SONET/SDH

Three levels of Synchronous Optical Networking / Synchronous Digital Hierarchy (SONET/SDH) ring hierarchy are presented and SONET/SDH were originally designed to transport circuit mode communications (eg, T1, T3) from a variety of different sources.

Level1
The edge rings span distances of a few kilometers to a few tens of kilometers and are composed of add/drop multiplexers (ADMs) that electronically aggregate traffic onto the fiber. Connectivity on these rings is mostly hubbed to a central office location (CO) denoted by a combination of ADMs and digital cross-connect switches (DCSs).

Level 2
DCSs are equipment that interconnect SONET/SDH edge rings to the larger Inter-Office (IOF) rings that span possibly tens to hundreds kilometers.

Level 3
IOF rings can also be interconnected using DCSs and can form mesh-connectivity optical networks or larger SONET/SDH rings.