SDH (Synchronous Digital Hierarchy)
It is an international standard for high speed telecommunication over optical / electrical networks which can transport digital signals in variable capacities. It is synchronous systems which intend to provide a more flexible, yet simple network infrastructure.
SDH (and its American variant- SONET) emerged from standard bodies somewhere around 1990.
These two standards create a revolution in the communication networks based on optical fibers, in their cost and performance.
The SDH gives the ability to create topologies with protection for the data transferred and enhanced management capabilities.