8.1. WDM LIGHTWAVE SYSTEMS 337
Figure 8.5: Schematic of the Lambdanet withNnodes. Each node consists of one transmitter
andNreceivers. (After Ref. [28];©c1990 IEEE; reprinted with permission.)
nected such that an optical signal sent by one node may require several hops through
intermediate nodes before reaching its destination. In each category, transmitters and
receivers can have their operating frequencies either fixed or tunable.
Several architectures can be used for all-optical multihop networks [6]–[11]. Hy-
percube architecture provides one example—it has been used for interconnecting mul-
tiple processors in a supercomputer [27]. The hypercube configuration can be easily
visualized in three dimensions such that eight nodes are located at eight corners of a
simple cube. In general, the number of nodesNmust be of the form 2m, wheremis
the dimensionality of the hypercube. Each node is connected tomdifferent nodes. The
maximum number of hops is limited tom, while the average number of hops is about
m/2 for largeN. Each node requiresmreceivers. The number of receivers can be
reduced by using a variant, known as thedeBruijn network, but it requires more than
m/2 hops on average. Another example of a multihop WDM network is provided by
theshuffle networkor its bidirectional equivalent—the Banyan network.
Figure 8.5 shows an example of the single-hop WDM network based on the use
of abroadcast star. This network, called theLambdanet[28], is an example of the
broadcast-and-selectnetwork. The new feature of the Lambdanet is that each node
is equipped with one transmitter emitting at a unique wavelength andNreceivers op-
erating at theNwavelengths, whereNis the number of nodes. The output of all
transmitters is combined in a passive star and distributed to all receivers equally. Each
node receives the entire traffic flowing across the network. A tunable optical filter can
be used to select the desired channel. In the case of the Lambdanet, each node uses a
bank of receivers in place of a tunable filter. This feature creates a nonblocking net-
work whose capacity and connectivity can be reconfigured electronically depending
on the application. The network is also transparent to the bit rate or the modulation
format. Different users can transmit data at different bit rates with different modulation
formats. The flexibility of the Lambdanet makes it suitable for many applications. The
main drawback of the Lambdanet is that the number of users is limited by the number