Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors.
There are multiple very closely related standards that describe synchronous optical networking:
SDH or Synchronous Digital Hierarchy standard developed by the International Telecommunication Union (ITU), documented in standard G.707 and its extension G.708
SONET or Synchronous Optical Networking standard as defined by GR-253-CORE from Telcordia
Both SDH and SONET are widely used today: SONET in the U.S. and Canada and SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide market dictate that SONET now be considered the variation.
Synchronous networking differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between elements in the network.
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/SDH (POS) networking. As such, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves, but rather as generic and all-purpose transport containers for moving both voice and data. The basic format of an SDH signal allows it to carry many different services in its Virtual Container (VC) because it is bandwidth-flexible.
