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

• SDH具有全世界统一的网络节点接口,标准化的信息等级结构,能够兼容现有的PDH准同步数字系列传输系统,使得全世界传输系统能够在STM-N级别上进行互联互通。