Figure: SDH Hierarchy at STM-4 shows the SDH hierarchy at STM-4 (OC-12).
An SDH multiplexing structure allows several combinations of signal transportation. For example, at the STM-1 (OC-3) level:
one STM-1 payload supports one administrative unit group (AUG)
each AUG can contain either three administrative units (AU-3s) or a single AU-4
For example, the hierarchical possibilities for a single AU-4 are:
each AU-4 transports data via a virtual container (VC-4)
a VC-4 consists of three tributary unit groups (TUG-3s), where either:
a single tributary unit (TU-3) can be multiplexed via a TUG-3, containing a VC-3 plus the VC-3 path overhead (POH) and TU-3 pointer
seven TUG-2s can be multiplexed via a TUG-3, where each TUG-2 can contain one TU-2, three TU-12s, or four TU-11s
the AU-4 structure addresses the data as (K, L, M), where K is the TUG-3 number, L is the TUG-2 number, and M is the TU-11/TU-12 number