Features of SDH Optical Transmission Equipment

The SDH transmission system has a unified frame structure internationally, with standard digital transmission rates and standard optical interfaces, enabling interconnection of network management systems. It therefore offers excellent horizontal compatibility, is fully compatible with existing PDH, and can accommodate various new service signals, forming a globally unified digital transmission standard that improves network reliability.

In the SDH access system, code streams of different levels are arranged in a highly regular manner within the payload area of the frame structure, and the payload is synchronized with the network. Using software, high-speed signals can be directly dropped and inserted into low-speed tributary signals in a single step, achieving one-step multiplexing. This overcomes the PDH plesiochronous multiplexing process of demultiplexing all high-speed signals level by level before regeneration and remultiplexing. Since DXC is greatly simplified and back-to-back interface multiplexing equipment is reduced, the transparency of service transmission over the network is improved.

Thanks to the adoption of advanced Add-Drop Multiplexers (ADM) and Digital Cross-Connects (DXC), the network's self-healing and reconfiguration capabilities are extremely powerful, resulting in high survivability. Because 5% overhead bits are allocated for signals in the SDH frame structure, its network management functions are particularly powerful and can be unified into a network management system, playing a positive role in network automation, intelligence, channel utilization, and reducing network maintenance costs while enhancing survivability.

Since SDH supports multiple network topologies, the networks it forms are highly flexible. It enhances network monitoring, operation management, and automatic configuration functions, optimizes network performance, and also makes network operation flexible, secure, and reliable, providing comprehensive and diverse network functions.

SDH possesses both transmission and switching capabilities. Its series of equipment can be freely combined through functional blocks to implement networks at different levels and with various topologies, offering great flexibility.

SDH is not exclusive to any particular transmission medium; it can be used over twisted pair and coaxial cable. However, fiber optics are required for SDH to transmit high data rates. This characteristic indicates that SDH is suitable for both trunk channels and branch channels. For example, China's national and provincial cable television trunk networks adopt SDH, and it is also convenient for compatibility with Hybrid Fiber-Coax (HFC) networks.

From the perspective of the OSI model, SDH belongs to the physical layer at the lowest level and does not impose strict restrictions on its upper layers, making it convenient to adopt various network technologies over SDH, supporting ATM or IP transmission.

SDH is strictly synchronous, ensuring the entire network is stable and reliable with few bit errors, and facilitating multiplexing and adjustment.

Standard open optical interfaces enable horizontal compatibility over basic optical cable sections, reducing networking costs.