The object provides Qln for the VTU-C 512 subcarriers.
Each subcarrier takes two octets to store the value.
The QLN(f) is represented as ( -23-n(i)/2), with n(i) in the range 0 to 254.
A special value n(i)=255 indicates that no measurement could be done for the
sub-carrier group. Each value in this array is 8 bits wide.