Administrative Units (AU)

The Administrative Unit in the STM Frame

In SDH and SONET, a Virtual Container (VC) contains the client signal and Path Overhead (POH) used for path monitoring. The VC belongs to the path layer and does not by itself define its phase position relative to the multiplex-section frame. In STM-1, the AU pointer in a fixed overhead position identifies the VC start. The receiving device reads this value and relates it to the beginning of the relevant frame.

The Administrative Unit (AU) combines a high-order VC with its position information in the multiplex section. G.707 defines AU-4 as VC-4 with an AU-4 pointer and AU-3 as VC-3 with an AU-3 pointer. The resulting structure can be placed in an Administrative Unit Group (AUG) and then in an STM-1 frame. G.707 defines the AU as the adaptation between the higher-order path layer and the multiplex-section layer.

The AU pointer carries the offset of the VC start relative to the multiplex-section frame. Changing this value accommodates the phase difference between the VC and STM-1 when a plesiochronous signal is adapted to the synchronous network. The pointer location remains fixed relative to the STM-N frame.

The AU Pointer and VC Position

The AU pointer identifies the VC position relative to the STM-1 frame. A client signal such as DS3 from a plesiochronous network can have a frequency and phase different from the SDH/SONET reference. Pointer processing handles this difference while the VC is transported through the multiplex section.

How the AU pointer works

• Location: The AU pointer occupies a fixed position in the STM-1 frame. In the first nine columns, rows 1 to 3 and 5 to 9 carry the Section Overhead, while row 4 is assigned to AU pointers. For AU-4, H1 and H2 carry the pointer value, while H3 provides a justification opportunity in that pointer area. For AU-3, G.707 defines three separate H1, H2, and H3 pointer sets, one for each AU-3.
• Function: H1 and H2 carry an offset that identifies the byte where the Virtual Container (VC) begins. The receiving device reads the pointer, locates the VC in the payload, and passes the container to the next processing function.
• Justification: Frequency and phase differences are handled by changes in the pointer value. During negative justification, the H3 opportunity can carry an additional VC byte. During positive justification, a stuff byte is used at the specified position. Pointer control bits tell the receiver which operation was applied and keep the buffer position within its intended range. Consecutive pointer operations are separated by at least three frames in which the pointer value stays constant.

AU-4 and AU-3 in SDH and SONET

G.707 defines two primary Administrative Unit types. AU-4 is associated with VC-4 and the SDH structure, while AU-3 is associated with VC-3 and the AU-3 structure used by SONET. Both include a pointer for the position of the related VC.

Forming the Administrative Unit Group (AUG)

The Administrative Unit Group (AUG) is the STM-1 payload structure placed before section overhead is added. In AUG-1, it can contain one AU-4 or three AU-3s, depending on the multiplexing structure. An AUG is a homogeneous assembly of AU-3s or a single AU-4.

An AUG matches the STM-1 payload area of 9 × 261 bytes, or $9 \times 261 = 2349$ bytes. Its composition depends on the AU structure:

• SDH with AU-4: AUG-1 consists of one AU-4. The relation is $\text{AUG} = 1 \times \text{AU-4}$.
• AU-3 structure: AUG-1 is formed by byte-interleaving three AU-3s. Three VC-3 paths can then be carried in one AUG-1. The relation is $\text{AUG} = 3 \times \text{AU-3}$.

The AUG composition changes how client signals are mapped into STM-1 and how AU-4 and AU-3 structures interwork. The pointer preserves the VC position, while byte interleaving fixes its place inside the AUG.