Single Mode Vs Multi Mode Fiber

Fiber optics are used in modern networks for high bandwidth and low latency in connections.

Fiber optics send information using light pulses, not electrical signals like copper cables. This allows for faster data transfer.

In this course, we will focus on the two main families of fiber optics:

Figure 1 - Single Mode vs Multi Mode Fiber

• Single-Mode Fiber (SMF)

• Multi-Mode Fiber (MMF)

The main difference between these two types lies in the core size, the light source, and the greatest supported distance.

Now that you know the two main families of fiber, let’s take a closer look at how a fiber optic cable is structured.

To begin with, you need to understand that a fiber optic cable is made up of several parts.
As you can see below, there are three main layers.

• On the outside, there is the Coating. This is the protective layer that shields the fiber from shocks or humidity. (Additional layers such as buffer, strength members, or outer jacket may also be added for extra protection.)

• Just below it is the Cladding. It surrounds the core and plays a crucial role: it keeps the light inside the core by reflecting it back through the phenomenon of refraction.

• Finally, at the center, we have the Core. This is the part where the light actually travels and where information is transmitted.

Figure 2 – Optical fiber structure

The size of the core changes depending on the type of fiber optic cable.

Figure 3 – Core sizes comparison: OM1, OM2, OS2

In Single-Mode Fiber

• OS1 / OS2 → about 9 µm

In Multi-Mode Fiber

• OM1 → 62.5 µm

• OM2, OM3, OM4, OM5 → 50 µm

These core sizes are important and should be memorized for the CCNA exam. Since the size of the core determines the fiber type, let’s now focus on single-mode fiber and see how it works.

In Single-Mode Fiber, light transmission is achieved using a laser. This device is expensive, but very precise.

The light travels in a straight line through the fiber core, which makes it possible to carry information across very long distances.

Figure 4 – Light transmission in Single-Mode fiber

Light in single-mode fiber experiences minimal loss and almost no dispersion.

The light travels in a straight line through the fiber core, which makes it possible to carry information across very long distances.

Ethernet Standards to Know

For the CCNA exam, you should be able to identify common single-mode Ethernet standards.

• 1000BASE-LX

  • “1000” = 1 Gb/s

  • “BASE” = baseband transmission

  • “LX” = Long-wavelength (~1310 nm)

  • Typical range: 5 to 10 km.

• 10GBASE-LR

  • “10G” = 10 Gb/s

  • “BASE” = baseband transmission

  • “LR” = Long Reach (~1310 nm)

  • Typical range: around 10 km.

LX and LR always refer to Single-mode fiber.

Single-Mode is great for long distances, as you can see, but what about shorter distances and cost-effective links? That’s where Multi-Mode fiber comes in.