The Concept: Packaging Small Streams for a Big Highway

Imagine the vast capacity of an STM-1 frame as a massive, high-speed freight train. Now, imagine needing to transport a small box (perhaps a single phone call's data, a T1 or E1 stream). You wouldn't dispatch an entire train for just one small box. Instead, you'd want a standardized way to package that small box so it can be located, tracked, and safely placed alongside many other small boxes on the train.

This is the precise role of a Tributary Unit (TU). A TU is a logical data structure that prepares a lower-speed tributary signal(like a T1 stream at $1.544 \text{ Mbps}$) for transport within a larger SDH payload. The TU acts as the complete, ready-to-multiplex package, containing both the data itself and the crucial information needed to find it. It is composed of two main parts: a Virtual Container (VC)carrying the payload, and a Tributary Unit Pointer.

The Key Component: The Tributary Unit Pointer (TU Pointer)

The magic of SDH's flexibility comes from its use of pointers. The TU Pointer is the "local delivery instruction" for a low-order payload. Its primary job is to indicate the exact starting position of the payload (the Virtual Container) within the structure that will carry the TU.

Structure and Function of the TU Pointer

The pointer itself is located in dedicated bytes at the beginning of the TU's structure. For the common TU-11 and TU-12, these are known as the V1, V2, V3, and V4 bytes:

• V1 and V2 Bytes: These two bytes together form the main pointer word.
  • Pointer Value (10 bits): The primary function is a 10-bit value that specifies the offset, in bytes, from the end of the V3 byte to the first byte (the V5 byte) of the Virtual Container payload. This allows the VC to "float" freely within its allocated space.
  • New Data Flag (NDF): Special bits that signal an impending change in the pointer's value, allowing the receiver to prepare.
• V3 Byte: This byte is used for justification. If the incoming client signal's clock is slightly slower than the network's clock, this V3 byte is filled with a dummy "stuffing" byte. If the client signal is slightly faster, this byte is used to carry an extra data byte. The pointer value in V1/V2 is then adjusted accordingly in the next multiframe to reflect this shift. This elegant mechanism absorbs timing differences.
• V4 Byte: This is a reserved byte, currently unused in most standard implementations, reserved for future extensions.

The Hierarchy of Tributary Units

Just like Virtual Containers, Tributary Units are organized in a hierarchy, with different sizes designed to efficiently carry the standard PDH client signals used around the world. There are two main categories: low-order and high-order.

Low-Order Tributary Units (LO-TU)

These are designed to carry the most common low-speed digital signals.

• TU-11: The fundamental TU for the North American hierarchy.
  • Carries: One VC-11, which in turn maps a T1 (DS1) signal at $1.544 \text{ Mbps}$.
• TU-12: The fundamental TU for the European hierarchy.
  • Carries: One VC-12, which in turn maps an E1 signal at $2.048 \text{ Mbps}$.
  • Structure: It is often visualized as a 9-row by 4-column block of bytes $(36 \text{ bytes})$ that is multiplexed over four STM-1 frames $4 \times 36 = 144 \text{ bytes}$ in total per 500 µs multiframe.
• TU-2: A less common TU for medium-speed signals.
  • Carries: One VC-2, used for transporting a T2 (DS2) signal at $6.312 \text{ Mbps}$.

High-Order Tributary Units (HO-TU)

This category is for higher-speed client signals.

• TU-3:
  • Carries: One VC-3, which can map a DS3 signal $(44.736 \text{ Mbps})$ or an E3 signal $(34.368 \text{ Mbps})$.

Building Bigger Structures: The Tributary Unit Group (TUG)

A single Tributary Unit is still too small to be placed directly into a large STM-1 payload. SDH uses an intermediate step of grouping TUs together into a larger structure called a Tributary Unit Group (TUG). If a TU is a small package, a TUG is like a pallet or crate that bundles several packages together for easier handling.

The process of creating a TUG involves byte-interleaving the bytes of several identical TUs. This creates a highly organized and scalable hierarchy.

The TUG Hierarchy

• Tributary Unit Group 2 (TUG-2): This is the first level of grouping.
  • Formation: A TUG-2 is formed by byte-interleaving either three TU-12s or four TU-11s. This structure neatly packages common tributary types.
  • Structure: A TUG-2 is represented as a block of 9 rows by 12 columns.
• Tributary Unit Group 3 (TUG-3): The highest level of tributary grouping.
  • Formation from TUG-2s: A TUG-3 is most commonly formed by byte-interleaving seven TUG-2s. This allows it to carry a large number of low-speed signals (e.g., $7 \times 3 = 21 \text{ E1 signals}$ or $7 \times 4 = 28 \text{ T1 signals}$).
  • Formation from a TU-3: A TUG-3 can also be formed directly from a single, high-order TU-3 (carrying a DS3/E3 signal).
  • Structure: A TUG-3 has the dimensions of 9 rows by 84 columns.

This hierarchical grouping of TUs into TUGs is what allows SDH/SONET to efficiently pack a large number of diverse, low-speed client signals into the payload of a single high-speed optical signal like STM-1. The entire structure, from the smallest C-11 to the largest TUG-3, is the key to providing flexible, scalable, and manageable digital transport.