They are mainly intended to be used as backbone transport networks. On top of this structure is then a more flexible switching protocol applied that handles routing of logical channels to provide more flexible services, such as multicast. However, this layer inherits the static nature of the underlying infrastructure and does not give a flexible and efficient use of the fibre capacity. In wavelength routing every sending node is connected to every receiver through a different wavelength. By tuning to a selected wavelength, the signal is routed to the selected receiver.
Several approaches exist.
Either the sender tunes to the fixed wavelength of the receiver or the receiver tunes to the senders fixed wavelength. Obviously some sort of pre-transmission coordination and synchronization is required between the parties that intend to exchange information. Both these schemes imply some sort of global knowledge of the other nodes in the network and at what wavelengths they are working. A severe problem with WDM is the synchronization, both bit synchronization and between wavelengths. Some kind of pre-transmission coordination and synchronization is needed between stations which intend to communicate.