AU/TU Adaptation Structure

The layered C, VC, TU, and AU structure

In SDH and SONET, a client signal is transported through defined adaptation layers. STM-1 and OC-3 have a line rate of 155.520 Mb/s, but the names refer to different frame conventions and overhead models. Examples of client signals are E1 at 2.048 Mb/s, E3 at 34.368 Mb/s, T1 at 1.544 Mb/s, and DS3 at 44.736 Mb/s.

In older PDH systems, hierarchy levels used regional rates and required several demultiplexing stages. SDH defines containers, path overhead, and pointers that organize client-signal mapping. A Container (C), Virtual Container (VC), Tributary Unit (TU), and Administrative Unit (AU) form successive adaptation levels. TUs serve lower-order structures, while AUs serve higher-order structures.

The adaptation process has several levels: a client signal is mapped into a C, receives Path Overhead (POH), and becomes a VC, followed by a TU or AU pointer. TUs are grouped into TUGs, while AU structures form an AUG that occupies the STM payload area.

The SDH multiplexing hierarchy

According to G.707, the path from a client signal to the STM payload area includes a Container (C), a Virtual Container (VC), a pointer, and either a TU or an AU. Two main paths are used: a higher-order path for VC-3 and VC-4, and a lower-order path for VC-11, VC-12, and VC-2, followed by TUG aggregation.

The operations are as follows:

Mapping into a Container (C): The raw client signal, such as a DS3 stream, is mapped into a container with a defined capacity. Justification matches its timing to the network clock and provides room for phase and frequency differences.
Forming a Virtual Container (VC): Path Overhead (POH) is added to the container. It carries information used to monitor the path and verify its continuity between path-terminating points. The result is a Virtual Container.
Creating a Unit (AU or TU): A is added to the Virtual Container. It specifies the position of the VC start in the parent structure.
- For higher-order VCs such as VC-3 and VC-4, this creates an Administrative Unit (AU). In SDH, VC-3 can also be formed directly from a TUG-3 as a path below VC-4.
- For lower-order VCs such as VC-11, VC-12, and VC-2, this creates a Tributary Unit (TU).
Grouping Units (AUG or TUG): Units are combined using .
- TUs are interleaved into a Tributary Unit Group (TUG).
- In AUG-1, one AU-4 or three byte-interleaved AU-3s form an Administrative Unit Group (AUG).
Placing the AUG in an STM frame: Finally, the Administrative Unit Group (AUG) occupies the STM payload area, while the Section Overhead (SOH) occupies its defined overhead fields. An STM-1 has 9 overhead columns and 261 payload columns.

The AU-3 and AU-4 structures

An Administrative Unit combines a higher-order Virtual Container with a pointer. G.707 defines AU-4 and AU-3 and their placement in AUG-1. In STM-1, one AU-4 or three byte-interleaved AU-3s occupy the area corresponding to one AUG-1.

AU-3: The VC-3 structure

Purpose: AU-3 provides a structure for transporting a $44,736 \text{ Mbps}$ DS3 signal.
Composition: It consists of a Virtual Container 3 (VC-3) and an AU-3 pointer.
Multiplexing into STM-1/OC-3: The payload of an STM-1 has roughly three times the capacity of one AU-3. Three individual AU-3s are therefore byte-interleaved to form one Administrative Unit Group (AUG). Each AU-3 has its own pointer, so the position of each VC-3 can be tracked independently.

AU-4: The VC-4 structure

Purpose: AU-4 is designed to transport a $139,264 \text{ Mbps}$ E4 signal.
Composition: It consists of a Virtual Container 4 (VC-4) and an AU-4 pointer.
Multiplexing into STM-1: One AU-4 is large enough to fill the STM-1 payload area. The AUG in this SDH structure therefore consists of one AU-4.

The lower-rate path: TUs and TUGs

Transporting individual lower-rate signals such as T1 or E1 uses several adaptation levels. Tributary Units (TUs) carry lower-order VCs, while Tributary Unit Groups (TUGs) combine them through byte-interleaving. The following example uses the SONET T1 path.

Step 1: Create a TU-11

First, the raw T1 client signal at $1,544 \text{ Mbps}$ is mapped into a VC-11. The TU-11 pointer, carried in bytes V1-V4, is then added. The result is a TU-11, a VC-11 with its position defined for the parent structure.

Step 2: Group into a TUG-2

Multiple TUs of the same type are combined into a Tributary Unit Group 2 (TUG-2) by byte-interleaving.

For SONET: Four TU-11s are interleaved to form one TUG-2.
For SDH: Three TU-12s (carrying E1 signals) are interleaved to form one TUG-2.

Step 3: Create a TUG-3

The TUG-2 groups are then combined into the larger Tributary Unit Group 3 (TUG-3), again by byte-interleaving.

Seven TUG-2s are byte-interleaved to form one TUG-3.

A single TUG-3 then contains a structured set of lower-rate signals. In the SONET example, one TUG-3 carries $7 \text{ TUG-2s} \times 4 \text{ TU-11s/TUG-2} = 28 \text{ T1 streams}$ . In the SDH example, it carries $7 \text{ TUG-2s} \times 3 \text{ TU-12s/TUG-2} = 21 \text{ E1 streams}$ .

Step 4: Map into a higher-order path

A TUG-3 is not equivalent to VC-3 or VC-4. In the SDH path, three TUG-3 structures are byte-interleaved in the VC-4 payload and receive VC-4 POH. The VC-4 and its AU-4 pointer form AU-4, which enters AUG-1 and the STM-1 frame. In a separate VC-3 path, a TUG-3 can be mapped into VC-3 and an AU-3 pointer can then form AU-3. This separation preserves the distinction between lower-order aggregation and higher-order transport.