DWDM for long-distance SANs

DWDM for long-distance SANs
Rick Cook

There are several ways to multiplex signals. Multiplexing is the process of somehow mixing different signals in one transmission medium (sometimes called a pipe) in such a way that the original signals all can be sorted out at the far end. One traditional multiplexing (MUX) technique is time-division multiplexing (TDM), in which the signals are chopped into little time slices, and each slice shoved down the pipe, where they are recovered at the far end. Frequency-division multiplexing, (FDM) the other standard technique, impresses the signals on different frequency carrier signals and frequency filters (demultiplexers) separate the signals at the far end of the pipe (AM and FM radio work this way).

This tip discusses the application of a new multiplexing technique, Dense Wavelength Division Multiplexing (DWDM) to SANs. DWDM is a way to multiplex optical carriers on a given pipe, and promises extremely wide bandwidth communications for SANs.

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When you think of upgrading your SANs, or maybe of installing one in the first place, be sure to ask about Dense Wavelength Division Multiplexing (DWDM). If it's available when you ask, then it could save you some money and give better performance that you are used to.

DWDM is a major technology for extending the range of SANs. It does not require a fiber dedicated to the SAN and it allocates bandwidth by what is needed rather than by fixed slots like Time Division Multiplexing (TDM). This means DWDM makes more efficient use of optical fiber than other systems, such as TDM.

DWDM works by assigning each signal to a separate band of frequencies and then using optical amplifiers to increase the signal strength of all of them simultaneously. DWDM can send as many as 80 signals simultaneously over a single fiber today, and systems that can divide a single fiber into 100 channels or more should be available soon. The beauty of DWDM for SAN applications is that the interface to the shared fiber can be bit-rate and format independent instead of converting all the signals into the same format as is done under most other shared-channel schemes. That means that a Fibre Channel signal from a SAN can share the fiber with higher-speed and lower-speed signals from other sources, using protocols such as ATM and SONET, without degrading the performance of any of the signals.

The International Engineering Consortium has a tutorial on DWDM available on its web site at: www.iec.org/online/tutorials/dwdm/index.html.

Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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This was first published in October 2001

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