FAQ

What is dark fiber?

Dark fiber refers to fiber-optic cables that have already been deployed either in underground conduits or on aerial utility transmission facilities but that have not yet been "lit"—that is, equipped with the optical electronics needed to transmit information over significant distances.

Dark fiber (aka “unlit” fiber) is usually dedicated to a single customer, where the customer is responsible for attaching the telecommunications equipment and lasers to "light" the fiber. During the late 1990s, so much fiber had been installed worldwide in anticipation of Internet demand that there is still, in 2010, a huge amount of fiber that has never been lit. Moreover, the ongoing development of the optical transmission technology known as dense wave division multiplexing (DWDM) allowed a single-fiber pair to carry many more channels of information in parallel. This development increased the capacity of the new fiber-optic infrastructure geometrically, with a single-fiber pair now capable of carrying well over a Terabit per second of aggregate data.

When deciding to use dark fiber, enterprises control the actual fiber and choose their own service provider they wish to connect to for services such as telephony, cable TV and Internet.

Professional third party companies who specialize in dark fiber systems take care of the actual installation of the fiber and also maintain it on behalf of the customer. These companies own the fiber, but grant IRUs (Indefeasible Rights of Use) for a contractual period for unrestricted use of the fiber.

There is no additional management complexity or overhead associated with dark fiber. In fact, in many cases dark fiber may be more reliable than traditional telecommunication services and easier to manage because it vastly simplifies the network architecture and allows the consolidation of network services to a central hub.

An IRU is an Indefeasible Right to Use the fiber. Most companies who offer dark fiber grant IRUs to the fiber. For regulatory reasons, generally only licensed carriers are allowed access to support structures and municipal rights of way. Rather than selling title to the fiber the fiber contractors grant a 5-20 year IRU. An IRU can be used as collateral, sold or traded and otherwise treated like a physically owned asset.

With dark fiber, simple laser devices, called transceivers, are all that is required to light the fiber; these are at the core of DWDM systems.

What can go wrong?

With these networks there are only 3 things that can go wrong with dark fiber - the source DWDM, the destination DWDM or the fiber itself.

How reliable is dark fiber?

Dark fiber can be more reliable than traditional telecommunication services, particularly if the customer deploys a diverse or redundant dark fiber route.

Because fiber has a greater tensile strength than copper or even steel, it is less susceptible to breaks from wind or snow loads.

For the greatest reliability, many customers will install 2 separate dark fiber links to 2 separate service providers. Even with the additional fiber for redundancy, dark fiber networks are cheaper than managed services from a carrier.

With dark fiber customers have a number of choices in terms of reliability and redundancy:

• They can have single unprotected fiber link and have the same reliability as it exists today with their current carrier;
• They can use alternative technology, such as a wireless link for backup in case of a fiber break; or
• They can install a second geographically diverse dark fiber link whose total cost is still cheaper than a managed service from a carrier.

Does dark fiber increase my network management costs and complexity?

Network cost and complexity is significantly reduced with dark fiber in a number of ways:

• Dark fiber has no active devices in the path as in a typical carrier network; there are fewer devices to manage and less things that are likely to go wrong.
• Repair and maintenance of the fiber is generally included in the service contract from the fiber provider. These companies are in many cases the same companies that carriers use to maintain their fiber.
• Dark fiber allows enterprises to centralize servers and/or out source many different functions such as web hosting, server management, etc.

A fiber network allows large enterprise customers, universities and schools to essentially extend their in house LANs across the wide area. Because there is no effective cost to bandwidth with dark fiber the long distance LAN can be still run at native speeds with no performance degradation to the end user. It is therefore very simple to relocate a server to a distant location where previously it required close proximity because of LAN performance issues.

Who manages and repairs the dark fiber in case of breaks or other problems?

In most cases management of the fiber is included in the service contract from the fiber provider. In many cases these are the same companies who maintain and repair for the major carriers. They offer the same terms and conditions to dark fiber customers as they do for the major carriers. In many cases the companies that installed the fiber are also the ones who maintain the fiber. These companies will also look after any on-going moves, adds and changes as well as relocating the fiber in case of road construction and so forth.

What about moves, adds, changes and fiber relocation services?

Moves, adds and changes are generally carried out by the fiber provider on a routine basis. Most moves, adds and changes only require breaking and fusing together existing fiber pairs. The work can be done on an hourly rate, or priced on a per move, add or change basis. The cost, terms and conditions for moves, adds, changes are usually included in your fiber maintenance agreement.

On very rare occasions fiber has to be relocated because of road construction or repair. Usually the city that had undertaken the road work will pay for the majority of the fiber relocation costs. However, if this not case, minor relocations of several hundred meters are usually included as part of the maintenance contract.

If a major relocation is required most contracts give the customer the option paying their share of the relocation expenses in proportion to the number of fibers they own in the cable or canceling the fiber contract.

Although uncompensated fiber relocations are very rare, it is a factor that should be taken into account when procuring dark fiber.

Is fiber the same fiber used by the carriers?

Yes. In most cases for metropolitan and long haul fiber the most common of fiber by carriers is Single Mode Fiber (SMF-28). This is usually adequate for most fiber installations. For particularly long spans or long distances, specialty Non-Zero Dispersion Shifted Fibers (NZDSF) are used. But given the cost difference, single mode fiber is generally the fiber of choice for most fiber installations.

All splices are performed with an industry-accepted fusion splicing machine and the customer is provided with testing documentation reflecting bi-directional losses by fiber and installed span loss by fiber.

What happens in case of fiber break?

Fiber breaks are very rare and typically any given span of 10 km of fiber will suffer an average of less than 5 minutes of outage due to breaks in a year.

If there is a fiber break, the fiber provider will specify an average repair interval. The repair interval for aerial fiber is usually much shorter than buried fiber. A normal fiber break outage for aerial is 4 hours and 6 hours or longer for buried fiber. However these are average repair intervals. With buried fiber, in particular, where there may be water and mud the repair time can be considerably longer.

The fiber provider will splice fibers tube by tube or ribbon by ribbon or fiber bundle by fiber bundle, rotating between tubes or ribbons operated by the separate users of the cable, including the customer, in accordance with a priority and rotation provided for in the maintenance contract. In general, priority among users of the fiber cable affected by a cut shall be determined on a rotating restoration-by-restoration and segment-by-segment basis, to provide fair and equitable restoration priority to all users of the cable, subject only to such restoration priority to which is contractually obligated by the fiber provider.