Sub-carriers mask. A bitmap of 64 bits that allows masking up to 64 downstream sub-carriers, depending on NSCds. If bit i (0 <= i < NSCus) is set to '1', the respective upstream 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 NSCus < 64, all bits i (NSCus < i <= 64) should be set to '1'.