This content originally appeared on DEV Community and was authored by Andrew Despres
Preamble:
This space will be utilized to synthesize my notes and help improve my learning process while I study for the CompTIA Network+ N10-009 certification exam. Please follow along for more Network+ notes and feel free to ask any questions or, if I get something wrong, offer suggestions to correct any mistakes.
Ever wondered how data crosses countries and oceans in the blink of an eye? While Wi-Fi and copper Ethernet cables are great for your home or office, the backbone of the internet and high-speed networks relies on a different technology: fiber optics.
Fiber optic cables are the undisputed champions of speed and distance in networking. They can handle vastly more data than traditional copper wires and carry it over incredible distances with less signal loss. This makes them perfect for everything from connecting continents to ensuring a data center runs at top speed.
Let’s break down how this amazing technology works.
How Does Fiber Actually Work? The Magic of Light
Unlike copper cables that use electrical signals, fiber optic cables use pulses of light to transmit data. This is what makes them immune to the electrical interference that can plague copper cables.
A single fiber optic strand is made of three key parts:
- Core: This is the super-thin glass or plastic tube in the very center that acts as the “waveguide,” or tunnel, for the light pulse.
- Cladding: This is a layer of glass or plastic wrapped around the core. The cladding has a different refractive index, which is a fancy way of saying it acts like a perfect mirror, constantly bouncing the light signal back into the core. This process is called total internal reflection, and it’s what keeps the light signal contained and moving forward.
- Buffer: This is the protective plastic coating on the outside that shields the delicate core and cladding from damage.
Cables often bundle multiple fiber strands together and add strength members like Kevlar to prevent kinking during installation. Some outdoor cables even have metal armor to protect them from damage.
The Two Main Flavors: Single Mode vs. Multimode Fiber
Think of the difference between a laser pointer and a flashlight. One creates a single, highly focused beam that can travel a long way, while the other creates a wider, more diffused light that’s best for shorter distances. This is the perfect analogy for the two main types of fiber optic cables.
Single Mode Fiber (SMF) – The Laser Pointer
SMF has a tiny core (around 8-10 microns) that allows only a single, straight path for light, much like a laser beam. It uses a powerful laser light source, which allows it to support incredibly high speeds (over 100 Gbps) over very long distances—we’re talking many kilometers.
- Best For: Long-distance links (WANs) and high-performance data centers where speed and distance are critical.
- Grades: OS1 (for indoors) and OS2 (for outdoors).
Multimode Fiber (MMF) – The Flashlight
MMF has a larger core (50 or 62.5 microns) that allows light to travel in multiple paths or “modes” at once—like a flashlight beam bouncing off the walls of a tunnel. It uses less expensive light sources (like LEDs or VCSELs), which makes it a more budget-friendly option. However, because the light is more scattered, MMF is limited to shorter distances, making it ideal for local area networks (LANs).
- Best For: Shorter network runs, like within a single building or on a campus.
- Grades: Rated by “OM” categories (OM1, OM2, OM3, OM4), where higher numbers support higher speeds. Laser-optimized MMF (OM3/OM4) can handle speeds like 10 and 40 GbE over a few hundred meters.
Plugging In: Common Fiber Connectors
You can’t just plug a bare fiber strand into a switch. You need a connector on the end. While there are many types, you’ll most likely run into these three:
- LC (Local Connector): This is the most popular connector today. It’s a small, “push/pull” connector that clicks into place. Its small size allows for high port density, meaning you can fit more connections into a switch or patch panel. It’s the go-to for modern high-speed Ethernet.
- SC (Subscriber Connector): Another “push/pull” connector, the SC is a bit larger and very sturdy. It was very common for Gigabit Ethernet and is still widely used.
Patch cable with duplex SC connectors (left) and LC connectors (right)
ST (Straight Tip): An older “push-and-twist” bayonet-style connector. You’ll still find it on older multimode networks, but it’s not common in new installations.
Fiber Optic Patch Cords
Patch cables for fiber optic can come with the same connector on each end (LC-LC, for instance) or a mix of connectors (LC-SC, for instance). Duplex patch cords must maintain the correct polarity, so that the Tx port on the transmitter is linked to the Rx port on the receiver and vice versa. The TIA/EIA cabling standard sets out a system of A to B polarity. Each element in the link must perform a crossover, and there must be an odd number of elements, such as two patch cords and a permanent link (three elements).
Transmitted optical signals are visible as bright white spots when viewed through a smartphone camera. This can be used to identify which adapter on an optical interface is transmitting and which fiber patch cord is receiving a signal from the other end of the cable.
A Note on Connector Polish: UPC vs. APC
The very tip of the fiber inside the connector, called the ferrule, must be polished perfectly to ensure light passes through with minimal loss.
- UPC (Ultra Physical Contact): The ferrule tip is polished flat. This is the most common type used for Ethernet. Connectors are typically blue.
- APC (Angled Physical Contact): The ferrule tip is polished at an angle. This creates a more precise connection with less signal reflection. It’s often used by service providers and for video applications. Connectors are typically green.
- Crucial Rule: You cannot mix and match APC and UPC connectors! The angle difference will cause a bad connection or even damage the fiber tips.
Also, by convention, cable jackets and connectors use the following color-coding:
Fiber Distribution Panels
A modern build or refurbishment might replace copper wiring with fiber optic cabling. Structured cabling links are installed in a manner similar to copper cabling. However, to avoid the wear and tear damage associated with continually reconnecting fiber optic cables, it’s essential not to frequently replace cable runs through conduit. Permanent cables are therefore routed through conduit to wall ports at the client access end, and to a fiber distribution panel at the switch end. To complete the connection, fiber patch cables are used to link the wall port to the network interface card (NIC) and the patch panel to the switch port.
Advanced Fiber: Doing More with Less
How do you push the limits of what a single cable can do? You use some clever tricks to pack more data into each strand.
MPO (Multi-fiber Push On)
Instead of a connector with one or two fibers, an MPO connector bundles 12 or more fibers into a single compact connector. This is like turning a two-lane road into a 12-lane superhighway. It’s commonly used in data centers to create ultra-high-speed links (like 40 Gbps or 100 Gbps) by combining several slower lanes.
Wavelength Division Multiplexing (WDM)
WDM is one of the coolest tricks in networking. It allows you to send multiple, separate data streams down a single fiber strand at the same time by using different colors (wavelengths) of light.
Imagine having several different color laser pointers all shining down the same mirrored tunnel. Each color is a separate data channel.
- CWDM (Coarse WDM): Uses fewer channels (up to 16) that are spaced far apart. It’s a cost-effective way to increase capacity.
- DWDM (Dense WDM): Tightly packs dozens or even hundreds of channels onto a single fiber. It requires more precise and expensive equipment but offers massive data capacity.
And there you have it—your guide to the world of fiber optics. We’ve journeyed from the fundamental magic of light traveling through a glass core to the practical differences between single mode and multimode cables. By understanding the common connectors like LC, SC, and ST, and even touching on advanced topics like MPO and WDM, you now have a strong foundation to appreciate how modern networks achieve such incredible performance. This knowledge is a cornerstone of networking, and you’re now one step closer to mastering the technology that connects our world.
This content originally appeared on DEV Community and was authored by Andrew Despres