Synchronous Optical Networking (
SONET) and
Synchronous Digital Hierarchy (
SDH) are standardized protocols that transfer multiple
digital bit streams synchronously over
optical fiber using
lasers or highly
coherent light from
light-emitting diodes (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the
plesiochronous digital hierarchy (PDH) system for transporting large amounts of
telephone calls and
data traffic over the same fiber without synchronization problems. SONET generic criteria are detailed in
Telcordia Technologies Generic Requirements document GR-253-CORE. Generic criteria applicable to SONET and other transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found in Telcordia GR-499-CORE.