SDH possesses the unified network node interface (NNI) and standardized information class structure in the world. SDH can be compatible with PDH. It makes all the transmission network systems can communicate with each other on the level of STM-N.

• SDH具有全球统一的网络节点接口,标准化的信息等级结构,能够兼容PDH准同步数字系列,使得全世界传输系统能够在STM-N级别上实现互联互通。