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Building up to transparent and reconfigurable optical networks

Posted: 01 Nov 2002 ?? ?Print Version ?Bookmark and Share

Keywords:agile optical network? optical-electrical-optical? oeo switching? photonic cross-connects? reconfigurable optical add/drop multiplexer?

Transparent and reconfigurable, or agile, optical networking promises both economy and flexibility, but its promise has yet to be fulfilled. Even transparent but static networking is challenging, as is evident in first-generation, ultralong-haul transport systems. These systems economized by eliminating many OEO conversions, but were inflexible and time-consuming to install, turn up. and manage. They extended the limits of capacity, reach, and performance - but offered only limited adaptive and management capabilities to deal with this environment.

Conversely, reconfigurable but nontransparent networks such as conventional-reach transport systems and OEO switching, are more feasible and flexible. They also cost more, due to the higher number of expensive per-channel OEO conversions and the expense of deploying those numerous devices. The number and types of drawbacks will increase as networks scale to higher capacities.

To achieve both economy and flexibility, transparent, and agile optical networking must adopt advanced switching and adaptive technologies as well as optical performance and fault management capabilities.

The first phase of agile optical networking is the transport layer, which consists mainly of optical amplifiers, DWDM muxes/demuxes and transponders. Optical amplifier and transponder-related technologies enable long optical reach by dynamically dealing with optical performance impairments. ROADMs and photonic cross-connects (PXCs) are also able to compensate for certain impairments.

The use of agile optical networking may significantly reduce optical reach. Loss and related optical signal-to-noise ratio (OSNR) penalties, as well as concatenated filtering effects that certain switching technologies introduce, account for this reduced reach. The longer reach allows for switching-related impairments, enables optical restoration on typically longer routes, and absorbs the effects of non-ideal network realities such as mixed fiber types.

In an agile network, individual wavelengths can be reconfigured and can transverse different fiber routes more than other wavelengths. Therefore, even if switching components were free of impairments, propagation penalties due to loss, OSNR, dispersion, and nonlinearity can change from wavelength to wavelength, depending on the different fiber parameters encountered. Dispersion is a major impairment, especially for higher rates like 40Gbps.

Thus, the compensation strategy must shift from being only a fixed bulk treatment across all wavelengths, as is used for static point-to-point transport systems. This type of shift is made possible by better slope-matched bulk dispersion compensation fiber modules that reduce the residual per-channel dispersion. The smaller residual can then be compensated for in transponders, for example, by the use of fiber grating technology.

To enable long reach, OSNR and nonlinearities must be balanced. Related technologies include variable optical attenuators (VOAs) or amplifiers and dynamic equalizers for flattening gain. These devices may operate over many wavelengths, but operating per-wavelength provides better resolution. From inherent wavelength visibility, such devices are now being integrated into certain switching technologies, a beneficial example of integrated transport and switching.

An important aspect of "switch-ready" transport is the ability to switch wavelengths in and out of a transport system without affecting in-service wavelengths. In uncontrolled (failure) situations, optical power transients are minimized by rapid automatic transient suppression mechanisms in optical amplifiers that maintain the power levels and performance of in-service wavelengths.

The final transport requirement is integrated optical performance and fault management, which speeds system installation and wavelength turn-up and simplifies ongoing maintenance. For example, the optical spectral analyzer allows per-wavelength performance monitoring (OSNR) to trigger pre-emptive maintenance action when performance degrades. Another tool that ensures optical path integrity before turning-up a new wavelength is an optical time-domain reflectometer (OTDR). The ability to remotely control this tool and view its results from any location simplifies and reduces the operational cost of service turn-up.

ROADMs are the next step in agile optical networking. Using ROADMs at two-way network sites can provide managed optical pass-through and serve as an economical means of adding and dropping traffic without full back-to-back terminals or PXCs. ROADMs can be highly integrated with the transport system, such as with two-fiber port liquid crystal or 1D MEMS technologies. And, apart from the greatly simplified fiber management advantage, integration also enables transport and switch interworking by including dynamic level equalization to achieve long optical reach and traverse practical numbers of such ROADMs and/or the PXCs.

The integrated approach is optimized for low-loss and low-cost pass-through traffic that can inherently grow with no network changes or service hits. Add/drop traffic has more loss and cost, but is only at end points and can also be applied on a 'pay-as-you-go' basis--consistent with nearly all static optical networks that can migrate to increase agility as service needs arise, technology matures and costs decrease.

- John Gruber

Director , Networking Architecture

Ceyba Inc.

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