What are pdh and sdh hierarchies?

PDH (Plesiochronous Digital Hierarchy) and SDH (Synchronous Digital Hierarchy) are two different systems for transmitting digital signals over telecommunication networks. In PDH, signals are transmitted at fixed multiples of the basic data rate, resulting in slight timing variations between signals. In contrast, SDH uses a standardized synchronous timing system, allowing for more efficient and reliable data transmission with fixed frame structures.