In 1985, the American National Standards Institute (ANSI) drafted an optical synchronization standard to enable interconnection of equipment through optical interfaces, naming it the Synchronous Optical Network (SONET). In 1986, the former CCITT formulated the SDH (Synchronous Digital Hierarchy) standard based on SONET, making the synchronous network applicable not only to optical fiber transmission but also to other transmission forms such as microwave and satellite.
The SDH frame structure overcomes the shortcomings of PDH. Compared with traditional PDH, SDH has the following obvious characteristics:
(1) Flexible add/drop function. SDH specifies strict mapping and multiplexing methods and adopts pointer technology. Branch signals can be flexibly added or dropped directly from line signals without the need for step-by-step multiplexing to achieve add/drop functions, reducing the number of equipment and simplifying the network structure.
(2) Strong network management capability. The SDH frame structure contains sufficient overhead bits, which not only meet the current requirements for alarms, performance monitoring, network configuration, switching, and official communication but also have room for further expansion to meet future monitoring and network management needs.
(3) Strong self-healing capability. The SDH network management system with intelligent detection and dynamic network configuration functions enables the SDH network to easily achieve self-healing. When equipment or systems fail, services can be quickly restored, improving network reliability and reducing maintenance costs.
(4) SDH has standard optical interface specifications. Equipment from different manufacturers can be interconnected on the optical path, truly realizing horizontal compatibility.
(5) SDH has compatibility. The STM-1 of SDH can not only multiplex 2 Mbit/s series PDH signals but also 1.5 Mbit/s series PDH signals, unifying the two major series in STM-1, facilitating international interconnection and a smooth transition from PDH to SDH.
In summary, the core characteristics of SDH are: synchronous multiplexing, standard optical interfaces, and strong network management capability.
Of course, SDH technology is not perfect; it also has some shortcomings:
(1) Due to the large number of overhead bits, its bandwidth utilization rate is not as high as that of PDH;
(2) It extensively adopts software technology. Once the computer system malfunctions or is maliciously attacked, the network management system cannot effectively monitor and manage the SDH network, and in severe cases, it may even cause the entire network to collapse;
(3) To be compatible with various rate signals and achieve horizontal connection, pointer adjustment technology is adopted, which generates relatively large jitter and causes certain transmission damage to signals.
