Concepts
Multiplexing units
SDH contains basic multiplexing units, including container (C-n), virtual container (VC-n), tributary unit (TU-n), tributary unit group (TUG-n), administrative unit (AU-n), and administrative unit group (AUG-n). Here, n stands for the number of the unit level.
Container
It is used to carry service signals that are transmitted at different rates. G.709 defines specifications for five types of standard containers: C-11, C-12, C-2, C-3, and C-4.
VC
It is used to support connections between channel layers of the SDH and is an information terminal of SDH channels. VCs are classified into lower-order VCs and higher-order VCs. VC-3 in AU-3 and VC-4 are higher-order VCs.
TU and TUG
TU provides adaptation between lower-order and higher-order path layers. One TU or a set of multiple TUs, occupying a fixed position in the payload of the higher-order VC, is called a TUG.
AU and AUG
AU provides adaptation between higher-order channel layer and multiplex section layer. One AU and a set of multiple AUs, occupying a fixed position in the payload of STM-N, is called an AUG.