NTT has been introducing the SDH transmission system conforming to the
SDH based interface since 1989. As a leader in this area, NTT aims at
the construction of a simple yet easy-to-operate transmission line ne
twork. In this paper, the current status of the SDH transmission syste
m, which forms the basis of the NTT transmission line network, is intr
oduced together with future trends in its development.