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Synchronous digital hierarchy (SDH)
SDH is a complete standard digital signal hierarchy that provides synchronous digital transmission, multiplexing, and cross-connection.
Technical background of SDH emergence
Fiber communications with inexpensive bandwidth features have become the dominative transmission method for communications networks.
The traditional PDH transmission system no longer adapts to development of modern communications networks.
Limitations of PDH
Interface standards
Standards for electrical interfaces are regional not worldwide. There are European series, North American series, and Japanese series definition of signal rate levels. They use different frame structures and multiplexing modes, which hinders interconnection.
Worldwide standards for optical interfaces are absent. Vendors develop their own line modulation formats for devices to monitor transmission performances on the optical lines. The modulation format and rate of optical interfaces of different vendors at the same rate are different. As a result, devices of different vendors cannot be horizontally compatible.
Multiplexing mode: Low-rate signals are multiplexed into or demultiplexed from high-rate signals level by level, which damages the signals and degrades transmission performance.
OAM: PDH signal frames do not have many overheads for better OAM functions such as layered management, performance monitoring, real-time service scheduling, bandwidth control, and alarm analysis and locating.
Lack of a unified NMS interface: It is difficult to form a unified telecom management network.
