What is the difference between fiber optic and dark fibers?

Question

If dark fibers are not using to transmit data, can I know what is the functionality?

Answer 1

Fiber optic cable is the cable used to transport data.

A fiber optic cable has numerous strands per cable. The labor costs of installation is the major install cost. Installers usually install several cables with numerous strands per cable - far more than now needed - because of the high labor costs of install and because the need will develop in the future.

Each cable that is put into service is "lit" that is they pass the data down the fiber with laser light and it is detected on the other end. Unneeded and excess strands are not lit and are called dark fiber or unlit fiber. At some future date, when demand requires added transport capacity, these strands will be lit.

Answer 2

Dark fiber is optical fiber infrastructure (cabling and repeaters) that is currently in place but is not being used. Optical fiber conveys information in the form of light pulses so the "dark" means no light pulses are being sent.


The transmission of data in the form of pulses of light. Fiber optics uses cables containing glass or silica fibers no thicker than a human hair. There is very little signal loss, and information can be transmitted at high speed over long distances

or in short :

Dark fibers are fiber optics cables that are NOT transmitting data

Fiber Optics are optic cables that are transmitting data

Answer 3

You surely have listened the term “dark fiber” and wonder what it is about. The term refers to optical fiber infrastructure that hasn’t been lit yet, meaning it is installed but it isn’t being used.

As it is known, fiber optic cables are made of thin strands of glass or plastic through which data moves as light pulses, caused by LEDs transmitters if it is multi-mode or a much more sophisticated technology if it is single-mode, so dark fiber is called like that because it isn’t active and there are not light pulses traveling through it.

Back in the 1990’s, companies installed fiber optic infrastructure all over U.S, especially in places with high-bandwidth demand like New York or San Francisco, with the hope that demand would keep growing, but at the time it didn’t and the cost was 60 bankruptcies.

Nevertheless, the demand has grown since 2009 due to video streaming and smartphones –back in 2011, AT&T an 8,000% growth in mobile data demand– and telecom industry started laying fiber optic networks again where they were needed as well as giving use to dark fiber by leasing it to clients.

What is it for?

Nowadays, Telecoms like Allied Fiber and Zayo Group rent dark fiber to local broadband and content providers, healthcare institutions, government agencies and businesses which need to control their own network, so instead of leasing a service they lease an infrastructure that allows them to build their own network with their own equipment.

Leasing dark fiber offers a lot of benefits to the end user, for example avoiding to spend time and capital in the construction, development and maintenance of their network. Also, in contrast with renting a bandwidth service, dark fiber provides low-latency with very high throughput.

According to Arch Fiber Network, dark fiber gives the client the capacity to increase bandwidth as needed without paying any monthly additional cost and to upgrade the equipment at their convenient time too, instead of waiting for a carrier.

The research Dark Fiber Lease Considerations says dark fiber providers price it per strand per mile and in a set period. Usually, they give the client the Indefeasible Right of Use (IRU) for 20 years in a one-time payment along with annual payments for Operation and Maintenance. Customers can also rent fiber for one, two or three years doing monthly payments with the option of leasing the infrastructure for other 20 years once the initial term is over.

Disadvantages

There are thousands of miles of dark fiber deployed across the country and still it isn’t everywhere because it is only available where companies installed it or where they got permission to do it, so not all hospitals or businesses have access to dark fiber.

In “The Dark Side of Dark Fiber” is explained that when dark fiber cover long distances it needs amplifiers and those amplifiers need maintenance which means spending more money. It also says that long-haul dark fiber is exposed to accidental damages that have to be fixed by the company that leases the fiber, so reparations might take time.

Substitute

Wavelength services are an alternative when companies don’t want to lease an entire fiber optic infrastructure, because it allows them to buy a specific wavelength of an existing fiber from a service provider and getting the exclusive use.

This method is known as Wavelength Division Multiplexing (WDM) and it transmits several wavelengths over a fiber optic, which means data traffic is shared, but each wavelength has a specific laser with a specific color, thus clients can get service from 1GBps up to 100GBps.

Renting dark fiber might be a solution to many businesses high-bandwidth demand, but being clear about its advantages and disadvantages is the best way assure a good decision is being taken.

Answer 4

"dark fiber" means it is not being lit up by a laser / LED.
As in, the fiber optic cable is just sitting there chillin' out.