The Concept: Preparing the Payload for Transport

We've learned that a Virtual Container (VC) is the standardized "shipping container" that holds our actual data payload along with its "shipping label" (the Path Overhead). However, this container is not yet ready to be loaded directly onto the high-speed STM-1 "freight train". It lacks one crucial piece of information: its exact position within the train's cargo hold.

This is where the Administrative Unit (AU) comes in. The AU is the final, complete package, ready for multiplexing into the main STM-1 frame. It is created by taking a high-order Virtual Container (either a VC-3 or a VC-4) and attaching a special navigational tool called an Administrative Unit Pointer (AU Pointer).

The AU Pointer's sole, critical job is to provide the "GPS coordinates" for the start of the VC within the STM-1's vast payload area. This mechanism provides the extraordinary flexibility that separates SDH/SONET from its rigid predecessor, PDH.

The AU Pointer: Enabling a "Floating" Payload

The AU Pointer is the linchpin of the entire synchronous system. Because the client data arriving (e.g., a DS3 stream from a plesiochronous network) is not perfectly synchronized with the SDH/SONET network's master clock, its timing will slightly drift. The pointer mechanism allows the payload (the VC) to "float" within the STM-1 payload space to accommodate this drift.

How the AU Pointer Works

• Location: The AU pointer is not located with the payload. Instead, it is always in a fixed, well-known position within the Section Overhead of the STM-1 frame (specifically in the 4th row, using the H1, H2, and H3 bytes).
• Function: The H1 and H2 bytes contain a value (an offset) that indicates the byte number where the Virtual Container (VC) begins. A receiving device reads this fixed pointer location, interprets the value, and then "jumps" to that precise starting position in the payload to begin processing the actual data.
• Justification: To handle the clock drift, the pointer's value can be changed. If the client payload arrives slightly faster, the pointer value is decremented (a process called negative justification), and the H3 byte is used to carry an extra payload byte. If it arrives slightly slower, the pointer value is incremented (positive justification), and a dummy byte is inserted after H3. This keeps the data flow smooth without overflowing or underflowing buffers.

The Two Flavors of Administrative Units: AU-4 vs. AU-3

To accommodate the different PDH hierarchies used globally, SDH and SONET define two primary types of Administrative Units. They are fundamentally different in size and how they are used.

Building the Full Payload: The Administrative Unit Group (AUG)

The final step before adding the main transport (Section) overhead is to group the Administrative Units together. The payload area of an STM-1 is technically filled not by an AU directly, but by an Administrative Unit Group (AUG). This concept neatly ties the AU-4 and AU-3 structures together.

An AUG is a structure with the exact size of the STM-1 payload ($9 \times 261 = 2349$ bytes). How it is formed depends on the standard:

• In European SDH: The AUG is incredibly simple. It consists of one single AU-4. So, for SDH, $\text{AUG} = 1 \times \text{AU-4}$.
• In North American SONET: The equivalent group is formed by byte-interleaving three separate AU-3s. This is how three independent DS3 signals are carried together within a single OC-3 signal. So, for SONET, $\text{AUG} = 3 \times \text{AU-3}$.

This crucial difference in the composition of the AUG is the primary reason why SDH and SONET, despite their similarities, are distinct hierarchies at their lower levels, each optimized for its native PDH signal structure. The Administrative Unit is the key element that provides the flexibility and structure to make this possible.