Given the differences in propagation lengths between the two directions, unidirectional rings are most frequently found in tightly-constrained geographical applications. If the default route takes a longer path, it can introduce undesirable timing characteristics as the distances and number of elements grow. Bidirectional rings Like unidirectional rings, bidirectional rings are arranged as closed-loop network elements connected by two or four fibers between each neighboring element. The major difference between the two implementations is the selection of the route taken by the working traffic. In bidirectional rings, the work traffic is selected from the shorter of the two routes.
Bidirectional rings provide the advantage of minimizing network delays.
Because of the smaller geographical applications employed by unidirectional rings, the four-fiber case is reserved for the case of the bidirectional rings. Only half of the fiber's capacity can be consumed for traffic. When the fiber count is increased to four fibers, the full capacity of each link can be employed. When a fault occurs, the receiving station immediately redirects its attention to the better of the two inputs. Line switching has been the preferred method of protection because it can be handled by any line-terminating equipment, and occurs closer to the location of the fault.