
With the advent of SONET, growth of private line and IP services, the legacy
transport stack became inadequate. During a transitional period, SONET rings
were introduced into the core network. However, examining Figure 13.6, the
impracticality of using SONET rings in a large core segment is obvious. Further-
more, early restoration studies showed SONET rings to be economically inferior to
mesh restoration with DCS-type equipment, such as the Intelligent Optical Switch
(IOS) [9]. Upon deployment of the IOS, the SONET ring layer was relegated to
reaching smaller COs (i.e., edge of the core segment) and is essentially capped in
most core network. The