The Logistics of the Digital Highway: An Analogy

Imagine a global logistics system. The high-speed STM/OC optical links are like massive freight trains or container ships, capable of carrying enormous amounts of cargo. The data we want to send (from a single phone call, a T1 stream, to high-speed internet traffic, a DS3 stream) is the cargo.

The older PDH system was like trying to ship everything using different-sized, non-standard boxes. It was chaotic and inefficient. SDH/SONET introduced a brilliantly organized, hierarchical system of "packing" this cargo. This system ensures that any type of data can be efficiently and flexibly loaded onto the high-speed "train." The combination of Administrative Units (AU) and Tributary Units (TU) is the heart of this logistics system.

This page explains the entire packing process: how small signals are bundled into medium-sized packages (TUs), how those are grouped onto pallets (TUGs), and how these, or larger single items (AUs), are finally loaded into the main STM container for transport.

The Overall SDH/SONET Multiplexing Hierarchy

The journey from a client signal to its final place in an STM frame follows a strict, layered hierarchy. Understanding this flow is key to understanding SDH/SONET. There are two primary pathways: one for large, high-capacity signals and one for aggregating numerous smaller signals.

The general process is as follows:

1. Mapping into a Container (C): The raw client signal (e.g., a DS3 stream) is first placed into a correctly sized Container. This step synchronizes the external signal with the network clock through justification.
2. Forming a Virtual Container (VC): Path Overhead (POH) is added to the Container. The POH acts as an end-to-end "shipping label," allowing for performance monitoring of that specific payload throughout its journey. The result is a Virtual Container.
3. Creating a Unit (AU or TU): A pointer is added to the Virtual Container. This pointer gives the payload its "address" within the larger frame.
   • For high-order VCs (VC-3, VC-4), this creates an Administrative Unit (AU).
   • For low-order VCs (VC-11, VC-12, VC-2), this creates a Tributary Unit (TU).
4. Grouping Units (AUG or TUG): Multiple smaller units are grouped together using byte-interleaving.
   • Multiple TUs are interleaved into a Tributary Unit Group (TUG).
   • Multiple AUs (specifically in SONET) or one large AU can be placed in an Administrative Unit Group (AUG).
5. Loading into STM frame: Finally, the Administrative Unit Group (AUG) is placed into the payload area of the STM frame, and the Section Overhead (SOH) is added to create the final, transmittable signal.

High-Speed Pathway: The AU-3 and AU-4 Structures

The Administrative Unit is designed for high-capacity payloads that will occupy a significant portion of the final STM-1 frame. There are two primary types, tailored to the North American (SONET) and European (SDH) hierarchies.

SONET: The AU-3 Structure (based on DS3)

• Purpose: The AU-3 is the standard way to transport a $44,736 \text{ Mbps}$ DS3 signal.
• Composition: It is formed by taking a Virtual Container 3 (VC-3) and adding an AU-3 Pointer.
• Multiplexing into the STM-1/OC-3: The payload of an STM-1 frame has roughly three times the capacity of a single AU-3. Therefore, to build an STM-1 (or its SONET equivalent, OC-3), three individual AU-3s are byte-interleaved to form a single Administrative Unit Group (AUG). Each AU-3 has its own independent pointer, allowing each DS3 payload to float independently.

SDH: The AU-4 Structure (based on E4)

• Purpose: The AU-4 is designed to transport a $139,264 \text{ Mbps}$ E4 signal.
• Composition: It is formed by taking a Virtual Container 4 (VC-4) and adding an AU-4 Pointer.
• Multiplexing into the STM-1: The capacity of a single AU-4 is large enough to fill the entire payload area of an STM-1 frame on its own. Therefore, the AUG in an SDH system consists of just one AU-4.

Low-Speed Pathway: Building Up with TUs and TUGs

Transporting individual low-speed signals like T1s or E1s requires a more intricate, multi-stage "packing" process. This is where Tributary Units (TUs) and Tributary Unit Groups (TUGs) are used. We'll use the SONET example of packing T1s.

Step 1: Create the Smallest Shippable Unit (TU-11)

First, the raw $1,544 \text{ Mbps}$ T1 client signal is mapped into a VC-11. Then, the TU-11 Pointer (bytes V1-V4) is added. The result is a TU-11, the smallest complete package ready for further multiplexing.

Step 2: Group onto a "Pallet" (TUG-2)

Multiple identical TUs are bundled into a Tributary Unit Group 2 (TUG-2). This is done via 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: Pack onto a "Crate" (TUG-3)

Finally, the TUG-2 "pallets" are grouped into an even larger structure, the Tributary Unit Group 3 (TUG-3).

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

After this process, a single TUG-3 now contains a highly structured collection of many low-speed signals. For instance, in a SONET context, one TUG-3 carries $7 \text{ TUG-2s} \times 4 \text{ TU-11s/TUG-2} = 28 \text{ T1 streams}$. For SDH, it's $7 \text{ TUG-2s} \times 3 \text{ TU-12s/TUG-2} = 21 \text{ E1 streams}$.

Step 4: Final Assembly into the High-Order Payload

A TUG-3 has a structure and capacity equivalent to a high-order VC-3. Thus, a fully assembled TUG-3 can now be mapped into a VC-3. Once it is a VC-3, an AU-3 pointer is attached, and it becomes an AU-3, ready to be multiplexed with two other AU-3s into the final AUG that fills the STM-1/OC-3 payload. This modular, recursive design is what gives SDH/SONET its power and flexibility.