Overview

This chapter describes several networks that use optical fiber as their underlying transmission mechanism. These networks can be thought of as client layers of the optical layer. As we saw in Chapter 1, the optical layer provides lightpaths to the client layers. To the client layer, these lightpaths look like physical links between client layer network elements. All the client layers that we will study process the data in the electrical domain, performing functions such as fixed time division multiplexing or statistical time division multiplexing (packet switching). These client layers aggregate and bring a variety of lower-speed voice, data, and private line services into the network. Each of these client networks is important in its own right and can operate over point-to-point fiber links as well as over a more sophisticated optical layer, using the lightpaths provided by the optical layer.

The predominant client layers in backbone networks today are SONET/SDH, IP, and ATM. SONET/SDH is particularly adept at dealing with lower-speed time division multiplexed streams, whereas IP and ATM are adept at dealing with statistically multiplexed packet streams. In many cases, IP and ATM use SONET/SDH as the underlying transport mechanism. With the emergence of high-speed interfaces on IP and ATM equipment, we are also seeing IP and ATM mapped directly into the optical layer, without requiring separate SONET/SDH equipment. In the metro network, we are seeing a proliferation of several types of client layers, such as Gigabit Ethernet, ESCON, and Fibre Channel. Many of the latter networks...