Tributary Units (TU)

The role of a Tributary Unit in SDH

A tributary signal has a lower bit rate than the STM-1 aggregate, so SDH places it in a defined structure before multiplexing. The Tributary Unit provides the position, alignment, and information required for further multiplexing.

This is the role of a Tributary Unit (TU). A TU is a logical data structure that prepares a lower-speed tributary signal such as a T1 stream at $1.544 \text{ Mbps}$ or an E1 stream at $2.048 \text{ Mbps}$ for transport within a larger SDH payload. A TU has two parts: Virtual Container (VC) carrying the payload and a TU pointer.

The Tributary Unit Pointer

The TU pointer identifies the first byte of a low-order VC within the structure that carries the TU. The VC phase can change relative to the TU multiframe, and the receiver obtains its position from the pointer value.

TU pointer structure and function

The TU pointer is carried in bytes V1 and V2. Byte V3 provides justification opportunities, while V4 is reserved. This byte arrangement is used by TU-2, TU-11, and TU-12:

• V1 and V2 Bytes: These two bytes together form the main pointer word.
  • Pointer value, 10 bits: Specifies the byte offset from V2 to the first byte of the VC. Pointer bytes are excluded from this offset.
  • NDF and S bits: NDF occupies bits 1-4 and indicates a change in alignment. S bits in positions 5-6 identify the TU type and range.
• V3 byte: A positive justification opportunity follows this byte. It can also be replaced by a data byte for negative justification. The pointer I/D bits indicate whether the opportunity was used. When V3 is not used for negative justification, its value is undefined. The mechanism is called justification.
• V4 byte: Reserved and currently carries no user information.

The pointer value occupies bits 7-16 of V1 and V2, and the offset is counted from V2 to the first VC byte. NDF uses bits 1-4. S bits take values 00 for TU-2, 10 for TU-12, and 11 for TU-11. The receiver can therefore identify the structure type before recovering its payload.

For a normal pointer, NDF has the value 0110; during an alignment change it has the value 1001. I/D bits signal use of a justification opportunity. Four successive STM-1 frames form a 500 µs TU multiframe, and byte H4 identifies its phase.

Tributary Unit hierarchy

Tributary Units have sizes corresponding to PDH client signals. The SDH hierarchy defines low-order TU-11, TU-12, and TU-2 units, together with the high-order TU-3.

Low-order Tributary Units (LO-TU)

These units map lower-rate PDH signals into fixed SDH structures.

• TU-11: The TU used in the North American T1 hierarchy.
  • Carries: One VC-11, which in turn maps a T1 (DS1) signal at $1.544 \text{ Mbps}$.
• TU-12: The TU used in the European E1 hierarchy.
  • Carries: One VC-12, which in turn maps an E1 signal at $2.048 \text{ Mbps}$.
  • Structure: A 9-row by 4-column block $(36 \text{ bytes})$ is multiplexed over four STM-1 frames. This gives $4 \times 36 = 144 \text{ bytes}$ in a 500 µs multiframe whose phase is indicated by H4.
• TU-2: A TU for the 6.312 Mbps T2 or DS2 signal family.
  • Carries: One VC-2, used for transporting a T2 (DS2) signal at $6.312 \text{ Mbps}$.
  E2 has a bit rate of $8.448 \text{ Mbps}$ and uses a separate mapping for the E-carrier hierarchy.

High-order Tributary Unit (HO-TU)

TU-3 carries higher-rate client signals.

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

Grouping Tributary Units into TUGs

A single Tributary Unit is grouped into a larger structure called a Tributary Unit Group (TUG) before it is placed in a higher-order container. Grouping uses byte interleaving of several TUs of the same type.

The process of creating a TUG involves byte-interleaving the bytes of several identical TUs. Each level has a defined number of units and a fixed byte layout.

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.
  • 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 formed by byte-interleaving seven TUG-2s. This carries $7 \times 3 = 21 \text{ E1 signals}$ 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.

Grouping places 3 TU-12s or 4 TU-11s in a TUG-2, and then 7 TUG-2s in a TUG-3. A TUG-3 can also contain one TU-3. This structure supports multiplexing E1, T1, E3, and DS3 signals into a VC-4.

TUG-2 has 9 rows and 12 columns, while TUG-3 has 9 rows and 84 columns. A VC-4 can contain three TUG-3 groups. Byte interleaving preserves the unit order at each level, which simplifies demultiplexing of a selected tributary.

In a typical mapping, VC-12 is combined with a TU-12 pointer, and three TU-12s form a TUG-2. Seven TUG-2 groups form a TUG-3, which can then be multiplexed into VC-4. This order separates signal mapping from subsequent grouping.