What do the fiber terms 9/125, 50/125 and 62.5/125 refer to?

What do the fiber terms 9/125, 50/125 and 62.5/125 refer to?

These terms refer to the diameter in microns of a fiber optic cable's core and cladding.

    The first set of numbers - 9, 50 and 62.5 refer to the diameter of the fiber cable's core.
    The second set of numbers - 125 refer to the diameter of the outside of the fiber cable's cladding.

The cladding is a special coating that keeps the light from escaping the glass core. 9/125 refers to a single mode fiber cable. 50/125 and 62.5/125 refer to multimode fiber cable.

Singlemode (SMF) vs. Multimode (MMF) Fiber Optic Cables

As bandwidth demand increases, a large number of data center managers may feel that singlemode cables are the definitive answer for the future. And to be fair, they do carry a lot more data over longer distances than multimode fiber cables. The real difference between the two is how they transmit light: singlemode fiber cables allow only one ray of light to be transmitted, while multimode fiber cables have several strands in a larger core that allow more "rays" of light to be transmitted simultaneously.

However, the key element in that phrase is "over longer distances." When it comes to enterprise-level data centers, multimode cables are just as effective for most applications where less distance is involved, and they cost a lot less than their singlemode counterparts.

This cost is felt in the needed transceiver as well. A singlemode optical cable has a small core size, meaning the beam of light it transmits must be much more focused than that needed for a multimode cable, thus leading to the more expensive transceiver.

Multimode cables, at least at the time of this writing, can still handle high-speed data demands at distances less than 500-600 meters. That means that almost any cable internal to an enterprise-level data center can still be multimode and function well.

That being said, there are less bend-sensitive and full-spectrum singlemode cables that offer more bandwidth and are less sensitive to handling of the patch cords. There are also more transceiver options as a result. As these cables get better and more affordable, they may become more common in shorter distance applications.