Optical Technologies
The WDM Architectures

WDM network architectures [53] can be classified into two broad categories: broadband and select architectures and wavelength routing architectures.

A broadband and select (B&S) WDM network has different nodes transmitting at different wavelengths. Their signals are broadcasting by a passive device in the middle of the network to all the nodes. In this case, this device is a passive optical star coupler. The coupler combines signals from all nodes and delivers a fraction of the power from each signal on to each output port. Each node employs a tunable optical filter to select the desired wavelength for reception.

This form of a network is simple and suitable for use in local- or metropolitan-area networks, such as access networks. The number of nodes in these networks is limited because the wavelengths cannot be reused in the network and because the transmitted power from a node must be split among all the receivers in the network.

A more sophisticated and practical architecture today is that employs a wavelength routing. The nodes in the wavelength routing (WR) WDM network are capable of routing different wavelengths at an input port to different output ports. This enables us to set up many simultaneous light paths using the same wavelength in the network; that is, the capacity can be reused spatially. These light paths all use the same wavelength on every link in their path. This is a constraint that we must deal with if we do not have wavelength conversion capabilities within the network. This architecture also avoids broadcasting the power to unwanted receivers in the network. Thus these networks are suitable for deployment in metropolitan- and wide-area networks, such as local-exchange and interexchange networks.

Ideally, all the functions inside the node would be performed in an optical domain, but in practice, certain functions, such as a processing the header and a controlling the switch, get relegated to an electronic domain. This is because of the very limited processing capabilities in an optical domain. The header itself could be sent at a lower bit rate than the data so that it can be processed electronically.