is a standardized technology used in telecommunications networks for transmitting large amounts of digital data over long distances. It provides a flexible and efficient way to multiplex and transport voice, data, and video signals in a synchronous manner. Here’s a detailed explanation of the SDH system:

1. Background: Before SDH, traditional telecommunications networks relied on PDH (Plesiochronous Digital Hierarchy) systems. PDH had limitations in terms of flexibility, scalability, and interoperability. SDH was developed to overcome these limitations and provide a more advanced and standardized solution.
2. Synchronization: SDH is based on synchronous operation, where all network elements are synchronized with a common clock source. This ensures accurate data transmission and allows for the multiplexing of different signals.
3. Multiplexing: SDH uses a hierarchical multiplexing structure to combine multiple lower-rate signals into higher-rate containers. The basic unit of multiplexing is the STM-1 (Synchronous Transport Module level 1), operating at a rate of 155.52 Mbps. Higher-rate containers, such as STM-4, STM-16, and STM-64, are formed by multiplexing multiple STM-1 signals.
4. Frames and Overhead: SDH frames consist of a payload area and overhead. The payload area carries user data, while the overhead contains information necessary for network management, error detection, and synchronization. The overhead includes section overhead, line overhead, and path overhead, each serving specific functions.
5. Virtual Containers (VCs): SDH allows for the flexible allocation of bandwidth through the use of Virtual Containers. VCs can be created within the payload area of an SDH frame to carry different types of traffic, such as voice, data, or video. VCs provide a means of logical separation and efficient utilization of the available bandwidth.
6. Path and Connection Management: SDH incorporates path and connection management functions to establish, monitor, and tear down connections within the network. This enables the provisioning of end-to-end connections between different network nodes, ensuring efficient routing and resource allocation.
7. Fault Detection and Protection: SDH provides robust fault detection and protection mechanisms to ensure high network reliability. It utilizes various error detection codes, such as CRC (Cyclic Redundancy Check), to identify errors and correct them. Additionally, SDH supports various protection schemes, including 1+1, 1:1, and 1:N, to ensure network resilience in the event of link or equipment failures.
8. Network Management: SDH incorporates comprehensive network management capabilities to monitor and control the network elements and ensure optimal performance. Network management systems allow operators to monitor alarms, perform diagnostics, provision new connections, and carry out maintenance activities.
9. International Standards: SDH is based on international standards defined by the ITU-T (International Telecommunication Union – Telecommunication Standardization Sector). The primary SDH standard is ITU-T Recommendation G.707, which provides detailed specifications for the SDH transport system.
10. Benefits of SDH: SDH offers numerous advantages, including high scalability, flexibility, and compatibility. It enables the efficient transport of different types of traffic over long distances, simplifies network management, and supports seamless integration with other network technologies like SONET (Synchronous Optical Networking).

In summary, SDH is a standardized technology used for high-capacity and efficient transmission of digital signals in telecommunications networks. It provides synchronization, multiplexing, fault detection, protection, and network management capabilities. SDH has revolutionized the telecommunications industry by offering a reliable and scalable solution for high-speed data transmission over long distances.