Just as a multiplexing structure is defined for the plesiochronous hierarchy PDH by ITU recommendations, so too is a multiplexing structure defined for the SDH hierarchy, also by these recommendations. This structure defines the standard transmission rates for synchronous signals and the way in which tributary signals from lower levels are transported.

SDH multiplexing   Multiplexing map   Containers (C-n): the transport interface   Path overhead (POH)   Virtual containers (VC-n): the unit of exchange   Pointers and payload shift   The STM-1 frame   STM-N   Forming STM-N frames   Concatenation of AU-4   Transporting low rate tributaries   Containers: C-3, C-2, C-12 and C-11   Virtual containers: VC-3, VC-2, VC-12 and VC-11   Tributary units: TU-12, TU-3 and TU-2   TU multiframes   Tributary unit groups: TUG-3 and TUG-2