Synchronous Digital Hierarchy (SDH) is an integrated information transmission network technology that integrates multiplexing, line transmission, and switching functions, and is operated by a unified network management system.
SDH has unified specifications for network node interfaces. The first unified elements in this specification are the interface rate levels and frame structure arrangements. In addition, SDH has also unified equipment types and equipment functions, making network composition more standardized. There are 4 types of SDH equipment (network elements):
(1) Terminal Multiplexer (TM): Used to multiplex and map various low-speed signals into line signals STM-N or perform the reverse processing, as shown in the figure.
(2) Add-Drop Multiplexer (ADM): Directly branches out or inserts low-speed signals from/to STM-N, as shown in Figure 1-4.
(3) Regenerator (REG): Realizes amplification and regeneration of STM-N signals to extend the communication distance, as shown in the figure.
(4) Digital Cross-Connect (DXC): Realizes cross-connection of signals with different rates at different ports, as shown in the figure.
