Getting on the road with a dark fiber network

The foundation for Wavelength Division Multiplexing (WDM) is the possibility to send all sorts of data over fiber networks in the form of light. WDM makes it simple to maximize the capacity and performance of a fiber network, but it can be challenging to understand and illuminate the concepts of WDM for others in an organization. Before going into the specific benefits of WDM, it’s helpful to understand the basics of fiber and the fiber network. This article introduces some of the important elements using an analogy that most people can visualize easily: an expressway network.

How a fiber network is like an expressway

Imagine you’re the owner of a road that links together several cities that are just starting to grow. You’re in charge of how much traffic can travel on it, so you want to make sure it’s the right size for what the people in those cities need. At first, it only needs a few lanes in each direction. Each one can carry a huge variety of traffic: high-performance sports cars traveling at 200 miles per hour, family sedans, buses, slow-moving construction equipment, and everything in between.

As the cities grow and get busier, you might want to add more lanes in each direction so that more people can travel to, from, and between them. You’ve built your road so that it’s possible to increase traffic capacity as needed, so there’s no need to start from scratch. You expand your multi-lane road into a full expressway, with dozens of lanes on each side, so that the road’s traffic capacity scales in line with the needs of the cities it connects. Now it can handle huge amounts of traffic, in both directions. That all makes sense, right?

Now instead imagine that you decided to keep the road in its original form, with just one lane in each direction to each handle a single lane of traffic in and out of the city. This would be a disaster: you’d have to choose which traffic you could accommodate, and which you wanted to disregard, and that might not align with the needs of the people who are actually using that traffic. To meet their requirements and keep those cities thriving, you’d need to build a new road for every type of traffic, starting over from scratch each time: planning, designing, excavating, paving, and then maintaining each road.

The cost of building and operating a single road is high enough, so why add a whole new set of running costs for each one?

Yet this is exactly how some organizations treat their precious fiber assets. A cable can be split into multiple traffic channels and simultaneously transport multiple data channels over it – much like adding new lanes on an expressway to make traffic more efficient. But many organizations aren’t making the most of what they have, or they’re using fiber network solutions built for industries that have very different needs.

However, by using Wavelength Division Multiplexing (WDM), organizations can get the most out of existing and new dark fiber investments. It’s a simple way to build a scalable expressway rather than a single road.

An introduction to dark fiber

A prerequisite for any kind of fiber networking is access to fiber. Without it, there is no chance of operating any kind of optical network on geographically separated sites.

In some cases, organizations choose to lay their own fiber, but in many cases, the fiber network is already there. Many organizations choose to lease dark fiber from a provider, which is a more accessible option, but also means that the type of fiber is in the hands of the provider.

Dark fiber is generally also referred to as unlit fiber. It’s like a road that is built and ready but isn’t yet open to traffic. As soon as traffic is run on it, however, the fiber is known as lit fiber.

It’s possible to create a WDM network on either a single or dual fiber.

Singel fiber strand

In a single fiber, two wavelengths are used to create a single communication channel. One wavelength is for transmission, and the other is for receiving. This type of fiber is similar to an undivided highway, with a dashed line down the middle that allows traffic to pass in either direction. In this single strand of optical fiber, we can consider that the traffic traveling in one direction is being transmitted (Tx), and the traffic in the other direction is the traffic being received (Rx).

Fiber pair

The most common way of transporting optical traffic, however, is by using a fiber pair. One of the fibers is used for transmitting the data and the other is used for receiving the data. In our analogy, it is like a road separated by a barrier in the middle. Now, instead of an undivided highway used for traffic in each direction, each direction of traffic now has its own highway.

 WDM adds more lanes to make the highway more efficient

Just as we can paint lines on the road to separate the traffic and increase the capacity of the motorway, WDM allows us to split the capacity of the fiber in several channels – like the “lanes.” Because the traffic is in the form of light, there can be as many channels as there are wavelengths in the spectrum that makes up white light. Typically, 40 or 80 channels are used in metro networks. 

In this way we can create a series of virtual fibers instead of actually having to use multiple fibers for each and every service. Fiber rental or purchase is often expensive, so if an existing fiber asset can be used to transport multiple traffic channels then the potential cost savings are huge. In this way, WDM maximizes the usefulness of that fiber and helps optimize network investments.

Explaining this article to a non-technical person:

Before delving into the details of how wavelength division multiplexing works, it’s important to get a basic grasp of what dark fiber is and how it works. A good analogy to use is that of a road connecting two fast-growing cities. It’s important to build the right road, to maintain and be aware of the quality and conditions of that road, and to make the most of its potential. The same goes for a fiber network, which, like a road, can carry traffic in both directions, and if used efficiently, can be scaled up or down in capacity without needing a new fiber network investment for every new traffic channel.