TABLE OF CONTENTS Initially, fiber optic systems were only used in point-to-point, or link connections. The SONET/SDM lengths listed in Table 7 2 are link lengths. A fiber optic link has the basic elements shown in Figures 1 2 and 7 1: source, fiber, and receiver. First and second-generation systems consisted only of these point-to-point link connections. The first generation systems used multimode fiber in the 800 nm window; the second generation used standard single-mode fiber in the 1300 nm window. Later, 800/1300 nm WDM and optical amplifiers were added, but the links still remained point-to-point and uncomplicated from a networking viewpoint. The link design considered these basic requirements:
Baseband signal type, analog or digital
Required performance, S/N for analog or BER for digital
Top frequency or data rate
Transmission distance
Sections 7.3.1 and 7.3.2 analyze two specific link designs. These illustrate many of the tradeoffs available in the design of optical fiber systems. Link power and rise time budgets are calculated for both systems. They ensure a desired system performance for the specific baseband signal. The system design variables are the operating characteristics of the three main system components: source, fiber, and receiver. Table 7 3 lists the variety of choices available for the parameters that affect system performance. Not all of these need be included directly in link analyses. Some parameter choices evolve from other choices. For example, choosing a SM fiber will always result in a NA in the region of 0.2.
