SDH
Synchronous Digital Hierarchy (SDH) standardized by ITU-T has
several improvements compared to the PDH:
- Direct pointer based access to data without need of layers of
hierarchical branching equipment
- Centralized remote control of network elements
- Increased use of the physical network
- Shorter delivery time for leased lines
SDH is a standardized multiplexing hierarchy for both
plesiochronous and synchronous purposes.
The multiplexing levels in the SDH can be divided into two groups:
multiplexing levels (virtual containers, VC) and line signal levels
(synchronous transport modules, STM).
STM-1 with transmission rate 155 Mbps is basic rate for ATM.