SDH Multiplexing Hierarchy

From STM-1 upward: STM-4/16/64 and mapping logical containers.

From a Single Stream to a Digital Superhighway

We've learned that the fundamental building block of SDH/SONET is the STM-1 (SDH) / OC-3 (SONET) frame, which provides a transport capacity of 155.52 Mbps. While a significant amount, modern demand requires far greater capacities. The genius of SDH/SONET lies in its Multiplexing Hierarchy (a clean, scalable, and standardized method for combining multiple data streams into a single, high-capacity optical signal).

This process involves two distinct concepts:

Mapping Tributaries: The process of packaging various lower-speed signals (like older PDH T1s and T3s) into the standard payload structure of a base-level STM-1/OC-3 frame.
Synchronous Multiplexing: The process of combining multiple base-level STM-1/OC-3 signals into higher-level signals like STM-4/OC-12, STM-16/OC-48, and beyond.

Part 1: Mapping Tributaries into the STM-1/OC-3 Payload

Before we can build faster signals, we must first fill our basic digital "cargo container" (the STM-1 payload). The process of taking external signals, known as , and formatting them to fit inside the STM-1 frame is called mapping. This is a multi-step procedure involving a hierarchy of logical containers.

The Mapping Container Hierarchy

To understand mapping, think of a set of Russian nesting dolls. Each layer provides structure and adds management information.

1
Container (C): The most basic "box." A block of bits designed to hold a specific type of tributary signal. For example, a C-11 container is sized to hold a T1 (1.544 Mbps) stream. The mapping process adapts the tributary's speed to the container's capacity, often using justification (stuffing) bits.
2
Virtual Container (VC): A Container with a "shipping label" added. This label is the , which contains information for end-to-end monitoring of the payload. The combination of payload (in the Container) and its POH creates a Virtual Container.
3
Tributary Unit (TU) / Administrative Unit (AU): A Virtual Container with a "locator" added. This locator is the . The pointer allows the VC to be flexibly positioned within a larger payload structure, which is the key to SDH/SONET's synchronous operation. Low-speed VCs are mapped into Tributary Units (e.g., TU-11 for a VC-11). High-speed VCs are mapped into Administrative Units (e.g., AU-4 for a VC-4).
4
Tributary/Administrative Unit Group (TUG/AUG): To fill a large container, you group smaller ones together. Multiple TUs are multiplexed together (using byte interleaving) to form a Tributary Unit Group (TUG). A group of AUs (typically just one AU in SDH) forms the Administrative Unit Group (AUG), which is the final structure that fills the payload area of the STM-1 frame.

Example Mapping Path: Transporting 28 x T1 signals

This is the most granular and fundamental mapping procedure in SONET, illustrating the full hierarchy for low-speed signals. The goal is to package 28 separate T1 (1.544 Mbps) signals into a single OC-3 (155.52 Mbps) signal.

Example: Packing 63 × E1 into one STM‑1

Step‑by‑step

Flow

What to notice

Three TU‑12 make one TUG‑2.
Seven TUG‑2 make one TUG‑3.
Three TUG‑3 fill the VC‑4 payload.
VC‑4 becomes AU‑4 via a pointer and sits in the STM‑1 payload.

Each T1 stream is mapped into a C-11 Container.
Path Overhead is added to each C-11, creating a VC-11 (Virtual Container-11).
A pointer is added to each VC-11, creating a TU-11 (Tributary Unit-11).
Four TU-11s are byte-interleaved to form a TUG-2 (Tributary Unit Group-2).
Seven TUG-2s are byte-interleaved to form a TUG-3 (Tributary Unit Group-3). At this point we have successfully multiplexed $7 \times 4 = 28$ T1 streams.
This TUG-3 structure is then treated as a higher-order payload. Path Overhead is added to form a VC-3 (Virtual Container-3).
An Administrative Unit Pointer is added, creating an AU-3.
Three AU-3s are byte-interleaved to form an AUG-3 (Administrative Unit Group-3). (Note: This step shows the complexity where 3*28=84 T1s could fit, but the example here is for filling a smaller container). In the context of OC-3, the 28 T1s are directly structured into the payload of its electrical equivalent, STS-1, which are then combined.
The final group (e.g., an AUG) is placed into the payload of the STM-1 frame.

This structured, hierarchical process allows network equipment to directly access any of the 28 T1 streams without having to demultiplex the entire 155 Mbps signal, which was the main limitation of PDH.

Part 2: Synchronous Multiplexing to Higher-Order Levels

Once we have a base-level STM-1 (or SONET OC-3) signal, creating higher-capacity signals is remarkably simple. The hierarchy is built on a fixed multiplication factor of 4.

4 × STM‑1 → STM‑4 (byte interleaving)

Interactive

Pick any column N: bytes N from STM‑1 #1, #2, #3, #4 are taken in turn to form STM‑4. Duration stays 125 µs.

Source STM‑1s

STM‑1 #1

STM‑1 #2

STM‑1 #3

STM‑1 #4

Resulting STM‑4 (column order)

STM‑1 #1STM‑1 #2STM‑1 #3STM‑1 #4

STM‑1 #1 • byte 1

STM‑1 #2 • byte 1

STM‑1 #3 • byte 1

STM‑1 #4 • byte 1

Click any number above to change the selected byte index. The STM‑4 column pulls bytes in order: #1, then #2, then #3, then #4, and repeats.

The process uses byte interleaving:

To create an STM-4 signal, a multiplexer takes four independent STM-1 signals.
It picks the first byte from STM-1 #1, then the first byte from STM-1 #2, then #3, then #4.
It repeats this for the second byte of each signal, and so on, until all bytes from the four STM-1 frames are interleaved into a new, larger STM-4 frame.
The STM-4 frame still has 9 rows, but it is now four times wider (1080 columns). Crucially, its duration remains exactly 125µs.

The Standard SDH/SONET Hierarchy

SDH Level	SONET Level	Bit Rate (Mbps)	Common Name
STM-1	OC-3	155.52	155M
STM-4	OC-12	622.08	622M
STM-16	OC-48	2,488.32	2.5G
STM-64	OC-192	9,953.28	10G
STM-256	OC-768	39,813.12	40G

This clean, scalable hierarchy allows network operators to easily increase capacity by upgrading to the next level, often without replacing the underlying fiber optic cable. This systematic approach revolutionized the economics and management of core transport networks.