Dark Wavelength vs Dark Fiber for 10 GigE

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 much larger 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.


Note: Spectrum of light wavelengths though a prism animation courtesy of Wikimedia Commons. 

Ethernet over Dense Wavelength Division Multiplexing (EoDWDM)

Organizations with very high bandwidth requirements, such as financial institutions, content delivery networks, video production houses, research labs, hospital groups, data centers, may need more performance than standard telecom line services can provide. One approach is to build-out a custom fiber network by installing your own fiber or leasing dark fiber strands. Another is to lease wavelengths on already lit fiber.

Your Own Fiber
Installing your own fiber on campus makes a lot of sense, since you control the property as well as the networking assets. Once you leave your premises, however, fiber construction gets very expensive very fast… even if you can obtain the necessary right-of-way permissions.

Using Dark Fiber
This is where dark fiber has found a niche. What is dark fiber? It is simply fiber optic cable that has been installed in the ground or on utility poles but never activated or “lit.” The cable may have been put in place in anticipation of future needs. Often, some strands in the cable are already in use, but there are other fiber pairs still dark.

Dark fiber has the advantage of already being in place and giving you the most flexibility in what you want to do with it. However, dark fiber assets aren’t always available where you want to go. You also have the responsibility of supplying the maintaining the terminal equipment that powers the fiber. This may be more costly that many organizations can afford and, perhaps, unnecessary.

Using Lit Fiber
The opposite of dark fiber is lit fiber. The fiber is not only installed, but the terminal equipment has been put in place and is operational. This is most often done by service providers who offer bandwidth to companies and other organizations that can use it and are located in the “service footprint” of the carrier.

Lit fiber is now popular for private connections and dedicated Internet access from 10 Mbps on up to OC-48 (2.4 Gbps) and Gigabit Ethernet (1 Gbps). A given carrier may offer either SONET, Ethernet or both protocols. All size organizations are moving from copper to fiber bandwidth services as fiber becomes more available and costs decline.

What if you need more bandwidth, higher security or more flexibility in protocols? Are you back to leasing dark fiber or installing your own?

Advantages of Wavelength Services
There’s an intermediate step that is perhaps the perfect solution to many demanding needs. This is wavelength services. While fiber cables have multiple glass strands or fibers, each fiber strand can also be divided into multiple colors or wavelengths. This is done by shining a few to dozens of different frequency laser beams down the glass fiber. Since each beam has its own wavelength, it operates independently of the other beams. It’s like fibers within fibers.

DWDM or Dense Wavelength Division Multiplexing technology can provide over 100 different wavelengths per fiber pair. This makes it very cost effective to transport private line connections for many users on a fiber network. It’s quite practical to lease one or more wavelengths for your high bandwidth applications.

Ethernet on Wavelength Services
Ethernet over DWDM or EoDWDM is a way of extending your high performance local network across town or around the country. It’s an Ethernet connection that’s available in bandwidths from 1 Gbps up to 100 Gbps and readily scaled up and down as needed. Popular service levels are 1 Gbps, 2.5 Gbps, 10 Gbps and 100 Gbps.

This is dedicated service because no one but you is using a particular wavelength. You have all the bandwidth to yourself with the security of not having other users data streams multiplexed on your circuit.

With wavelength services, such as EoDWDM, the burden of running the network is left to the service provider. Wavelengths are often called managed wavelength services because the service provider maintains the fiber, terminal equipment & repeaters plus monitors operation 24/7. You simply connect at each end. You can opt for protected service that switches to a new wavelength or fiber if the circuit fails and even diverse routes so that no one disaster is likely to knock out service completely.

Do you find yourself in an increasing bandwidth situation and wanting to consider all the options available for your particular locations? If so, get fiber optic network services and pricing, plus recommendations from expert consultants.

Dark Fiber Services to 400 Gbps

It’s clear that bandwidth demands are on the rise. Even the smallest companies are swapping out their 1.5 Mbps T1 lines for 10 Mbps Ethernet over Copper. Medium size companies are increasingly moving up to 50 or 100 Mbps connections. Large companies are climbing the Gigabit ladder higher and higher.

Usually we expect available bandwidth to top out at 5 or 10 Gbps. Sometimes carriers can support 40 Gbps on their networks while they move up to 100 Gbps themselves. Now, one competitive carrier is setting a new benchmark at 400 Gbps.

Integra Telecom is one of the country’s leading providers of fiber optic networking solutions. They own and operate a long-haul fiber network that stretches for 5,000 route miles. This is expanded by 3,000 miles of metropolitan fiber connecting more than 2,200 enterprise buildings and data centers.

This describes only the lit and operational fiber assets provided by Integra. Among their 160,000 miles of fiber assets are dark fiber pairs just waiting to be pressed into service. If your requirements go beyond the usual WAN requirements, dark fiber may be the answer to your high bandwidth needs.

What is dark fiber? It is essentially fiber optic strands in a fiber bundle that are not in use. Fiber itself is cheap compared to the cost of burying it in the ground. That’s why anyone installing fiber cables over any distance makes sure that there are many, many extra fiber strands beyond those they expect to use. If you install too small a cable and see your traffic grow in the future, it gets very expensive to duplicate your network by installing more fiber. Since fiber strands are about the size of thick hairs, why not install a cable that has 100 or more pair even if you are planning to light only a dozen?

Integra has lots of fiber beyond their immediate or expected needs. Rather than let all of that unused capacity sit idle, they are offering it to large enterprises, healthcare organizations, governmental agencies and educational institutions that can put it to good use. Their options include bundles, lease and Indefeasible Rights of Use (IRU) that offer complete fiber pairs dedicated to your exclusive use.

Why would you choose to use dark fiber instead of other bandwidth services? Bandwidth is certainly a driver. A single pair of dark fiber supports up to 400 Gbps of lit transport. A pair is used to provide protection against failure in a single fiber or the equipment it connects.

How is it possible to get this much bandwidth from one fiber pair? It has to do with how you transmit your data. The glass fiber itself has nearly unlimited bandwidth, at least for any applications we can envision now. The inefficient way to transport data is to feed one end of the fiber with a laser of a certain color and modulate that beam with your data. Sure, you could get 5 Gbps or more using this technique. You certainly won’t get 400 Gbps. For that massive capacity you need to use the fiber more efficiently.

Right now, the go-to technology for increasing fiber capacity is called DWDM or Dense Wavelength Division Multiplexing. Instead of sending one beam down the fiber core, you send dozens or even a hundred or more beams simultaneously. Each is a different wavelength or color called a Lambda. You split up your data stream at the sending end and recombine it at the receiving end.

Of course, you can treat each wavelength as a separate and individual transmission path. This is most often how DWDM systems are set up. Each channel is a high bandwidth communications link completely independent of the others. There is no requirement that the channels use the same protocol or run at the same speed.

Another big advantage of dark fiber is network security. You and you alone have use of the fiber strands. There are no other customer’s signals multiplexed with yours as they generally are on the usual data services. This can be especially advantageous for companies with very high security requirements, such as banks and governmental agencies.

You should also know that there is another step between fiber optic bandwidth services and dark fiber. This involves leasing a complete wavelength on a lit fiber link. You have the security of being the only traffic on that wavelength and avoid the cost of buying and operating your own terminal equipment.

Are your bandwidth needs well beyond the services that are being offered commercially? Dark fiber pairs may be the solution to your demanding requirements. It’s readily available in many business locations.



Note: Picture of glowing fiber optic strands courtesy of Wikimedia Commons.