The "Intelligent Shipping Containers" of SDH/SONET

If the SDH/SONET network is a high-speed, synchronized railway system, then Virtual Containers (VCs) are the standardized, intelligent shipping containers that carry the actual cargo (your data, voice calls, or video streams). A simple container just holds things, but a Virtual Container is special because it comes with its own built-in tracking system and "shipping label." This "label" is known as the Path Overhead.

The fundamental role of a VC is twofold:

1. To provide a structured space, perfectly sized to hold a specific type of data stream (the payload).
2. To attach a set of management and monitoring data (the Path Overhead) that travels with the payload from end-to-end, ensuring its integrity and proper handling across the entire network path.

From Container (C) to Virtual Container (VC)

The creation of a Virtual Container is a simple but crucial step. It begins with a basic Container (C), which is simply a raw block of bits designed to hold a tributary signal. The transformation into an "intelligent" Virtual Container happens when we add the Path Overhead.

$\text{Container (C)} + \text{Path Overhead (POH)} = \text{Virtual Container (VC)}$

For example, the high-capacity VC-4 is formed by taking a C-4 container, which is a block of 9 rows by 260 columns of bytes, and prepending one additional column for the 9-byte VC-4 Path Overhead. This creates the final VC-4 structure of 9 rows by 261 columns.

The Path Overhead (POH): The VC's "Shipping Label"

The Path Overhead is what makes a Virtual Container so powerful. It's a collection of bytes, each with a specific monitoring or management function, that stays with the data payload from the moment it enters the network until it exits. This allows the network operator to ensure the quality of a specific customer service, regardless of the physical route it takes.

Key Bytes of the High-Order POH (e.g., in VC-4/VC-3)

• J1 - Path Trace: Think of this as the "To/From" address on a package. This byte is part of a longer 64-byte message that is continuously repeated. It contains a unique identifier for the path, allowing the receiving equipment to confirm that it is connected to the correct source. This prevents misconnections, like accidentally sending a bank's data to a different company.
• B3 - Bit Interleaved Parity (BIP-8): This is the path's "tamper-evident seal." It's a single byte used for error detection. The sender calculates a parity value over all the bytes in the payload of the previous VC frame and puts the result in the B3 byte of the current frame. The receiver performs the same calculation on the received data and compares it to the received B3 value. If they don't match, it means a bit error occurred during transmission along the path.
• C2 - Signal Label: This byte acts as a "contents" label, specifying what kind of data is being carried inside the Virtual Container. This is crucial for the receiving equipment to know how to process the payload. For example, C2 might indicate the payload is a stream of ATM cells, a packet-over-SONET/SDH stream, or an older PDH signal.
• G1 - Path Status: This is the "return receipt" or "delivery status" byte. It allows the equipment at the end of the path to send status information back to the source equipment. It has two critical functions:
  • REI (Remote Error Indication): Informs the sender that errors have been detected (via the B3 byte). It essentially says, "The data you sent me arrived corrupted."
  • RDI (Remote Defect Indication): Informs the sender that a failure has been detected along the path. It says, "There's a problem somewhere on the line that is preventing me from receiving a good signal."
• H4 - Multiframe/Position Indicator: This byte serves as a pointer for payloads that are themselves composed of smaller, multiplexed structures. When a VC-4 is used to carry multiple lower-speed streams (like T1s), the H4 pointer helps locate the start of the pattern for those streams.

Lower-order Virtual Containers, like the VC-11 (for T1 signals), have a much simpler POH condensed into a few bytes (e.g., the V5 byte), but it performs similar core functions of error monitoring (BIP-2), status reporting (REI, RDI), and signal labeling.

A Container for Every Need: The Virtual Container Hierarchy

SDH/SONET defines different sizes of Virtual Containers, each tailored to efficiently transport standard tributary signals used in telecommunications. The most common types in the SONET hierarchy used in North America are:

*SPE stands for Synchronous Payload Envelope, which is the SONET term for a Virtual Container.

Preparing for Transport: Adapting VCs into TUs and AUs

A Virtual Container with its payload and POH is a complete, self-managing data unit. However, before it can be placed into the larger STM/STS transport frame, it needs to be made positionally flexible. This is achieved by wrapping the VC into another structure that includes a pointer.

• Low-Order VCs (like VT-1.5) are adapted into Tributary Units (TU).
• High-Order VCs (like STS-1 SPE or STS-3c SPE) are adapted into Administrative Units (AU).

This final adaptation step, creating the TU or AU, is what gives SDH/SONET its unique flexibility. The pointer allows these payload "pallets" to be placed anywhere within the high-speed frame, enabling the network to seamlessly handle timing variations and providing the simple add/drop capability that was a revolutionary improvement over PDH. The detailed mechanics of pointers are covered in a dedicated topic.