6.1.1 Connections in WDM Networks

WDM networks based on wavelength routing and or add/dropping can offer:

• Optical transparency. This means that a variety of traffic formats and rates can be transmitted on the physical layer without modification to the optical node equipment.

• Smaller processing load and reduced latency in intermediate nodes by bypassing optical traffic not destined to the node at the WDM layer level in both circuit and packet-switch networks.

• Terminal equipment savings as only a subset of wavelengths are necessary at each node.

The overall network is generally described as a set of superimposed layers of connections: physical layer (with fiber sublayer and optical layer), and logical layer (logical connection and logical path). The "virtual" connections between the end systems are supported by a network of logical connections in the logical layer. The connections are one to one, many to one, one to many, or multicast. In a transparent optical network the routing uses wavelength paths. If the wavelengths can be changed along an optical path, the path is called a "virtual" wavelength path.

Designing a full-transparent optical network may imply many unsolved problems. Full transparency means that no digital electronic processing is used and that the network optically performs all filtering, and space and wavelength switching with or without wavelength conversion along all network routes. It can be penalizing to pass through cascades of multiplexers, demultiplexers, passive or active routers, cross-connectors, add/droppers, and different kinds of switches at the different nodes, but moreover, all-optical operation,...