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and the backbone network. A metro wavelength-division-multiplexing (WDM) network adopts
the WDM technology [13-14] to provide multiple wavelength channels for this purpose.
Figure 1 shows one of the popular architectures for metro WDM networks. In this
architecture, a metro hub connects the local networks to the backbone network via optical
fibers. There are N nodes where node i is connected to the local network in region i
( 1
≤≤ − ) and node N is connected to the wide area backbone network. Each node can
send data to any other nodes through the metro hub. It is desirable that the metro hub only
uses passive optical components for low cost and high reliability. In the literature, two types
of hubs were proposed and they are described in the following.
iN
1
Figure 1. Metro WDM network in which each node carries the traffic of many users.
The first type of metro hubs is called passive-star-coupler hub [15-17]. This metro hub
uses a passive star coupler to provide W wavelength channels to its input-output pairs [15-
17], where the passive star coupler is a passive optical component and W is the number of
wavelength channels in a fiber. Figure 2 shows an example of a passive-star-coupler hub with
W =4. The hub can provide W wavelength channels to the node pairs. If the number of nodes
N is equal to the number of wavelength channels W , then node i can be assigned a dedicated
wavelength channel at λ i for reception (this is shown in the example in Figure 2). If node j
wants to send data to node i , node j transmits at wavelength λ i . On the other hand, if N is not
equal to W , the N nodes can share the W wavelength channels through a multiple access
control protocol.
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