Dark fiber

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In fiber-optic communications, dark fiber or unlit fiber (or fibre) is the name given to individual fibers that have yet to be used within cables that have been already laid. They are hence not yet connected to any device, and are only there for future usage.

The term was originally used when talking about the potential network capacity of telecommunication infrastructure, but now also refers to increasingly common practice of leasing fiber optic cables from a network service provider.

1 Dark fiber for capacity expansion
2 Dark fiber overcapacity
3 The dark fiber market
4 References
5 See also
6 External links

[edit] Dark fiber for capacity expansion

One reason that dark fiber exists in well-planned networks is that much of the cost of installing cables is in so-called civils - the civil engineering work required in order to get the cables installed. This includes planning and routing, obtaining permissions, creating ducts and channels for the cables, and finally installation and connection. This work accounts for more than 60% of the cost of developing fiber networks, with only a relatively small proportion actually being invested in the optical fiber cable and high-tech networking infrastructure. ^{[citation needed]}

It therefore makes sense to plan for and install significantly more fiber than is needed for current demand, to provide for future expansion and provide for network redundancy in case any of the cables fail.

Additionally, many fiber optic cable owners such as railroads or power utilities have always added additional fibers for lease to other carriers.

In common vernacular, dark fiber may sometimes still be called "dark" if it's been lit by a fiber lessee and not the cable's owner.

[edit] Dark fiber overcapacity

In the dot-com bubble, a large number of telephone companies (or telcos) built optical fiber networks, each with the business plan of cornering the market in telecommunications by providing a network with sufficient capacity to take all existing and forecast traffic for the entire region served. This was based on the assumption that telecoms traffic, particularly data traffic, would continue to grow exponentially for the foreseeable future.

Unfortunately for them, the collapse of the dot-com boom left fiber supply greatly exceeding demand of even the least optimistic forecasts by a factor of up to 30 in many areas. The availability of wavelength-division multiplexing further reduced the demand for fiber by increasing the capacity that could be placed on a single fiber by as much as a factor of 100. As a result, the wholesale price of data traffic collapsed. A number of these companies filed for bankruptcy protection, or went bankrupt, as a result.

Just as with the Railway Mania, the misfortune of one market sector became the good fortune of another, and this overcapacity created a new telecommunications market sector.

[edit] The dark fiber market

For many years, incumbent local exchange carriers would not sell dark fiber to end users, because they believed selling access to this core asset would cannibalize their other, more lucrative services. Incumbent carriers in the US were required to sell dark fiber to competitive local exchange carriers as Unbundled Network Elements (UNE), but they have successfully lobbied to reduce these provisions for existing fiber, and eliminated it completely for new fiber placed for fiber to the premises (FTTP) deployments.

Competitive local carriers were not required to sell dark fiber, and many do not, although fiber swaps between competitive carriers are quite common. This increases the reach of their networks in places where their competitor has a presence, in exchange for provision of fiber capacity on places where that competitor has no presence. This is a practice known in the industry as "coopetition".

Meanwhile, other companies arose specializing as dark fiber providers. Dark fiber became more available when there was enormous overcapacity after the boom years of the late 1990's through 2001. The market for dark fiber has slowly tightened up with the return of capital investment to light up existing fiber, and with mergers and acquisitions resulting in consolidation of dark fiber providers. Dark fiber has been, and still is, available for sale on the wholesale market for both metro and wide area links. Prices for dark fiber may sometimes be lower than the price of a high speed leased line rental.

There is a market segmentation between dark fiber providers and lit service providers, although in some cases both models may be used by the same entities.

Dark fiber capacity is typically used by network operators to build SONET and DWDM networks, usually involving meshes of self-healing rings. Now, it is also used by end-user enterprises to expand Ethernet local area networks, especially since the adoption of IEEE standards for Gigabit and 10 Gigabit Ethernet over single-mode fiber. Running Ethernet networks between geographically separated buildings is a practice known as "WAN elimination".

Managed dark fiber is a form of wavelength-division multiplexed access to otherwise dark fiber where a simple "pilot" signal is beamed into the fiber by the fiber provider for management purposes.

Wavelength multiplexing allows a service provider to offer virtual dark fiber to customers, offering individual wavelengths or lambdas (λ) as individual dark fibers.

There is also a market in "colors" where access to a dark narrowband WDM optical channel is provided over a wavelength division multiplexed fiber network that is managed at the physical level, but unlit by the network provider.