ring in local and long distance networks, as well as undersea cables. Virtu- ally all were digital: While optical fiber could be used for both analog and digital applications [22], it was clear by then that the telecommunications industry needed digital transmission systems. The RBOCs were fairly quick to complete the upgrade of their metropoli- tan networks to fiber optics. (Analog microwave radio systems remained in some rural areas well into the 1990s, and some rustic locations may remain on copper or microwave radio links, but the latter have since been digitized.) These early fiber-optic systems were, however, vendor-proprietary. If one end of a fiber- optic cable was fed by an AT&TLTMA fiber-optic terminal, then the other end had to be an LTMA as well. Fujitsu, Northern Telecom, and others raced each other to improve the capacity of their systems. Many of the first links went at the DS3 rate (44.736 Mbps), or at multiples of DS3 such as 135 Mbps. Many of the first fibers pulled were of the multimode variety, whose light-conducting cores were 50 to 100 microns in diameter. But by the mid-1980s, single-mode cable, with a core diameter under 10 microns, became the norm for most appli- cations. Multimode fiber was easier to connect to but it had a shorter range, which declined with the speed at which it was run. Single-mode fiber had virtu- ally no physical limit on the maximum speed at which it could operate; its range was determined by the purity and chemical composition of the glass. Distances of 30 miles between repeaters were commonplace, enabling most links within the RBOC networks to be accomplished without repeaters. (The most recent fiber-optic cables, developed in the late 1990s, are capable of carrying high- speed signals for hundreds of miles without repeaters.) The long distance networks also adopted fiber optics, but not all did SO as quickly. There were many interexchange carriers in place for the beginning of equal access, but most were actually resellers, either with their own switching attache