SONET Rings

SONET can run as a straight point-to-point line between sites, or in a ring topology. When fiber in a point-to-point arrangement is cut, service is lost. However, the higher speeds attainable on fiber make reliability critical. When a medium such as copper carries a conversation from one telephone subscriber, a copper cut only impacts one customer. Fiber cuts in networks can put hundreds of locations out of service. For this reason, the majority of telephone companies deploy bidirectional ring topology.

In the bidirectional SONET/SDH ring, one set of fiber strands is used for sending and receiving; the other is the protect ring (spare ring). If one set of fiber strands is broken, the spare (protect) ring reroutes traffic in the other direction. In addition, if one multiplexer on one set of fibers fails, the backup multiplexer on the fiber running in the other direction automatically takes over.