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- The Multiplexing Principles of SDH
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Multiplexers can be used to connect PBX, Hot line and other devices of network from central site to user site through fiber optical cable.
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Multiplexer Knowledge
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The Multiplexing Principles of SDH
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Mapping - A process used when tributaries are adapted into Virtual Containers (VCs) by adding justification bits and Path Overhead (POH) information.
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Aligning - This process takes place when a pointer is included in a Tributary Unit (TU) or an Administrative Unit (AU), to allow the first byte of the Virtual Container to be located.
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Multiplexing - This process is used when multiple lower-order path layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into a Multiplex Section.
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Stuffing - SDH has the ability to handle various input tributary rates from PDH. As the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SDH frame to provide enough space for all these various tributary rates. Therefore, at certain points in the multiplexing hierarchy, this space capacity is filled with "fixed stuffing" bits that carry no information, but are required to fill up the particular frame.
The Figure1 at the below illustrates the ITU-T SDH multiplexing structure. The notations in the boxes, such as C-1, VC-3, and AU-4, are explained in the Figure2.
ITU-T SDH multiplexing structure
Notations Illustration
At the lowest level, containers (C) are input to virtual containers (VC). The purpose of this function is to create a uniform VC payload by using bit-stuffing to bring all inputs to a common bit-rate ready for synchronous multiplexing. Various containers (ranging from VC-11 at 1.728 Mbit/s to VC-4 at 139.264 Mbit/s) are covered by the SDH hierarchy. Next, VCs are aligned into tributary units (TUs), where pointer processing operations are implemented.
These initial functions allow the payload to be multiplexed into TU groups (TUGs). As the figure illustrates, the xN label indicates the multiplexing integer used to multiplex the TUs to the TUGs. The next step is the multiplexing of the TUGs to higher level VCs, and TUG-2 and TUG-3 are multiplexed into VC-3 (ANSI mappings) and VC-4. These VCs are multiplexed with fixed byte-stuffing to form administration units (AUs) which are finally multiplexed into the AU group (AUG). This payload then is multiplexed into the Synchronous Transport Module (STM).