Redundancy One of the virtues of a ring topology is that it allows for a certain amount of redundancy. Typically, the network will consist of at least two independent parallel fibre rings. The network can therefore continue to operate if there is a break in one of the ring sections. Where a network consists of more than two ring fibres, operators can choose to have just a single redundant line to which traffic can be switched over from any of the others in the event of a failure, or have a dedicated backup for each and every line. The latter approach provides greater protection, but is also much more expensive.
Rings can also be bi-directional.
This kind of arrangement is the most flexible, as data can be routed to follow the shortest path between two nodes. This means data does not have to traverse the entire network just to reach a destination node that is immediately upstream from the originating node. With SDH, all the network elements are synchronised to a central clock. This is generated from a high-precision primary reference clock unit. The clock can be propagated along the same cabling used for the SDH signals.