A SONET/SDH topology that employs either two or four fibers.The single-ring configuration comprises two fibers, one of which is active and the other of which is held in reserve.Traffic moves in one direction across the active fiber. In the event of a network failure, such as a node failure or a cable cut, the backup ring is activated to enable transmission in the reverse direction, as well. A fourfiber configuration duplicates this approach in a second ring. Line-switched rings up to 1,200 kilometers in route distance offer standard restoral intervals of 50 milliseconds or less. See also optical fiber, path-switched ring, SDH, SONET, and topology.