Synchronous Digital Hierarchy

SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission
on optical media. It is the international equivalent of Synchronous Optical Network. Both technologies
provide faster and less expensive network interconnection than traditional 
PDH (PlesiochronousDigitalHierarchy) equipment.

In digital telephone transmission, "synchronous" means the bits from one call are carried within one  
transmission frame. "Plesiochronous" means "almost (but not) synchronous," or a call that must be extracted 
from more than one transmission frame. 
SDH uses the following SynchronousTransportModules (STM) and rates:
STM-1 (155 megabits per second), 
STM-4 (622 Mbps), 
STM-16 (2.5 gigabits per second), and 
STM-64 (10 Gbps).