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Assigning wavelength reach affects all-optical performance

Posted: 17 Feb 2003 ?? ?Print Version ?Bookmark and Share

Keywords:photonic switching? all-optical switching? wavelength reach? optical networking? optical-electrical-optical interfaces?

During the past three years, some of the most significant advances in optical networking have been made in the areas of system reach and photonic - or all-optical - switching. The combination of increased reach and photonic mesh connectivity results in reach that can be applied to the (A-Z) traffic across a network, rather than just a single link. The resulting 'transparent network' extends the concept of regenerator elimination from the line to the nodes.

The elimination of unnecessary intermediate optical-electrical-optical (OEO) interfaces between DWDM systems results in greater than 50 percent network cost reduction. However, this photonic mesh does not completely eliminate OEO interfaces, since signal degradation due to distance and intermediate switching elements still results in some connections requiring reach extension or wavelength conversion at the photonic node. A close look at the A-Z traffic demand shows that unique start and endpoints drive a wide variety of reach requirements for individual wavelengths in a given network, a term we call 'network-derived' reach.

When technologies used in DWDM systems, such as EDFAs, Raman amplifiers, and dispersion compensators are combined with different transmission fiber properties, channel performance is affected in different ways across the DWDM spectrum. As a result of these spectrally dependent effects, individual wavelengths have different reach capabilities from one another.

This causes problems in traditional DWDM setups that focus on reducing line regenerators between endpoints of the system. The idea is to ensure that all wavelengths reach the maximum limit of the system, but this design typically lowers the average reach of the system to accommodate the poorest performing wavelength. This compromises the performance potential of the best channels.

Since transparency allows wavelengths to extend beyond the limits of a single link, it presents an opportunity to remove that compromise, by dynamically assigning wavelengths based on connection-length requirements. This way, the overall system capacity and reach can be used more efficiently. Longer connections would use better performing wavelengths, and therefore, achieve longer effective reach. Naturally, shorter connections use the poorer performing wavelengths. The result is a net reduction in the number of regenerators across the network.

The "agile reach" model requires that the system be equipped with full wavelength tunability to enable unconstrained selection of a wavelength upon setup of the connection. Furthermore, intelligent control and routing systems are required to maintain information about network attributes such as performance and interaction (due to system fill) allowing for the intelligent assignment of future wavelength setup requests. Due to the variability of a network's characteristics, it is imperative that the system be capable of dynamically measuring and updating this information to ensure optimal wavelength selection in all cases.

By allocating the A-Z connection distance to a wavelength of comparable reach performance, the overall system capacity and reach can be used more efficiently. This method allows longer demands to use better performing wavelengths, and therefore achieve longer effective reach. Naturally, shorter A-Z requirements utilize the poorer performing wavelengths. When the full performance range of the system can be fully utilized, the result is a net reduction in the number of regenerators across the network.

- Greg Penz

Director, Product Management

Innovance Networks Inc.

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