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Dark fiber
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In fiber optic communications, dark fiber or unlit fiber (or fibre) refers to unused fibers, available for use.
The term was originally used when talking about the potential network capacity of telecommunication infrastructure, but now also refers to the increasingly common practice of leasing fiber optic cables from a network service provider.
reason that dark fiber exists in well-planned networks is that much of the cost of installing cables is in the civil engineering work required to install the cables.
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Encyclopedia
In fiber optic communications, dark fiber or unlit fiber (or fibre) refers to unused fibers, available for use.
The term was originally used when talking about the potential network capacity of telecommunication infrastructure, but now also refers to the increasingly common practice of leasing fiber optic cables from a network service provider.
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 the civil engineering work required to install the cables. 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.
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.
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 has been lit by a fiber lessee and not the cable's owner.
Dark fiber overcapacity
In the dot-com bubble, a large number of telephone companies (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.
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 a factor of as much as 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.
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 cannibalise 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 specialising 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 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.
In the U.S. Level(3) Communications acquired a number of dark fiber providers in 2005–2006, and took their dark fiber off the market. AboveNet still markets dark fiber and is one of the only national providers left.
Dark fiber has been, and still is, available for sale on the wholesale market for both metro and wide area links, but it may not be available in all markets or city pairs. Prices for dark fiber may sometimes be lower than the price of a high speed leased line rental. In the UK there are few dark fiber providers, with the majority of carriers providing leased lines and network services over fiber. AboveNet owns and operates a London-based dark fiber network with lit services also offered. In addition, FiberSpan sell dark fiber metro and wide area network links, becoming a dominant player due to its microduct technology.
Dark fiber capacity is typically used by network operators to build SONET and dense wavelength division multiplexing (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 Ethernet and 10 gigabit Ethernet over single-mode fiber. Running Ethernet networks between geographically separated buildings is a practice known as "WAN elimination".
Other variations
- 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 using a transponder tuned to the assigned wavelength. DWDM systems generally require central management because their closely spaced wavelengths are subject to disruption by signals on adjacent wavelengths that are not within tightly controlled parameters, especially if amplification is required for signal transmission over 100 km.
- Virtual dark fiber using wavelength multiplexing allows a service provider to offer individual wavelengths ("lambdas" or "colours"), where access to a dark narrowband wavelength-division multiplexing (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. This is typically done using coarse wavelength division multiplexing CWDM because the wider 20 nm spacing of the wave bands makes these systems much less susceptible to interference.
Rate of Expansion
According to Gerry Butters, the former head of Lucent's Optical Networking Group at Bell Labs, Moore's law holds true with fiber optics. The amount of data coming out of an optical fiber is doubling every nine months. Thus, excluding the transmission equipment upgrades, the cost of transmitting a bit over an optical network decreases by half every nine months. The availability of dense wavelength-division multiplexing DWDM and coarse wavelength division multiplexing CWDM is rapidly bringing down the cost of networking, and further progress seems assured.
See also
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