Getting on the road
to dark fibre

Imagine building a motorway linking together several cities. One capable of transporting huge amounts of traffic, up to say 80 lanes  in each direction. Then imagine that each of these lanes is completely independent to each other and  able to carry high-performance sports cars at 200mph (assuming that 200mph is permissible!), family sedans travelling at 80mph, and slow-moving trucks at say 50mph, and everything in between. Ideal situation, right? Then imagine that you owned that motorway and could control how much traffic could travel on it. In the early days there might only be one or two lanes of traffic in use. But that’s ok. The connection between these cities is in its infancy but will grow. Since  the cost of the motorway is the same if it is built for one lane or 80  in each direction, then this really doesn’t matter anyway. There is a likelihood that the route will get busier with more traffic going over that route.  What if you didn’t have the lanes on this motorway  and instead  had one large lane in each direction where you could only transport a single traffic channel?  It would be a disaster because then you would have to build a second, third or potentially up to 80 motorways to get the required amount of traffic between sites.

Yet it’s the way some people  use their precious fibre assets. A fibre cable, commonly referred to as a dark fibre, has the ability to be split into multiple traffic channels and simultaneously transport multiple data channels over it. There are only a few basic building blocks required to build such a network, and they are not as difficult as you may think. You need:  access to the fibre (the motorway), a multiplexer (the separate lanes on the motorway) and the individual motorcars to travel in each of the lanes (transceivers).

The motorway

The first step is to stop using the motorway for  a single vehicle, or in networking terms, stop using an ELWL circuit to link sites. ELWL is an extra-long wavelength circuit which is usually limited to 25km and has a wideband signal which takes up all the motorway (fibre). These ELWL modules (referred to as transceivers) are then replaced with narrow band modules (similar price point to ELWL) but are able to travel on individual motorway lanes (or wavelength channels, in optical terms).

At the start and end of the motorway, there is a  termination point. This termination point is a multiplexer in optical terms. A multiplexer is a unit which combines the individual traffic signals on to a single dark fibre network and allows multiple traffic channels and types to be connected together and transported simultaneously.

The motor car

Now that the multi-lane motorway is in place (the multiplexer provides the individual lanes), the different traffic types need to be added.  Let’s continue the motorway analogy for a moment and imagine that the individual vehicles have a tank of fuel and that tank of fuel determines how far they can travel. These fuel tanks typically have the capacity to travel anywhere between 100meters and 200km. (In optical terms this refers to the power budget of the transceiver, and instead of a fuel tank they have an optical transmitter that determines the distance the signal can be transmitted.) So first of all we need to determine how far the motorway stretch is. A tank of petrol capable of travelling 50km shouldn’t be used if the sites are 100km apart. Then it needs to be determined if the traffic is made up of sports cars, trucks, family sedans, motorbikes, and if so, how many of each. These vehicle types should be considered as the types of traffic (data, voice, storage, video, data, etc). Once it is known how much traffic is required both now and in the future, the right amount of traffic lanes can be printed on the motorway (or in optical terms the right multiplexer can be selected depending on number of channels). Remember, the motorway lanes (multiplexer channels) are completely independent from each other, so the fibre can be used for any specific and unique traffic requirements. These vehicle types are the form factors of the transceivers (typical versions are CFP, QSFP28, QSFP, SFP+, XFP, Xenpak). Since they need to travel on specific channels of the motorway, they need to be channel specific, or in optical terms, wavelength specific, which is where the term DWDM comes from: dense wavelength division multiplexing.

Turbulent weather

There can be external conditions that can affect the performance of the system. Using the motorway analogy again, if the road goes downhill in a straight line with the wind from behind, then the car will travel much further than if it has to go uphill over a bumpy road with road work and a headwind.  In optical terms this is why it is vitally important to understand the length of the fibre AND the losses in the fibre. Just as a driver wouldn’t travel on an unknown journey with the wrong amount of fuel in the tank, then a network shouldn’t be built without knowing the fibre conditions. It is vitally important to understand the losses in the fibre, so that the correct building blocks can be put in place. If a length of fibre is 80km but has many patches and high losses, then the signal may need some amplification to bridge the gap. Think of the potholes in the road as the losses in the fibre. This is overcome by amplification to give the signal more strength. Think of the amplifier as the refuelling station which can be located at the front of the network to give the tank an extra fill, in the middle of the network when the tank runs low, or at the end of the network to bring the traffic over the line.

There you have it. A simple and powerful optical network designed to maximise the potential of the dark fibre asset. The benefit:  There is no impact on the monthly opex bill, and given its  small footprint with no power, noise, fans or emissions, it provides the ultimate green data center networking solution. The same principle as single channel ELWL connectivity, but with many lanes of traffic instead of one.

Embedded WDM – It really is that simple


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