Today's end-to-end OC-192-and-beyond carrier technologies call for a best-of-breed mix of OEO and photonic elements. All-optical switching solutions are effective for optical add/drop multiplexers (OADMs), network nodes where most traffic is expressed without processing; or in network nodes where part of the traffic needs to be dropped and continued to other nodes.
All-optical switching is also crucial in optical cross-connects (OXCs) where fibers carrying large number of wavelengths need to be switched. Ideally, OEO conversion should occur only at the exact network nodes where that information is to be processed, not at the many interconnect points on the way.
That said, the ideal optical network that fueled most of the late 1990s telecom hype is not really that far from reality. It will probably happen 10 to 15 years later than anticipated as a slow evolution of the current networks.
Optical performance monitoring
All of these components are available today at different levels of maturity. For some, the performance is still not sufficient; for others, the reliability might not be proven, and in some cases the entry-price level is too high. Nevertheless, as all these factors improve with time and development effort, they will be designed into existing networks, transforming them piece-by-piece into the fully optical network.
Consider two specific examples of the gradual evolution occurring these days: the optical add/drop multiplexer and the optical cross-connect. In both examples, the target is to push OEO to the edge of the network and increase the network flexibility as new technologies mature and become available.
The ability to add and drop channels to and from a DWDM link along the network is one of the basic requirements for a DWDM optical network. The emphasis is on dropping some but not all of the traffic at each node. The ultimate requirement would be to drop and add any one of the 200 existing channels at any point.
To achieve that requires large port-count filters-that is, arrayed waveguide grating (AWG), and large switching fabrics. Currently, fibers carry up to 40 channels, and adding or dropping is done using fixed-wavelength filters such as thin-film filters or fiber Bragg gratings. These constitute the static OADM (S-OADM). In a system based on S-OADM, channels within the DWDM network are pre-assigned between fixed nodes at the time the network is set up, leaving no flexibility to accommodate changes in the traffic load or new required services.
One of the key elements for adding flexibility to S-OADM is an optical switch that can instantly modify the optical connectivity. Adding stand-alone optical switching units to an existing S-OADM gives flexibility to the whole network, migrating to reconfigurable OADM (R-OADM) and later on to dynamically reconfigurable OADM (DR-OADM).
Having an R-OADM in place allows for adding several more wavelengths on the existing fixed ones. These new wavelengths can be remotely configured to connect any node to any other node within the network, to accommodate new services or relieve congestion. Furthermore, using optical switches with multicast capabilities enables features such as drop-and-continue, where a small part of the optical power is dropped and the remaining power continues to the next node.
Moving to DR-OADM further increases flexibility, allowing routing of specific wavelengths to specific ports or customers. Again, using multicast-capable switches would allow dropping the same signal to several different customers. Although not the ideal solution, this example shows one possible step in the right direction.
The second example employs an OXC that connects several input fibers, each containing many DWDM channels, to several output fibers and allows switching of any channel within any of the input fibers to any channel within any of the output fibers. Taking, for example, four input fibers with 80 channels in each and four output fibers would require a 320-by-320 optical switch.
On top of that, to allow full connectivity and avoid channel conflict, we would need wavelength conversion to cover the cases where two channels with the same wavelength have the same destination fiber. Several technological barriers are still present in the technologies for high port-count switching and wavelength conversion.
Moreover, the entry-level price is too high to justify implementing these large systems. Instead, a simpler solution for an OXC that's available today uses a WS-OXC having limited connectivity, compared to a fully blown OXC. In a WXC, you can switch any channel in any of the input fibers to the same channel (wavelength) in any of the output channels, but no wavelength conversion is possible.
Although limited in connectivity, the suggested solution is built on existing components. It uses 80-channel multiplexers/demultiplexers (such as AWG) and M number of small N-by-N ( e.g. 4-by-4) switch matrices. When wavelength conversion becomes available the N-by-N matrices would be replaced by (N+1)-by-(N+1) matrices, thus allowing one channel per wavelength group to go through wavelength conversion. This approach removes blocking and enables a completely flexible OXC.
Reuven Duer is director of physics and customer support at Lynx Photonic Networks (Calabasas Hills, Calif.).
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