Feature

Dense wavelength division multiplexing for disaster recovery

Ezine

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The case for DWDM
DWDM hardware sits between your existing networking equipment and the fiber optic cable(s) extending to your remote location (see "DWDM can transport multiple protocols" on this page). Without a doubt, the greatest benefit of this technology is its ability to use an optical multiplexer to gather incoming light signals from your voice and data networks, and then provision them onto a multiwavelength, single-mode fiber optic cable in different wavelengths to be identified, split and placed onto the appropriate protocol interface in the DWDM equipment at the other end of the fiber optic cable. Signal integrity and performance is maintained because light signals are gathered at their source and then multiplexed, amplified and sent across the fiber optic cable at levels greater than 80 wavelengths with current product offerings.

What this ultimately means is that instead of each application requiring a separate fiber optic cable to support each extended E_Port, for example, each application would be provisioned a different wavelength on the same fiber optic cable. If you need more capacity or performance on the long-distance SAN link, then increase the number of varying wavelengths sent over the glass cable and that's exactly what you will get.

And if the ability to add capacity and performance to the long-distance SAN link by increasing the number of wavelengths isn't enough,

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some DWDM hardware solutions come with the ability to provide enhanced resource management or QoS to applications by allowing the network engineer to prioritize traffic by protocol or by endpoints. With this QoS feature, not only will you be able to assign multiple applications to the same physical cable, you will also be able to control which wavelength will emerge from the other end of the cable first. That way, both critical and noncritical applications can share the same long-distance optical medium.

Not only can DWDM systems accommodate multiple protocols within a single box, but they can do so over the same fiber cable as well. By giving the user the ability to send IP, FC, ESCON and ATM, for example, over the same cable without sacrificing performance and without propagating error conditions across frequencies, hardware engineers have engineered the ultimate bridge in connecting voice and data networks across distances. However, like all new technologies that solve a multitude of problems, only the rich can afford DWDM systems. But as time passes and more solutions come to market, prices are likely to continue to fall.

What's best for you?
Choosing a long-distance SAN link solution isn't a simple decision. More than just a messaging network, this extended SAN link will transport n times more data than a messaging network placed simply for remote management or e-mail access from some central repository. For this reason alone, a sufficient amount of time must be allotted to gather trending information with regard to the data that will be read and written across the link by your production applications.

Scalability in the realms of capacity and performance using DWDM is achieved by increasing the number of wavelengths sent over the fiber optic strand. However, with an IP optical pipe, bandwidth can only be increased by your carrier, and only in the steps defined by the "OC" levels (OC-3, OC-48, etc). Depending on the carrier, that may happen with the click of a button or take days. And although IP optical pipe is sufficient for the majority of long-distance SAN solutions, DWDM's ability to place up to 80 different wavelengths on a fiber strand can't be matched when considering fiber exhaust.

For SAN links that are fewer than 120 kilometers and which require support for transporting multiple protocols, again, DWDM technology should be given top consideration. However, for longer links in which an optical repeater is not a viable solution, an expandable optical WAN pipe supporting IP will have to do the job. Next month, we'll look at your options for that scenario.

This was first published in September 2003

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