Choosing Between Dark Fiber and Wavelengths Comparing fiber optic options that include SONET, Ethernet, wavelengths and dark fiber.
By: John Shepler
Fiber optic services are popular for high bandwidth metro and long haul connections. They typically start about 100 Mbps and go on up to 10 Gbps, 40 Gbps and occasionally 100 Gbps bandwidths. But what if you find that fiber optic bandwidth services are too restrictive for your needs? There are a couple of alternatives. One is leasing dark fiber routes and the other is wavelength services. Let’s see how they compare and why you might find one more attractive than the other.
Traditional Fiber Optic Services
Fiber optic bandwidth services are standardized, packaged services that offer either SONET or Ethernet connectivity from point to point. You may think of them as private line services, but chances are that you are not the only customer riding on the fiber strand. At lower bandwidths, such as OC3, you are probably being electrically multiplexed with other users to create a much larger bandwidth signal that is more cost effective to transport. If you are using a 10 Gbps or 40 Gbps service, you are still sharing the strand with other users but optically multiplexed using wavelengths.
Why Use Dark Fiber?
One problem with these pre-packaged fiber optic services is that they generally limit you to a particular protocol and speed. If you change your mind, it can be time consuming and costly to change or upgrade your service. For this reason, some companies have decided to become their own bandwidth providers by leasing dark fiber and lighting it up with equipment they own. This gives you the ability to have multiple protocols running on the same fiber at whatever bandwidth makes sense. When you want something different, your own engineering staff reconfigures or changes out the equipment one each end. There’s no waiting around for carrier personnel because you are the carrier. The dark fiber is just an empty pipe, as the name implies.
Wavelength Division Multiplexing
The trick to having multiple protocols on one fiber is called WDM or Wavelength Division Multiplexing. It’s somewhat akin to the idea of electrical multiplexing in that many different signals can share the same transport mechanism. However, technically it is quite different. WDM uses multiple wavelengths or colors of laser light that are separate enough in frequency to be distinct. All of these colors, also called lambdas, exist in the infrared portion of the light spectrum that is seen as nearly transparent through glass fiber.
Think of WDM as shining a whole rainbow of laser beams down a fiber optic cable where they are detected separately at the other end. Each beam is independent of the others. One color doesn’t care that it is running Ethernet and the next color up is carrying SONET or ESCON. They don’t see each other and don’t interfere.
Wavelengths Are Available
Since WDM is an established technology running on just about all carrier fiber optic networks, there is no reason why you can’t just lease an entire wavelength instead of a service running on that wavelength. That’s fairly easy to do these days because many networks have excess wavelength capacity available. You can have your own wavelength running a high bandwidth Ethernet service (10 GigE) or 10 Gbps or 40 Gbps SONET.
Wavelengths vs Dark Fiber
Why lease a wavelength instead of dark fiber? For one thing, you may not need the entire capacity of the fiber or all those wavelengths you can create yourself. There’s considerable capital cost in purchasing the WDM systems for each end of the fiber that you can avoid by just leasing the wavelengths you need by the month. Capital expense and network management then become the service provider’s problem.
Wavelengths Until Dark Fiber is Available
Dark fiber may not be available for the entire route you have in mind or you may be planning to install your own fiber in a particular area. Until it is ready, you can make good use of existing capability by leasing wavelength services as long as needed.
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