8.1. WDM LIGHTWAVE SYSTEMS 335
Figure 8.3: An example of a wide-area network in the form of several interconnected SONET
rings. (After Ref. [19];©c2000 IEEE; reproduced with permission.)
not limited by the electronic-speed bottleneck and may help in reducing the cost of
installing and maintaining the network. The nodes in a transparent WDM network (see
Fig. 8.3) switch channels using optical cross-connects. Such devices were still in their
infancy in 2001.
An alternative topology implements a regional WDM network in the form of sev-
eral interconnected rings. Figure 8.4 shows such a scheme schematically [20]. The
feeder ring connects to the backbone of the network through an egress node. This ring
employs four fibers to ensure robustness. Two of the fibers are used to route the data in
the clockwise and counterclockwise directions. The other two fibers are called protec-
tion fibers and are used in case a point-to-point link fails (self-healing). The feeder ring
supplies data to several other rings through access nodes. An add–drop multiplexer can
be used at all nodes to drop and to add individual WDM channels. Dropped channels
can be distributed to users using bus, tree, or ring networks. Notice that nodes are not
always directly connected and require data transfer at multiple hubs. Such networks
are called multihop networks.
Metro networks or MANs connect several central offices within a metropolitan
area. The ring topology is also used for such networks. The main difference from the
ring shown in Fig. 8.4 stems from the scaling and cost considerations. The traffic flows
in a metro ring at a modest bit rate compared with a WAN ring forming the backbone
of a nationwide network. Typically, each channel operates at 2.5 Gb/s. To reduce the
cost, a coarse WDM technique is used (in place of dense WDM common in the back-
bone rings) by using a channel spacing in the 2- to 10-nm range. Moreover, often just
two fibers are used inside the ring, one for carrying the data and the other for pro-
tecting against a failure. Most metro networks were using electrical switching in 2001
although optical switching is the ultimate goal. In a test-bed implementation of an opti-
cally switched metro network, called themultiwavelength optical network(MONET),
several sites within the Washington, DC, area of the United States were connected us-