同义词
1)Plesiochronous Digital Hierarchy(PDH),准同步数字架构2)Synchronous Digital Hierarchy(SDH),同步数字架构3)hierarchy,synchronous digital (SDH),同步数字层阶架构4)synchronous optical network/synchronous digital hierarchy (SONET/SDH),同步光纤网络/同步数字架构5)PDH,准同步数字体系6)PDH(Plesiochronous digital hierarchy),准数字同步系列
