1, were used to deliver interconnectivity
between voice-switching exchanges. Here, associated bit-rates were tied to older plesiochronous
digital hierarchy (PDH) standards such as E1 (2.048 Mbps). Subsequently, the
late 1980s and early 1990s saw concertive carrier efforts to standardize SONET/SDH
technology, with interface speeds scaling from the low tens of megabits (STS-1, E3) to
multigigabits (OC-192/STM-64). These advances were enabled by two key factors??”
high-speed electronic hardware and improved single-mode fiber (SMF) media, as
shown in Figure 8.2. The telecom-bubble era of the late 1990s to early 2000s saw an
even more profound evolution with the commercialization of WDM technology. This
approach delivered unmatched terabits-per-fiber scalability by transmitting multiple
channels (called wavelengths) of light in unused SMF spectral bands. Indeed,
dense WDM (DWDM) now forms the foundation of modern optical networks, and key
advances have come in crucial enabling component technologies, for example, passive
elements (fibers, couplers, filters) and active elements (lasers, amplifiers, switches)
[1]. Although the DWDM market has experienced severe realignment in the post-bubble
era, it has since returned to normalcy and is now experiencing steady growth [2].
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