SONET and SDH

Synchronous Optical Network (SONET) adopts optical transmission. It is a synchronous transmission system defined by the ANSI and is an international standard transmission protocol.

ITU-T Synchronous Digital Hierarchy (SDH) uses a SONET rate subset. SDH adopts synchronous multiplexing and a flexible mapping structure. It can add or drop low-speed tributary signals to or from SDH signals without a large number of multiplexing/demultiplexing devices. This reduces signal attenuation and decreases device investments.