The Synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the PDH system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors.
The more recent Synchronous Digital Hierarchy (SDH) standard developed by ITU (G.707 and its extension G.708) is built on experience in the development of SONET. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world.
SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.
SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized to network based clocks. Thus an entire network can operate synchronously, though the presence of different timing sources allow for different circuits within a SONET signal to be timed off of different clocks (through the use of pointers and buffers.) SDH was made possible by the existence of atomic clocks.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET networking. As such, it is inaccurate to think of SONET as a communications protocol in and of itself, but rather as a generic and all-purpose transport container for moving both voice and data.