Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) are now applied worldwide both in carrier backbone networks, by metropolitan service providers and in large scale WANs by end-user organizations.

• 同步数字体系(SDH)和同步光纤网络(SONET)在世界范围广泛应用于城域主干网,长途通信网和广域网。