= 3, 4), an assembly of tributary unit groups (TUG-2s), or an assembly of TU-3s, together with virtual container POH appropriate to that level. Administrative Unit-n (AU-n). An administrative unit is the information structure that provides adaptation between the higher-order path layer and the multiplex section. It consists of an information payload (the higher-order virtual container) and an adminis- trative unit pointer, which indicates the offset of the payload frame start relative to the multiplex section frame start. Two administrative units are defined. The AU-4 consists of a VC-4 plus an administrative unit pointer, which indicates the phase alignment of the VC-4 with respect to the STM-N frame. The AU-3 consists of a VC-3 plus an administrative unit pointer, which indicates the phase alignment of the VC-3 with respect to the STM-N frame. In each case the administrative unit pointer location is fixed with respect to the STM-N frame (Ref. 6). One or more administrative units occupying fixed, defined positions in a STM payload is termed an administrative unit group (AUG). An AUG consists of a homogeneous assembly of AU-3s or an AU-4. Tributary Unit-n (TU-n). A tributary unit is an information structure that provides adap- tation between the lower-order path layer and the higher-order path layer. It consists of ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS an information payload (the lower-order virtual container) and a tributary unit pointer, which indicates the offset of the payload frame start relative to the higher-order virtual container frame start. The TU-n (n = 1, 2, 3) consists of a VC-n together with a trib- utary unit pointer. One or more tributary units occupying fixed, defined positions in a higher-order VC-n payload is termed a tributary unit group (TUG). TUGs are defined in such a way that mixed-capacity payloads made up of different-size tributary units can be constructed to increase flexibility of the transport network. A TUG-2 consists of a homogeneous assembly of identical TU-1s or a TU-2. A