Introduction: Two Dialects of the Same Language

When discussing synchronous optical networking, two acronyms immediately come to mind: SONET and SDH. While often used interchangeably, they are distinct standards that evolved to solve the same fundamental problem, but for different parts of the world. Think of them as two dialects of the same language (largely mutually intelligible but with differences in vocabulary and grammar).

SONET (Synchronous Optical Network) is the North American standard, while SDH (Synchronous Digital Hierarchy) is the international standard used in Europe and most of the rest of the world. Understanding their relationship, their core similarities, and their specific differences is crucial for anyone working with global transport networks. This page explores why these two standards exist and how they elegantly coexist to form a single, seamless global optical backbone.

Historical Origins: Solving the PDH Problem

The parallel development of SONET and SDH was a direct response to the limitations of the older Plesiochronous Digital Hierarchy (PDH). Critically, the PDH world was split into two incompatible camps.

• North American PDH (T-carrier): Based on the T1 signal, which multiplexed 24 voice channels into a $(1,544 \text{ Mbps})$ stream. Higher rates like T3 $(\approx 45 \text{ Mbps})$ were built by multiplexing T1s.
• European PDH (E-carrier): Based on the E1 signal, which multiplexed 30 voice channels into a $(2,048 \text{ Mbps})$ stream. Higher rates like E3 $(\approx 34 \text{ Mbps})$ and E4 $(\approx 140 \text{ Mbps})$ were built from E1s.

This incompatibility led to two separate standardization efforts in the 1980s:

1. SONET Emerges: In the United States, Bellcore (now Telcordia) and ANSI led the development of SONET, designing it specifically to efficiently transport the existing T-carrier signals (T1, T3) over a new, high-speed, and manageable optical infrastructure.
2. SDH Adapts: Internationally, the ITU-T (driven by European telecommunication bodies) recognized the genius of the synchronous, pointer-based SONET concept. Instead of inventing a completely new standard, they chose to adapt SONET to better fit their E-carrier based infrastructure. This adaptation was named SDH.

Fundamental Similarities: The Shared DNA

Before diving into the differences, it's essential to understand that SDH and SONET are more alike than they are different. They share the same core technological principles that made them a revolutionary leap over PDH:

• Synchronous Operation: Both systems are fully synchronous, relying on a master-slave clocking hierarchy traceable back to a Primary Reference Clock (PRC) to ensure all network elements operate in perfect unison.
• Basic Frame Rate: Both use a fundamental frame duration of $(125 \mu \text{s})$, directly tied to the 8,000 samples per second required for digital voice.
• Pointer Mechanism: Both use a sophisticated pointer system to locate payloads within the transport frame. This allows for the flexible and efficient add/drop of tributary signals and accommodates timing differences between networks.
• Layered Overhead (OAM): Both dedicate a significant portion of their frame structure to a rich set of Overhead bytes for advanced Operations, Administration, and Maintenance (OAM), enabling robust performance monitoring, fault management, and protection switching.
• Byte-Interleaved Multiplexing: Both build higher-rate signals by byte-interleaving lower-rate signals, ensuring a predictable and easily accessible structure.

Key Difference 1: The Base Rate and Hierarchy

The most fundamental difference lies in their basic building blocks and the resulting hierarchy of data rates.

Key Difference 2: Terminology

While the underlying concepts are the same, SONET and SDH use different names for many of their structural components. Understanding this different vocabulary is essential when working with international circuits or equipment from different vendors.

Key Difference 3: Multiplexing Structures

Because SONET was built around the T1 stream and SDH around the E1 stream, their internal structures for multiplexing low-speed signals into the main STM-1/OC-3 payload are different.

• In SONET: The smallest tributary is the Virtual Tributary (VT). A T1 stream is mapped into a VT1.5. Then, 28 VT1.5s are multiplexed into a Tributary Unit Group, which is then mapped into the main STS-1 payload (SPE). To build the OC-3 signal, three such STS-1 payloads are byte-interleaved.
• In SDH: The smallest tributary is packaged into a Tributary Unit (TU). An E1 stream is mapped into a VC-12 and then given a pointer to become a TU-12. These are then grouped into TUG-2s and TUG-3s before being mapped into a single, large VC-4, which then gets an AU-4 pointer and fills the entire STM-1 payload.

The end result is the same (a $(155,52 \text{ Mbps})$ frame), but the internal arrangement of the cargo is different, reflecting their different origins.