Editing Synchronous optical networking (section)

==Difference from PDH==
SDH differs from [[Plesiochronous Digital Hierarchy]] (PDH) in that the exact rates that are used to transport the data on SONET/SDH are tightly [[synchronization (computer science)|synchronized]] across the entire network, using [[atomic clock]]s. This [[synchronization in telecommunications|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 [[encapsulation (computer science)|encapsulate]] earlier digital transmission standards, such as the PDH standard, or they can be used to directly support either Asynchronous Transfer Mode (ATM) or so-called [[packet over SONET/SDH]] (POS) networking. Therefore, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves; they are generic, all-purpose transport containers for moving both voice and data. The basic format of a SONET/SDH signal allows it to carry many different services in its virtual container (VC), because it is bandwidth-flexible.