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154 Chapter 7

Starting in 1988 up through 1996, Synchronous Optical Networking
(SONET) and Synchronous Digital Hierarchy (SDH) commanded all
the investments from manufacturers and carriers alike. Both were
touted as the backbone standards for future telecommunications.
Therefore, there was no lack of manufacturers, research and devel-
opment, or implementation schedules. As always, networks mature.
Standards undergo changes, and many fall by the side of the road.
SONET and SDH are no different. Recently, one manufacturer made
a comment that SONET was dead.
In 1997, wave-division multiplexing (WDM) began its stellar rise
in popularity. Manufacturers and technical wizards saw the benefit
of using multiple wavelengths to dramatically increase the capacity
of existing fibers without the problems created by time-division mul-
tiplexing (TDM) systems at these speeds. Both SONET and SDH
standards were designed to carry TDM digital signals in the middle
to late 1980s. Using TDM, a higher-speed signal is created by multi-
plexing many lower-speed channels. Problems surface when we plan
to upgrade to OC-768 and above because TDM has trouble operating
at these speeds.TDM on SONET or SDH still needs electrical signals
and electronic switching systems. Conversion of these TDM signals
may be too expensive at these higher speeds.
On the other hand, WDM carries multiple data speeds on a single
fiber. Dense WDM (DWDM) is a fiberoptic transmission technique
that employs light wavelengths to transmit data parallel-by-bit or
serial-by-character. The all-optical networks using WDM with add-
drop multiplexers and cross-connects permit this. DWDM systems
multiplex up to 128 wavelengths in the 1,550-nanometer (nm) win-
dow. The money for research and development has shifted to WDM
and DWDM because, as we have seen, this is where the action is.
This addresses the importance of scalable DWDM systems in
enabling service providers to accommodate consumer demand for
ever-increasing amounts of bandwidth. DWDM is a crucial compo-
nent of optical networks that enables the transmission of e-mail,
video, multimedia, data, and voice-carried in Internet Protocol (IP),
Asynchronous Transfer Mode (ATM), and SONET/SDH, respectively,
over the optical layer.