Is DWDM the "next big thing" for SANs?

Alan Earls

Analysts take note of the bandwidth boosting powers of Dense Wavelength Division Multiplexing

Add another buzzword to your storage lexicon...Dense Wavelength Division Multiplexing (DWDM) may be about to burst from the arcane terminology of communications and straight into the mainstream of storage-speak, according to analysts.

Briefly, DWDM packs so much speed and bandwidth into optical transmission systems that storage assets at a distance -- perhaps even hundreds of miles away -- can be accessed as easily as on-site devices.

Conceptual Roots

In the early days of fiber-optic transmission -- the 1970s and early 1980s -- telecommunications-network developers were attracted by optical fiber's low loss, low weight and by its inherent protection against tapping (no one, then, had been able to tap an optical fiber). Optical fiber allowed developers to bridge long distances with a small number of repeater stations. These repeaters, however, required optical/electrical conversion and vice versa. The fiber's transmission capacity -- the maximum number of bytes that can be transmitted over a connection within a certain time slot -- seemed to be unlimited when compared to the number of bytes per second involved in global voice/data communications traffic at the time.

Today, with the commercial availability of optical amplifiers, it's a standard practice to use single-mode, optical fibers for long-distance transmission. Due to the evolution

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of the Internet and the growing development of international electronic commerce and communications, however, the availability of single-wavelength, long-distance links has been exhausted.

Developers chose to address this problem by deploying more fibers, and by using cost-effective, wavelength-division multiplexing (WDM) technology. WDM technology is currently in use to retrofit and boost current fiber-transmission capacities.

Previously -- in long-distance links -- the optical amplifiers already included in the system were used to determine the wavelength range available for multiplexing. In order to offer more multiplexing options for data transmission, engineers then developed Dense Wavelength Division Multiplexing (DWDM).

With DWDM, a number of signals coming from different transmitters with different wavelengths are combined into one fiber using an optical multiplexer. At the signals' destination, an optical demultiplexer splits the various wavelengths into different fibers and guides them to their distinct receivers for conversion back into electrical signals.

The scheme has the noticeable advantage of being transparent for data protocols. Basically, it is possible to mix analog and digital signals or, more commonly, digital signals of different protocols. The scheme has the noticeable advantage of being transparent for data protocols. Basically, with DWDM, it is now possible to mix analog and digital signals or, more commonly, digital signals of different protocols.

DWDM, is the future here?

Jeff Harrow, in his Compaq-sponsored technology newsletter, "The Rapidly Changing Face of Computing," notes that DWDM has been violating Moore's law -- the proposition that computer power will continue to double every 18 months -- by doubling much faster. DWDM, "and other advances, have led to the doubling of fiber capacity every SIX months -- far faster than both Moore's Law and storage's [capacity] increases," he writes.

Dataquest, for their part, says in a report issued in July 2000, that they expect global telecommunications carriers will use DWDM to increase the capacity of their system in the near future. They estimate that current DWDM technology can create up to 80 separate, 10-Gbps channels on a single fiber strand. Furthermore, Report Author Colin Hay reckons DWDM is perhaps the best of several alternatives for telecommunications carriers trying to keep up with bandwidth demand. Advantage include its range -- up to at least 1,000 kilometers, its protocol independence, and its ability to support existing fiber networks while leaving the door open to other technology options such as ADSL or Gigabit Ethernet.

Dataquest concludes that "in the three to five-year time frame, the combination of 10-Gbps Ethernet and DWDM could be the next logical step in the evolution of the public telecommunications network."

Furthermore, a white paper recently posted by EMC says that their company's start in DWDM came as a result of efforts by its major financial and dot-com customers to marry storage technologies with the DWDM technology provided by Nortel Networks. Now, the two firms have combined key technologies to pursue a vision of "fault-tolerant, scalable, and distance-insensitive storage offerings." Still, white papers aren't the same as product, so we asked analysts what they see on the horizon.

Bottom-line predictions

What does the advent of DWDM mean in terms of storage planning? Steve Duplessie, senior analyst at the Enterprise Storage Group, in Milford, Mass., says that DWDM will allow for very inexpensive bandwidth in emerging Metropolitan Area Networks. This, in turn, should extend SAN distance and capabilities. And, he adds, "We expect people to start rolling out DWDM SAN installations before year-end."

"The fact that you can now use 60 or more frequencies at a time permits all kinds of economies," notes Dan Tanner, an analyst with the Boston-based Aberdeen Group. "That means you can expect a great many more storage networks connecting campuses and metropolitan regions," he says. However, he warns, connections over longer distances may still be problematic due to the fact that DWDM still cannot erase latency issue.

"When economics make something available, suddenly people find ways to use it," adds Tanner. "Clearly it will ratchet thing up a bit."

Alan Earls is a freelance writer in Franklin, Mass.
Related Topics: SAN management, VIEW ALL TOPICS

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