the objective minimum bit rates for ldsl systems are 192 kb s downstream and 96 kb s upstream in each of the 80 test scenarios we find a significant performance advantage for the selectable mask system in a number of test cases the u201c single mask system u201d uses a single upstream and a single downstream mask based on 0j 074 and are respectively referred to as u 2 and d 2 herein this is a non overlapped psd scenario where the upstream channel ends at tone 23 and the downstream begins at tone 33 the u201c mask selectable system u201d uses two upstream masks u 1 and u 2 and two downstream masks d 1 and d 2 upstream mask u 1 ends at tone 13 and the downstream mask d 1 is a shaped overlap mask derived from spectrum management class 5 in t1 417 the u201c mask selectable system u201d selects the best upstream and downstream mask combination for each test case according to some criteria optimality criterion is left to the discretion of the operator who may want to force a mask set up according to the operator 39 s field knowledge or give priority to upstream minimum rate or downstream minimum rate up to certain margin etc this degree of freedom is a keystone of the selectable mask system in the same spirit adsl overlap mode is left today to the discretion of the operator neither g 992 1 nor g 992 3 define criteria to.