Page 210 - Optical Switching And Networking Handbook

09_200023_CH08/Batesx 1/17/01 8:20 AM Page 195

High-Speed Applications 195

SONET/SDH is designed to do. Figure 8-8 shows the ring topology
with dual fibers run (bidirectional service) between the ADMs.
Ring topologies also may be used as access methods to provide
robust services to the Internet. Figure 8-9 shows a ring of OC-3s
installed for a large organization looking for redundancy and
resiliency in the network. In this case, the access methods are
installed by the carriers to different carriers (AT&T World Net and
UUNet). If a single failure occurs, the network will begin to immedi-
ately close the circle and offer the customer high-speed and reliable
communications all at the same time. Regardless of the application,
this redundant-ring topology assures the customer of availability
without the wait. The network will likely heal and begin the alter-
nate routing in less than 10 ms. At one point, this form of internet-
working would have been too expensive for customers to afford and
for carriers to install. With all the changes in bandwidth allocation
and the technology improvements, networks like this are becoming
common.

Access Methods

One of the major issues always facing end users and local carriers is
that of access. Users are now more sophisticated in their desires.
They no longer want to settle for just any service. Instead, they

Figure 8-8
Ring architecture of
SONET/SDH
multiplexers ADM
ADM