The fiber guide

It’s quite common for network engineers not to be as familiar with the hardware side of networking. One of the most common hardware questions I get is about optical network links, something that has become the new standard for network interconnects. So I figured I would do a writeup on the optics topic.

Back to basics

The goal of a data transfer is to communicate some type of data across distance. The distance can be very short, like for example between servers in a rack, but it can also be very very far, spanning entire continents. One of the best options for data transfers, especially across longer distances, is fiber optical links. Using fiber optics, light is essentially bounced inside of a glas cylinder in such a way that the transmitted light from one end of the fiber can be received on the other. The physical phenomenon that is used to “send” the light through the fiber is called Total Internal Refraction, and allows for almost all of the light to be contained inside the glas media.

In order to sustain bidirectional communication you must be able to send light in both directions, to both be able to transmit and receive data across the fiber. Most data links nowadays have a data transfer rate of 1Gbit/s or faster, which requires the communication to be full-duplex across a data link. This means that the devices on both sides of a link, must be able to both send and receive data at the same time. For most fiber links, this is solved by simply running two fibers, one for sending from device A to device B, and one for sending from device B to device A.

When you are using a single fiber patch cable to go between two devices, the fiber cable is almost always a crossover cable. But, when you are using multiple fiber patches or Multi Core Fiber (MCF) then you may need to manually swap the RX and TX at one point in your fiber path. This is what we usually refer to as performing an “RX/TX” operation on the fiber.

Different types of light

There are a lot of different variations on what type of light is used in fiber optical links, in fact, there are practically unlimited variations. One key attribute of a fiber optical link is the wavelength used. The wavelength that is transmitted from device A, must be received by a receiver that can read that specific wavelength on the device B side.

Wavelength is measured from the start to the end of a wave, or to be a bit more technical, when a full wave rotation (2π).

There are generally two main types of fiber optical links, which partially has to do with their wavelength, but also has to do with the size of the glas inside the fiber optic and the light emitter source. Fiber optical network links use a range of light that is outside of the visible light spectrum, namely the Infra red (IR) light spectrum.

IMPORTANT disclaimer: Even though the light is not visible, it can still blind or harm you, so always be careful when handling fiber optical links. Especially when using the longer range transmitters.

Multimode

When you are using a multimode fiber link, the light is transmitted and received at a wavelength of 850nm. This is done using a sending and a receiving fiber optic cable respectively. Please notice that in this diagram we are using what would equate to a crossover cable. This is necessary since we are using a single fiber for sending and a single fiber for receiving.

The maximum range of a multimode link is usually 500m, but this can vary greatly depending on what link speed you are aiming for, the quality of fiber, amount of bends in the fiber and these bends respective radius, the “cleanliness” of the fiber ends, and more. The light emitter for multimode is an LED.

Historically multimode has been much cheaper than singlemode, because of the significant cost different in manufacturing singlemode fiber and transceivers. However, nowadays the price gap is quite small, so you should ideally choose the type that works best for your application.

Singlemode

When you need more range than 500m, you should always choose a singlemode link. With that being said, singlemode can be run at super short ranges as well so don’t be lead into believing that singlemode is only used for longer fiber links.

Singlemode typically uses a wavelength of 1310nm and the typical range is 10km, but there are common transceivers that can to up to 120km without any light amplification along the fiber path. Again, the range depends on the same factors as with multimode: what link speed you are aiming for, the quality of fiber, amount of bends in the fiber and these bends respective radius, the “cleanliness” of the fiber ends, and more. Singlemode uses a laser as a light source, which creates more directed light than the multimode LED. There is a newer type of emitter called a VCSEL, but traditionally LED was used.

Fiber optic cables

When you have selected the correct transceiver, ie. multimode/singlemode, speed and distance, you can move on to selecting the actual fiber optics you need. It is important to note that there are different quality of fiber optical cables, which will dictate the range you can run a given link speed across.

Multimode cables

For multimode links, you should use multimode fiber cables.

You should not mix OM1 with the other fiber types, since they have different sizes for the glas diameter in the fiber. Now, with that said, you can still mix the qualities but be aware of the potential signal and data loss from mixing different cable qualities, especially the ones that have different sized cores.

OM5 uses WDM and is therefore a bit special as well. Most common is the use of OM3/4.

Here is an example of a OM3 multimode cable:

A tip for when you are trying to identify if a cable is multimode or singlemode is to look at the connector. Typically multimode connectors are gray, while singlemode connectors are blue.

Singlemode cables

For singlemode links, you should use singlemode fiber.

Most fiber nowadays is OS2 and should be suitable for most of your needs. It is most common to have two fibers, for a single link, just as with multimode. However, there is the option of using something called Bi-directional (BiDi) links. These use two different wavelengths, one for sending and one for receiving, which allows you to establish a link using just a single fiber.

Most network access services that run to residential complexes are BiDi GPON. Which allows for less fiber to be needed, or for more spare fibers to be available.

Here are some examples of singlemode fiber cables:

Here is an example of a BiDi link:

Wave Digital Multiplexing

Wave Digital Multiplexing (WDM) is a technique used to run multiple physically separate links across the same physical fiber. This is achieved by using a specific wavelength (also referred to as color or channel) for each link and then using lenses, mirrors and prisms to combine these different light beams and send it across a single fiber.

There are two main types of WDM techniques: Coarse WDM (CWDM) and Dense WDM (DWDM). In short, CWDM is cheaper but has fewer available channels, while DWDM is more expensive, but allows for running more links across a single fiber.