Introduction to SDH Multiplexing Structure

                                                         Figure 1 SDH G.709 multiplexing mapping structure

Basic Multiplexing Units
Standard Container C: An information structure used to carry service signals of various rates, mainly performing adaptation functions (such as rate adjustment) so that commonly used plesiochronous digital hierarchy signals can enter a limited number of standard containers.
C-11, C-12, C-2, C-3, and C-4 serve as the information payload of virtual containers.
Virtual Container (VC): An information structure that supports SDH path layer connections, composed of C + POH.
VC-n = C-n + VC-n POH
VC-1 and VC-2 are low-order, while VC-3 and VC-4 are high-order, serving as the information payload of TU and AU respectively.
Tributary Unit and Tributary Unit Group (TU and TUG)
An information structure that provides adaptation between the low-order path layer and the high-order path layer, i.e., TU-n = VC-n + TU-n PTR. A collection of one or more TUs is called a TUG.
Administrative Unit and Administrative Unit Group (AU and AUG)
An information structure that provides adaptation between the high-order path layer and the multiplex section layer, i.e., AU-n = VC-n + AU-n PTR (n=3,4). A collection of one or more AUs is called an AUG.
Synchronous Transport Module (STM-N)
STM-N = AUG + SOH

Concepts of Mapping, Alignment, and Multiplexing

Mapping: The process of adapting tributary signals into virtual containers at the SDH boundary.
Alignment: The process of incorporating frame offset information into tributary units or administrative units.
Multiplexing: The process of adapting multiple low-order path layer signals into high-order paths, or adapting multiple high-order path layer signals into the multiplex section layer.

Explanation of Related Abbreviations

POH: Path OverHead — path overhead, bytes used for path performance monitoring, management, and control;

PTR: Pointer — pointer, bytes used to continuously monitor the position of low-speed signals;

SOH: Section OverHead — section overhead, bytes that provide network operation, administration, and maintenance.