336 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.4: A WDM network with a feeder ring connected to several local distribution networks.
(After Ref. [20];©c1999 IEEE; reproduced with permission.)
ing a set of eight standard wavelengths in the 1.55-μm region with a channel spacing
of 200 GHz [21]. MONET incorporated diverse switching technologies [synchronous
digital hierarchy (SDH), asynchronous transfer mode (ATM), etc.] into an all-optical
ring network using cross-connect switches based on the LiNbO 3 technology.
8.1.3 Multiple-Access WDM Networks
Multiple-access networks offer a random bidirectional access to each subscriber. Each
user can receive and transmit information to any other user of the network at all times.
Telephone networks provide one example; they are known as subscriber loop, local-
loop, or access networks. Another example is provided by the Internet used for con-
necting multiple computers. In 2001, both the local-loop and computer networks were
using electrical techniques to provide bidirectional access through circuit or packet
switching. The main limitation of such techniques is that each node on the network
must be capable of processing the entire network traffic. Since it is difficult to achieve
electronic processing speeds in excess of 10 Gb/s, such networks are inherently limited
by the electronics.
The use of WDM permits a novel approach in which the channel wavelength itself
can be used for switching, routing, or distributing each channel to its destination, re-
sulting in an all-optical network. Since wavelength is used for multiple access, such
a WDM approach is referred to aswavelength-division multiple access(WDMA). A
considerable amount of research and development work was done during the 1990s for
developing WDMA networks [22]–[26]. Broadly speaking, WDMA networks can be
classified into two categories, called single-hop and multihop all-optical networks [6].
Every node is directly connected to all other nodes in a single-hop network, resulting
in a fully connected network. In contrast, multihop networks are only partially con-