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Choosing Dark Wavelength vs Dark Fiber
Tradeoffs between dark wavelength and fiber options for high bandwidth fiber optic service.

By: John Shepler

Organizations that require extremely high MAN (Metropolitan Area Network) bandwidth or need to run a multitude of protocols have looked to dark fiber as a technical solution. Dark fiber is still a good choice for the most demanding applications, but there is a competitive service you should also be aware of. It’s called dark wavelength or dark lambda. Let’s look at what each has to offer and how they compare with the more common point to point and MPLS network connections.

What is a Dark Wavelength?
We think of wavelengths as colors of light, not something dark. A wavelength with no color would essentially be turned “off” and would be the same as a dark fiber, right?

Not really. The difference is that dark fiber is nothing but the glass fiber strand itself. There is no equipment attached. Somebody, and that somebody is likely you, has to install terminal equipment at each end and turn on one or more laser beams to “light” the fiber. Sometimes "managed" dark fiber is available, where the carrier will provide the terminal equipment but you'll still have exclusive use of the fiber strand.

With a dark wavelength, this has already been done. It’s not a fiber with a single color laser at one end and a detector at the other. Instead, the equipment that has been installed is DWDM or Dense Wavelength Division Multiplexing gear. What DWDM does is send multiple non-interfering laser beams down the same fiber strand to create what amount to additional channels equivalent to multiplying the number of fiber strands available.

With DWDM there is still only a single physical glass strand. The multiplexing or creating of multiple virtual fibers out of one is based on the fact that glass is transparent to more than a single color or wavelength of light. You can easily see this with a common prism. Shine white light in one side and bands of color appear on the other side. The prism shows that a number of separate colors can travel through the glass without interfering or canceling each other out.

Say you want to create a dozen different wavelengths on a single fiber. You’ll need a DWDM system that contains a dozen laser transmitters, each tuned to a slightly different wavelength or color. In practice, all the colors are in the infrared part of the spectrum and not visible colors. Nonetheless, they are referred to as colors, wavelengths or lambdas (the Greek letter used to denote wavelength).

Why go to all this trouble and expense? Simple: To multiply the capacity of a fiber cable. There are two ways to get more bandwidth from a fiber bundle. Either add more strands or use more of the inherent capacity of each strand. Adding strands means running an additional cable along the same route or replacing the cable you have with a larger diameter one that has more of the hair-thin glass fibers. Both options are incredibly expensive. Getting more from the infrastructure you already have is very attractive by comparison.

This is why DWDM is so popular. Why pull many miles of new cable at a cost of millions of dollars when you can upgrade your terminal equipment for a fraction of the cost? DWDM is a well established and standardized technology. Why not let technology save you the cost and delays involved in upgrading the physical cable?

The Dark Wavelength vs Dark Fiber Tradeoff
If you lease a dark fiber strand, you have exclusive use of that piece of glass. You have the security of knowing that only your traffic will travel over that strand. There will be other customers using other fiber strands, but there is a physical separation between you and them.

Dark fiber also gives you the flexibility of using any protocol you want and even setting up multiple protocols on the same fiber. How do you do that? By installing DWDM equipment of your own to create multiple independent wavelengths.

You can see how this can get to be expensive fast. A simple one wavelength fiber strand is one thing. You can probably get 10 Gbps bandwidth on that strand with simple equipment. DWDM is another matter. Now you need a fairly sophisticated piece of equipment at each end that you have to install, pay for and manage. For that, you can create multiple independent channels of, say, 10 Gbps each.

Instead, why not let somebody else bear that expense? That’s the basis of dark wavelength services. Someone, the carrier or service provider, has already lit the fiber with DWDM equipment that they own and operate. However, they don’t need all of the wavelength capacity themselves. If there is only enough traffic to use half the wavelengths available, the others can be leased to help pay for the system.

A dark wavelength is simply an unused wavelength on an existing fiber optic system. To be truly dark, the laser for that wavelength may be turned off or there is no card in the system for that particular channel. Either way, once someone agrees to lease the wavelength, service can be turned up fairly quickly. After all, the fiber and the DWDM chassis are already in place and operating.

Why Choose Dark Wavelength?
One reason to opt for dark wavelength service is that it may be all that is available. The owner of the fiber network may not be willing to lease an entire strand. That’s especially true if they have already lit their strands and are using some of the wavelengths.

Another attraction of dark wavelengths is equipment cost and maintenance. In theory, a provider could require you to purchase the channel cards that are compatible with their DWDM system and light the wavelength yourself. More likely, they will handle that themselves. You may or may not be asked to pay a one-time installation fee that includes the cost of the wavelength cards.

If the common carrier turns up the wavelength and maintains the system, you will have exclusive use of that particular wavelength at a bandwidth of 1, 5 or 10 Gbps. Sometimes wavelengths can be aggregated to create higher bandwidths up to 100 Gbps. Either way, only your traffic will be carried on that wavelength as whatever protocol you choose. Other customers will have their traffic on other wavelengths, but the different color beams will not interact.

Other Options
Not everyone needs or wants dark fiber or wavelength service. Many businesses only need 100 Mbps or Gigabit service. Both traditional SONET or the newer Carrier Ethernet protocols are generally available as point to point connections, ports to muchlarger MPLS optical networks, or Dedicated Internet Access.

What bandwidth option is best for your applications? Why not discuss your needs and get competitive quotes from multiple carriers for a range of services that can meet your needs. Then you can compare costs and benefits and pick the high bandwidth fiber optic service that best meeds your requirements.

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