Introduction: The Standardized "Shipping Boxes" of SONET/SDH

Imagine a global logistics network. To ship goods efficiently, you need standardized containers that fit on any train, ship, or truck, regardless of what's inside. In the world of SONET/SDH, this role is played by the Container (C).

A Container is a logical, standardized block of data with a defined size and structure. Its primary function is to encapsulate a client signal, adapting its bit rate and format into a uniform structure that the synchronous network can handle. This process is the very first step in preparing external data for transport. It is the fundamental solution to the "digital jungle" of varying PDH rates and formats from the legacy world.

The Mapping Process: Fitting Cargo into the Container

The process of placing a client signal into a Container is called mapping. Because client signals are often plesiochronous(their clocks are not locked to the SONET network's master clock), the mapping process must compensate for these slight timing variations. This is achieved by adding extra bits, called stuffing or justification bits, to the client data stream to precisely fill the fixed capacity of the Container.

Composition of a Mapped Container

• Payload Data: The actual client signal's bits.
• Justification/Stuffing Bits: These are filler bits used to accommodate clock differences. Information about whether these bits are actual data or just padding is carried in dedicated overhead bits.
• Overhead Bits: Control bits included within the container structure itself to manage the justification process and sometimes carry other basic path information.

This mapping process ensures that the output of the Container is a synchronous data stream, perfectly aligned with the SONET network's timing, ready for the next stage of processing—being placed into a Virtual Container.

The Hierarchy of Containers

Just as shipping containers come in different standard sizes, SONET/SDH defines a hierarchy of containers, each tailored to a specific client signal bit rate. The choice of container is determined by the "cargo" it needs to carry.

High-Order Containers (for high-capacity signals)

• Container-4 (C-4): The largest standard container, primarily used in the SDH world.
  • Purpose: Designed to carry a high-speed signal of approximately $140 \text{ Mbps}$. It is the perfect size for a European E4 signal ($139.264 \text{ Mbps}$).
  • Structure: A logical block of data consisting of 9 rows by 260 columns of bytes, totaling $2340 \text{ bytes}$. The client signal is mapped directly into this structure.
• Container-3 (C-3): A medium-capacity container, used in SONET for DS3 signals.
  • Purpose: Designed to transport signals around $45 \text{ Mbps}$. This perfectly accommodates a North American DS3 signal ($44.736 \text{ Mbps}$).
  • Structure: A logical block of 9 rows by 84 columns of bytes ($756 \text{ bytes}$), with additional space reserved for justification bits and overhead.

Low-Order Containers (for common tributary signals)

• Container-11 (C-11): The workhorse of the North American SONET hierarchy.
  • Purpose: Designed specifically to carry a $1.544 \text{ Mbps}$ North American T1 (DS1) signal.
  • Structure: C-11 uses a multiframe structure that repeats every $500 \text{ µs}$. This structure contains payload bytes from the T1 stream, plus dedicated justification control bits (C-bits) and stuffing opportunity bits (S-bits) to precisely match the rate.
• Container-12 (C-12): The European equivalent to the C-11.
  • Purpose: Designed to carry a $2.048 \text{ Mbps}$ European E1 signal.
  • Structure: Very similar in concept to the C-11, it is a multiframe structure of 139 bytes that repeats every $500 \text{ µs}$ and includes justification overhead.
• Container-2 (C-2): Used for transporting $6.312 \text{ Mbps}$ PDH signals (DS2), but it is far less common in modern networks than C-11 or C-3.

The Purpose of a Dedicated Container Structure

By defining these standardized "boxes," SONET/SDH accomplishes several key goals at the very first step of entering the network:

• Rate Adaptation: It provides a clean, standardized way to absorb the small clock variations of external PDH networks, turning a plesiochronous input into a perfectly synchronous output.
• Protocol Transparency: The container doesn't care about the content of the client data; it only cares about its bit rate. This allows SONET/SDH to transport a wide variety of different services, from legacy voice circuits to modern packet-based data.
• Standardization: It creates uniform building blocks. Once a T1 is mapped into a C-11, the network no longer sees a "T1"; it sees a standard C-11 structure, which can be handled by any compliant network equipment, regardless of manufacturer.

A Container (C) is simply a standardized "box" filled with bits. By itself, it has no information about its journey. To make it a manageable entity, a "shipping label" must be attached. In SONET/SDH, this label is the Path Overhead (POH). The combination of a Container and its POH creates the Virtual Container (VC), which is the subject of the next topic.