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82 Chapter 4

As discussed in Chapter 3, Synchronous Optical Networking
(SONET) is the high-speed optical-based architecture for carriers
and users alike. This optical-based networking strategy was devel-
oped in North America by the American National Standards Insti-
tute (ANSI). While the ANSI committees were working on SONET,
though, another movement was underway. In Europe, the standards
committees also were wrestling with the logical replacement to the
Plesiochronous Digital Hierarchy (PDH), which is an asynchronous
multiplexing plan to create high-speed communications channels.
The Europeans came up with a separate multiplexing hierarchy
called Synchronous Digital Hierarchy (SDH) in support of the
SONET standards.
Ever since the standards bodies approved the recommendations
for the SDH (and SONET), the services have been used effectively to
improve and revolutionize the industry. Significant cost efficiencies
and performance improvements have been shown. SDH provides a
means for the rest of the world to use the capabilities of fiber-based
transport systems and multiplexing architectures to improve on the
older PDH, which was inefficient and expensive.The PDH evolved in
response to the demand for plain old telephone services (POTS) and
was not ideally suited to promote the efficient use of bandwidth and
high-speed services.
Digital networks continue to expand in complexity and penetra-
tion within the carriers’ networks, now moving closer to the con-
sumers’ premises. High-speed communications prior to the
formulation of SDH in 1990 operated at speeds of up to 139.364
megabits per second (Mbps). However, the carriers implemented
coaxial and radio-based systems operating at 140 to 565 Mbps. The
networks were severely constrained due to the high cost of the trans-
mission medium (coaxial cable especially). The multiplexing rates
used plesiochronous rates, which led to the European PDH.
After the development of fiber and the enhancements of inte-
grated circuitry, the newer transmission speeds and complex net-
working architectures became realistic. In Europe, the evolution and
deployment of Integrated Services Digital Networks (ISDN) also led
to proliferation of the Broad Band ISDN (B-ISDN) standards, which
enable a simple multiplexing technique. In the United States, the
breakup of Bell Telephone prompted the local carriers to look for