A multiplex section refers to a maintenance segment (including two regenerator section terminals and the fiber between them) within the Synchronous Digital Hierarchy (SDH) standard. It represents the path between two multiplex section termination functions, which is essential for managing high-speed data transmission across long distances. The multiplex section plays a critical role in network protection, allowing for rapid rerouting of traffic in case of failures.
The implementation of the multiplex section function involves sub-layers that manage protection mechanisms, making it relatively complex. Network protection is one of the most important features of SDH, requiring careful design to ensure reliability and efficiency. From an atomic model perspective, the multiplex section protection function divides the path termination into smaller, manageable sub-functions. This allows for more precise control and monitoring of the signal path.
Currently, the four-fiber bidirectional multiplex section switching function is widely used in trunk networks. Its basic functions are still primarily focused on terminal and adaptation functions, while the protection function is mainly realized through connection management. This structure ensures robustness and flexibility in handling various network scenarios.
SDH employs time-division multiplexing to combine lower-speed signals into higher-speed ones. For example, a single bit per second can be transformed into multiple bits per second by combining them in a structured way. Common SDH levels include STM-1, STM-4, STM-16, STM-64, and STM-128. These levels allow for the efficient transport of data over optical networks.
In China, the SDH system uses the 2Mbit/s PDH signal as the payload, with the AU-4 multiplexing route being the standard. This structure enables the integration of various low-speed signals, such as 2M, 1.5M, 34M, and 45M, into the basic STM-1 signal. The mapping process involves adapting these signals into containers like C4, C3, and C12, followed by the addition of overhead bytes to form virtual containers (VCs).
For instance, a 140Mbit/s signal is adapted into a C4 container, which is then mapped into a VC4. The VC4 is further encapsulated into an AU-4, which is part of the STM-1 frame. This hierarchical structure ensures that different signal rates can be efficiently transported and managed within the same network.
Similarly, a 34Mbit/s signal is mapped into a VC3, which is then grouped into a TUG3. This information is eventually combined into a VC4, forming part of the STM-N signal. For 2Mbit/s signals, they are first mapped into a C12 container, then into a TU12, and finally aggregated into a VC4.
The multiplexing structure for 2Mbit/s signals follows a 3-7-3 pattern: three TU12s make up a TUG2, seven TUG2s form a TUG3, and three TUG3s create a VC4. This arrangement ensures that all 63 2Mbit/s channels can be efficiently packed into a single STM-1 frame.
Understanding how SDH reuses bandwidth is crucial for optimizing network performance. By carefully managing the mapping, alignment, and multiplexing processes, SDH ensures that diverse data streams can coexist on the same physical infrastructure, maximizing utilization and minimizing latency.
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