SONET defines interface standards at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (OC-1) to 9.95 Gbps (OC-192). Prior rate standards used by different countries specified rates that were not compatible for multiplexing. With the implementation of SONET, communication carriers throughout the world can interconnect their existing digital carrier and fiber optic systems.

The international equivalent of SONET, standardized by the ITU, is called SDH.

SDH/SONET Tutorial
A short tutorial on SDH and SONET from Queen's University of Belfast.

SONET presentation notes
This student page provides a good overview of SONET, including historical notes, terms, protocols, frames and how it works.