document.write('SDH(同步数字pdQSynchronousDigitalHierarchySDH也是一U数字传输制式。它其实是光U传输技术和|络技术结合的产物。最早提出SDH概念的是国贝尔通信研究所Q当时称为光同步|络(SONET)1988q_国际甉|电话咨询委员?CCITT)接受了SONET的概念,重新');
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