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

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 SDH/SONET, 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 SDH network can handle. This process is the very first step in preparing external data for transport across the synchronous network. It is the solution to the "digital jungle" of varying PDH rates and formats.

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 SDH network 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.

• Payload Data: The actual client signal's bits.
• Stuffing/Justification Bits: Extra bits added to fill the container capacity and accommodate clock differences. Information about whether these bits are actual data or just padding is carried in overhead bytes.
• 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 SDH network's timing, ready for the next stage of processing.

The Hierarchy of Containers

Just as shipping containers come in different standard sizes, SDH/SONET 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.
  • Purpose: Designed to carry a high-speed signal of approximately $140 \text{ Mbps}$. It is the perfect size for an E4 signal from the European PDH hierarchy ($139.264 \text{ Mbps}$) or a North American DS4 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, bit by bit.
• Container-3 (C-3): A medium-capacity container.
  • Purpose: Designed to transport signals in the $34 \text{ to } 45 \text{ Mbps}$ range. This perfectly accommodates a European E3 signal ($34.368 \text{ Mbps}$) or 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-12 (C-12): One of the most common low-order containers, crucial for voice telephony in regions using the E1 standard.
  • Purpose: Designed specifically to carry a $2.048 \text{ Mbps}$ European E1 signal.
  • Structure: C-12 has a more complex structure that repeats every $500 \mu \text{s}$ (spanning four STM frames). It consists of 139 bytes, which include payload bits from the E1 stream as well as justification control and overhead bits.
• Container-11 (C-11): The North American equivalent to the C-12.
  • Purpose: Designed specifically to carry a $1.544 \text{ Mbps}$ North American T1 (DS1) signal.
  • Structure: Similar in concept to the C-12, it has a multiframe structure that repeats every $500 \mu \text{s}$ and uses justification to align the T1 signal.
• Container-2 (C-2): Used for transporting $6.312 \text{ Mbps}$ PDH signals (E2 or T2). It is less commonly used than C-12 or C-3.

The Next Step: Adding the "Shipping Label"

A container (C) is simply a standardized "box" filled with bits. By itself, it has no information about its journey through the network. To make it a manageable entity, a "shipping label" must be attached. In SDH/SONET, this label is called the Path Overhead (POH).

The combination of a Container and its Path Overhead creates a new, more powerful structure: the Virtual Container (VC).

The Path Overhead contains crucial end-to-end monitoring information, such as error-checking codes (BIP-8) and a signal label indicating the type of cargo inside. This allows the network to monitor the health of the connection from the point where the client signal enters the network to the point where it leaves, regardless of how many nodes it traverses in between. The detailed structure and function of Virtual Containers are the subject of the next topic.