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Shining Light on Dark Fiber

Posted by Dee McVicker on 3/13/15 1:37 PM

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Lighting up Dark FiberDark fiber would make a great name for a cereal brand.

At least then we’d know what to expect, unlike the leased dark fiber options companies are faced with these days.

Just because fiber optical cabling is labeled “dark fiber,” it doesn’t necessarily mean that it is so. Not in the true sense of the term, anyway. 

Dark fiber is commonly used to describe the optical cabling that was laid down and left unused during the dotcom glut when these communication links were built to overcapacity. This is the fiber that is literally “dark.” There are thousands of miles of dark fiber available in the U.S., and it’s becoming an affordable and popular way for companies and universities to add to their data capacity without having to trench in their own optical fiber. The cost is relatively inexpensive these days. 

All you have to do is lease dark fiber and connect to – or light it up. We know of many companies that connect to dark fiber using wavelength division multiplexing (WDM) for combining, say, 16 sources of data on a single fiber pair at data rates from 155 megabits to 3 gigbits per channel.

But, not all dark fiber is the same. The term dark fiber has been hijacked by some providers who actually lease unused capacity on their existing fiber networks; it is not at all the stuff that we’ve come to know as unlit, virgin fiber. 

Knowing the difference is important, as it will affect the scalability of your leased communication link. True dark fiber gives you access to the full fiber pair for multiplexing at full capacity several channels onto that pair using WDM. The other, or managed dark fiber, only provides you with a slice of that multiplexed capacity, sometimes at the same price or for more than you’d pay for the full enchilada. 

For most organizations, this is a critical point. True dark fiber can be much more cost effective because as your bandwidth requirements grows, your leased “dark” fiber can grow too and provide lower incremental costs per Mb. 

The reason is that true dark fiber lets you multiplex multiple channels onto it, whereas managed dark fiber has already been multiplexed onto and therefor doesn’t offer end-user scalability. If you tried to connect a wavelength division multiplexer onto managed dark fiber to run multiple data channels down that cable, you would be disappointed. It just simply won’t work. The multiplexer needs all of the available wavelengths for aggregating channels. WDMs cannot be added to managed dark fiber for the simple reason that these cables already have active devices attached to them in order to allocate the bandwidth that you’re leasing! 

How can you tell the difference? If your service provider starts talking about OC transmission rates, such as OC12, OC48 or any other OC designator or discuss datarates at all, you can expect that the leased fiber line is managed dark fiber. A true dark fiber connection will require that you set up the transmission yourself, typically using a WDM on both ends of the cable. 

If you understand the difference between the two, at least you’ll know what to expect.


Topics: Fiber 101, WDM