The architecture used in SONET technology. SONET rings, known as "self-healing rings," use two or more transmission paths between network nodes, which are typically digital cross-connects (DCSs) or add/drop multiplexers (ADMs). If there is a break in one line, the other may still be available, providing the second is not in close proximity to the first and also damaged. For the best security against failure, when possible, different physical routes are used for the two lines. The most fault-tolerant architecture is the four-fiber bi-directional ring. See SONET
Two Fiber Unidirectional
This is the simplest SONET ring topology. All data are transmitted on the working or active path, while the standby path (protection path) lies in waiting. When a failure in the active path occurs, the two network nodes affected immediately switch to the standby line. Four fibers may used in a unidirectional system, but it is not usually done.
Two Fiber Bidirectional
In this architecture, traffic flows in both directions, but half the capacity of the line is used as a data channel and the other half for protection. In the event of failure, the alternate line takes up the slack.
Four Fiber Bidirectional
This is the most robust architecture which can withstand multiple failures providing the lines are routed in different locations. Both active and standby paths are duplicated in this topology, which is common in large carrier networks that cannot afford a breakdown.