tributary unit (TU)
In the Synchronous Digital Hierarchy ( SDH), feeder signals can originate from different levels of the Plesiochronous Digital Hierarchy ( PDH). The individual feeder signals are first packed into a container nominally synchronous to the STM-1 frame according to an ordering principle.
A Tributary Unit (TU) is a virtual container ( VC) extended with the pointer pointing to it. Tributary Units are grouped into Tributary Units Group (TUG) according to a specific scheme and form an Administrative Unit ( AU).