Fibre Channel SANS can be extended over wide areas using optical networking, specifically dense wave division multiplexing (DWDM). To understand why we must peer into optical networking for a few paragraphs.

Telcos use optical networks to carry their traffic across continents and cities, so-called metro distances. An optical network pulses light signals down a glass fibre cable and the pulses carry bits of information.

Fiber Optic Transmission Systems (FOTS) is the generic term for systems which transmit data using electromagnetic energy in the form of light waves. Such systems are termed to be photonic, although they actually are electro-opto-electronic (EOE) in nature, as the signal both originates and terminates in systems that are electrically based. FOTS generally are digital in nature, although in rare instances they can be analogue.

FOTS in long-haul carrier applications make use of optical fibres that cause very little distortion or attenuation (loss of signal strength). That means that the optical repeaters (amplifiers) that clean up and boost the signals can be spaced at intervals of tens, or hundreds, or even thousands of kilometers in some cases.

FOTS can support huge bandwidth, which in contemporary systems can be in the range of Gbit/s or even Tbit/s. Each fibre in a multifibre cable is a self-contained world of bandwidth. Carriers routinely deploy hundreds of fibres along the same route at the same time; the dark fibres can be lit as necessary. The issues of limited radio spectrum, line-of-sight, EMI, and signal attenuation that so dramatically plague the alternative transmission systems really don't apply to FOTS.

Dense Wavelength Division Multiplexing (DWDM) is loosely defined as a transmission technique involving eight or more wavelengths of light. Contemporary systems in commercial use employ as many as 32 wavelengths, although 160 wavelengths have been deployed in select systems, and many more have been demonstrated under laboratory conditions.

As each wavelength functions at a different frequency each, in effect, is a totally separate carrier. This minor miracle is achieved by tunable lasers which emit light signals at different wavelengths, with each wavelength being introduced through a separate window into a common optical fibre. Matching light detectors at the receiving end of the system each sense a given wavelength, ignoring the others.

The advantages of pure DWDM are several. First, DWDM provides the ability to increase the bandwidth of a single fiber to incredible proportions. For example, 32 wavelengths (lambdas), each operating at an OC-192 rate of 10 Gbit/s, add up to a total of 320 Gbit/s.

DWDM has at least one clear disadvantage. Each wavelength must be treated individually. For example, if 32 wavelengths share one optical fibre and a repeater must be placed at some point to clean up and boost the signal, the 32 must be separated, repeated individually, then recombined and sent on their way.

Back to SANs
Given this bandwidth DWDM provides its possible to have SANs that span intercontinental distances. You could have your servers in Boston, New Work, Houston, Los Angeles and Seattle and the drive arrays in Phoenix and Kansas.

Of course you have to rent the cable infrastructure's bandwidth and that means all the repeaters and amplifiers en route. A supplier called Optovia reckons it can drop the number of amplifiers needed.

Optovia says that by achieving twice the spacing of any amplifier on the market today, it's Hut-Skip optical amplifiers allow network operators to reduce the number of amplifiers in their networks by 50 percent, along with the remote outside structures (known as "huts") that house them. In doing so, Optovia's Hut-Skip amplifiers significantly reduce the initial capital expenses and ongoing operational expenses associated with optical networking.

The Hut-Skip Line System, Optovia's flagship solution, features an industry-leading cascadeable site spacing of up to 180 km. The solution delivers a best-in-class feature combination of high gain and low noise, which enables Hut-Skipping at 10 Gbit/s and 40 Gbit/s for metro and long haul networks.

Optovia and MCI completed a field trial of Optovia's new Hut-Skip Line System. The companies demonstrated the high-speed network application with standard fibre over 1600km with amplifiers placed 160km apart, not the traditional 80km apart.

By increasing amplifier spacing from the traditional 80 km to 160 km, and eliminating every other hut, the trial proved how to dramatically reduce the costs of building and operating a long haul network.

These repeaters/boosters are also changing to ones that can be remotely managed. A Telco does not just carry SAN traffic. Instead telcos have to switch traffic between various nodes in their network. Optical add/drop multiplexers (OADMs) are used to add new colours at a switching node or to drop colours.

Thus a purple colour carrying data for a TV broadcast can be set up to be routed from London to Paris and then Milan. Meanwhile, a yellow colour carrying voice traffic can also go from London to Paris but then be switched to Berlin. Previously a man in a van had to go out to a switching node and reset the OADMs. By having remote OADMs (ROADMs) this resetting can be done from a central site by sending out digital control signals.

Back to SANs again
What this means is that a telco like KPN or MCI or Verizon or BT can offer cross-country networking fast enough to build and operate a SAN. The Fibre Channel traffic is overlaid on the DWDM signals. Or, conceivably, IP is used to carry the Fibre Channel signals. Of course it will be costly. But through evolving optical technology reducing the number of repeaters/boosters and making them remotely configurable the infrastructure costs and infrastructure operating costs are decreasing.

Given sufficient demand then larger enterprises can build cross-country SANs. This means that disaster protection can be inherent. You make sure that two drive array sites in a SAN back each other up through replication or mirroring. It means that wide area file systems (WAFS) also become built-in.

For the cost to reduce enough for 'everyday' enterprises to use this kind of technology will take years. But there is a kind of intercontinental and cross-country SAN creep going on with a growing number of enterprises finding that such large scale wide-area SANs are affordable and are worth having.