As optical networks evolve, performing restoration at the optical layer can provide one of the greatest potential cost savings. By implementing a restoration scheme at the optical layer, optical nodes can perform protection for all the wavelengths on a path, with switching times similar to that of current electrical SONET rings (see Figure 15). Because protection is performed in the optical layer, the electrical systems do not need the extensive protection architectures that have been required historically, which provides tremendous cost savings to network providers. In addition, optical-layer restoration allows better wavelength utilization by implementing 1:N protection in the SONET/SDH layer.

Figure 15. Optical Reconfiguration Performance

Several methods of protection can be implemented in the optical network, all of which are logically similar to their electrical counterparts.

Link

Link restoration is perhaps the simplest to implement in the optical network. A link restoration routes the optical path across an alternate link between sites, providing protection in case of a fiber or equipment failure. Although providers can dedicate fibers for a protection link, it is usually not cost effective to do so. While a link restoration scheme can provide full restoration for a single link failure, when fibers are shared for working or protection, it provides less than full protection for multiple failures.

Path

Using a 1:N path-restoration scheme allocates a disjointed path for an end-to-end connection, but the alternate path is not dedicated for each connection. So again, although this method provides full restoration for single link failures, it can provide less than full protection for multiple link failures.

Hybrid

A restoration similar to link protection is hybrid restoration. Hybrid restoration provides protection for each link but attempts to improve fiber utilization by eliminating the backhauling of traffic. To accomplish this, the switching near the failed link takes place on nodes that might not be adjacent to the failure.

Ring

Perhaps the most robust protection architecture for optical networks is the optical ring. Optical rings operate identically to their electrical ring equivalents, with the same architectures and alternatives available. Although they require more fiber than other restoration schemes, optical rings provide the highest level of availability. By partitioning wavelengths into groups, network planners can switch certain wavelengths in the optical layer while still performing switching for existing systems in the SONET layer. The partitioning allows a smooth evolution to optical rings.