSCds) is set to '1', the respective downstream sub-carrier i is masked, and if set to '0', the respective sub-carrier is unmasked. Note that there should always be unmasked sub-carriers (i.e., the object cannot be all 1's). Also note that if NSCds < 512, all bits i (NSCds i <= 512) should be set to '1'.