3.1. Introduction

In the previous chapter, we described TDM and how it has been utilized in SONET and SDH standards. We noted that when SONET and SDH were developed, they were optimized for carrying voice traffic. At that time no one anticipated the tremendous growth in data traffic that would arise due to the Internet phenomenon. Today, the volume of data traffic has surpassed voice traffic in most networks, and it is still growing at a steady pace. In order to handle data traffic efficiently, a number of new features have been added to SONET and SDH.

In this chapter, we review some of the advanced features of SONET and SDH. Specifically, we describe the different ways of concatenating SONET and SDH signals, and different techniques for mapping packet data onto SONET and SDH connections. We also address transparency services for carrier's carrier applications, as well as fault management and performance monitoring capabilities. The subject matter covered in this chapter will be used as a reference when we discuss optical control plane issues in later chapters. A rigorous understanding of this material, however, is not a prerequisite for dealing with the control plane topics.