Mapping PDH in SDH | Teleinf Edu

Purpose of PDH mapping

Mapping defines how the bits of a PDH signal occupy an SDH container. The input has a nominal bit rate, but its clock is not phase-locked to the synchronous network clock. Without rate adaptation, the number of client bits associated with successive SDH frames could vary over time.

Such an input is described as. The mapper distributes data among payload positions and uses justification opportunities to absorb the clock difference. Control information allows the receiver to determine whether a designated position carries client data.

Stages from a container to an STM-N frame

The client signal is mapped into the corresponding C-n container.
Adding Path Overhead creates a VC-n virtual container.
A pointer associates a low-order VC with a TU, or a high-order VC with an AU. The pointer identifies the virtual-container start position.
Tributary units are combined into TUG structures and then carried in VC-4. Administrative units form an AUG transported within STM-N.

These operations are not interchangeable. Mapping concerns client bits, POH creates a supervised path, and multiplexing combines completed units into a structure with greater capacity.

E1 mapping

E1 has a bit rate of $2.048\ \text{Mbit/s}$ . In the European SDH hierarchy it is mapped into C-12. Low-order path overhead creates VC-12, and a pointer then forms TU-12.

\text{E1} \rightarrow \text{C-12} \rightarrow \text{VC-12} \rightarrow \text{TU-12}

Three TU-12 units form one TUG-2.
Seven TUG-2 groups form one TUG-3.
Three TUG-3 groups occupy the corresponding area in VC-4.

The product $3 \cdot 7 \cdot 3 = 63$ explains why one STM-1 can carry 63 E1 signals in this multiplexing structure. The result does not mean that C-4 directly contains 63 untreated streams. Each E1 first has its own VC-12 and a defined position in the TU and TUG hierarchy.

E3 mapping

An E3 signal at $34.368\ \text{Mbit/s}$ is mapped into C-3. Adding path overhead forms VC-3. In the high-order tributary-unit route, a pointer associates VC-3 with TU-3. TU-3 is placed in TUG-3 and then transported within VC-4.

\text{E3} \rightarrow \text{C-3} \rightarrow \text{VC-3} \rightarrow \text{TU-3} \rightarrow \text{TUG-3} \rightarrow \text{VC-4}

C-3 can also carry a DS3 signal at $44.736\ \text{Mbit/s}$ . Use of the same container designation does not imply an identical bit layout. The mapping procedure must match the client bit rate, clock tolerance, and input format.

E4 mapping

An E4 signal at $139.264\ \text{Mbit/s}$ is mapped directly into C-4. Adding path overhead creates VC-4. The AU-4 pointer identifies its position relative to the frame, and AU-4 is carried in the AUG of STM-N.

\text{E4} \rightarrow \text{C-4} \rightarrow \text{VC-4} \rightarrow \text{AU-4} \rightarrow \text{AUG}

This route is shorter than the E1 path because E4 enters a high-order container directly. TU-12 and TUG-2 grouping is not involved. This is a substantive difference between the mapping paths rather than another description of the same container hierarchy.

Client recovery at the receiving end

The receiver performs the operations in reverse order. It first locates the VC from its pointer, extracts the container, and interprets justification control information. It then removes stuffing positions and reconstructs the PDH timing.

Correct control-bit interpretation preserves the number and order of client bits. Loss of multiframe alignment or an incorrect justification decision introduces errors into the recovered stream even when the STM-N frame itself remains synchronized.