You may know the basics of fiber: glass or plastic fibers that serve as conduit for pulses of light to travel over distances. The light carries data, allowing fiber optic cables to be used as an alternative to traditional metal cabling. Optical fiber has benefits including less signal loss and immunity to electromagnetic interference (EMI).

But not all fiber is created equal. One choice that you may find yourself encountering is Single Mode (SMF) or Multi Mode (MMF) fiber. What do these qualifiers mean? Good question. Let's dig deeper.

 

Judging by name alone, you might assume that Multi Mode is “better” than Single Mode, because Multi means more, and more is better, right? Not necessarily! Single Mode fibers have a smaller core and narrower wavelength than Multi Mode. It's referred to as “Single” Mode because it only allows one mode of light to pass along its length. This gives them a greater tolerance for bandwidth and allows them to travel over greater distances due to the narrower focus of the light pulses. A Single Mode cable can reliably travel up to 10,000 meters. They're more expensive and fragile than Multi Mode, but necessary when utilizing fiber over great distances.

 

Multi Mode fiber features a thicker core diameter and a longer light wavelength. This limits the distance it can travel (up to around 600 meters), but also makes it less expensive. They allow multiple modes of light to travel along the fiber length (“propogate”) and since the connections are simplified due to the larger core diameter, it means the equipment used in conjunction with it can be simpler, and thus less costly. Multi Mode fiber has a high capacity and reliability, but sacrifices traveling distance and thus is better suited to short run applications, such as a network within a building or data center.

 

One of the key things to keep in mind is that Single and Multi Mode fibers are not compatible with each other. You can't mix them together between two end points, and Multi Mode equipment will not work with Single Mode cables, and vice versa.

Another thing to keep in mind is that, while both types of fiber can handle speeds up to 10G, higher speed capabilities arriving just around the corner may exceed what Multi Mode fiber can handle. Thus, it may be a better option to invest in SMF today, as it is more “future proof” and will prevent you from having to replace MMF in the future.