2.1. Introduction

SONET and SDH define technologies for carrying multiple digital signals of different capacities in a flexible manner. Most of the deployed optical networks are based on SONET and SDH standards. Consequently, a basic understanding of SONET and SDH is a prerequisite for understanding the optical control plane issues discussed in later chapters.

The basis for signal transport under SONET and SDH is a technique called Time Division Multiplexing (TDM). Multiplexing in general refers to the process of combining individual signals for transmission. The term de-multiplexing is used to denote the process of separating individual signals from a multiplexed stream. While there are many techniques for doing these, TDM is commonly used in telecommunication networks. This chapter begins with an introduction to TDM. Following this, the basic features of SONET and SDH are described. The next chapter deals with more advanced features of SONET and SDH.