SONET and SDH
*** also see the IEC's SONET Tutorial and the great Kalyanaraman SONET Presentation
SONET (Synchronous NETwork) and SDH (Synchronous Digital Hierarchy) are the two worldwide cousins of high-speed fiber data transmissions. SONET is used in North America, while SDH is used in the ROW (Rest Of World). We will explain the differences in detail - however, they are very similar.
The original T-carrier system ran on copper wires, and was known as the the Plesiochronous Digital Hierarchy (PDH).
SONET 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 Telecom Synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element 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 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.