Page 209 - Optical Switching And Networking Handbook

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194 Chapter 8

multiplexer can be hubbed into a digital cross-connect, where it is
concentrated and then forwarded onto the next node. This is used in
many larger organizations where regional offices are located and dis-
trict or branch offices are tied into the network through the hub.
Again, the flexibility is available if a major change occurs in the net-
work architecture or in case of major campaigns in the organization.
Hubs will act as the cross-connect points to link the various echelons
in the network together. These may be developed in a blocking or
nonblocking manner. Typically, some blocking may be allowed. The
hub-and-spoke arrangement is shown in Figure 8-7.

Ring

In a ring architecture, where SONET/SDH automatic protection
switching is employed, the best of all worlds comes to fruition. The
ring topology uses ADMs throughout the network, and a series of
point-to-point links is installed between adjoining neighbors. The
bidirectional capability places the most robustness into the network;
however, unidirectional services also can be installed. The primary
advantage of the ring architecture is survivability in case of a cable
cut or a failure in a network node. The multiplexers have sufficient
intelligence to reroute or reverse direction in case of a failure. If more
than one fiber link is installed, the systems could use alternate
paths, but they must recover in milliseconds (ms) which APS on

Figure 8-7 OC-1
Hub-and-spoke OC-1
arrangement in a
SONET/SDH
multiplexer OC-1 OC-3 OC+1
network

OC-1 OC-1