Just like how network security used to be simple with just a firewall and the live wire, network connectivity used to be simply copper cabling and the popular RJ45 connector.

While copper won’t be leaving the data center any time soon, even the most die-hard copper fans can’t deny the growing need for fiber.

Fiber modes and cable specifications can be a lot for network architects to absorb; but there are a few fiber facts you should know before planning your network connectivity and network TAP visibility fabric—single-mode fiber vs. multi-mode fiber; fiber wavelengths explained; and light loss budget.

Single-mode vs. Multi-mode Fiber

Unlike trusty copper twisted pair cabling, fiber cabling is divided into two categories before you even decide what speed you need—single-mode and multi-mode. Single-mode fiber is more expensive than multi-mode, but you have to understand the complete picture before making a connectivity decision based on cost alone. The differences between these two types of fiber are rooted in attenuation.

Attenuation is the weakening of fiber optic signals as distance increases through the cable and is commonly referred to as dB loss. Single-mode fiber is significantly better than multi-mode in terms of dB loss, which is why it is so much more expensive. But why does single-mode perform so well and what does it mean in terms of your networking?

Single-mode fiber cores are just 9micron in diameter, which means the light passing through the cable won’t create many reflections and ultimately minimizes attenuation. This means you can use single-mode fiber for long-range connectivity and faster speeds.

Multi-mode fiber cables, on the other hand, generally come with either a 50micron or 62.5micron core (depending on which mode you choose). Attenuation is minimized at short distances, but long range becomes a challenge as the large core creates many light reflections.

Knowing when to use multi-mode and when to use single-mode fiber is important, but wavelength is another factor that affects attenuation with either choice.

Fiber Wavelengths and Their Effects on Attenuation

There are three main wavelengths used for fiber optics—850 nm and 1300 nm for multi-mode and 1550 nm for single-mode (1310 nm is also a single-mode wavelength, but is less popular). These three wavelengths happen to present near-zero absorption, which is when water vapor collects in the glass and causes attenuation. Choosing amongst these wavelengths comes down to cost and another attenuation factor: scattering.

Scattering is when the light from the signal bounces off atoms in the glass. At shorter wavelengths (850 nm), scattering is fairly high. However, scattering decreases steadily as wavelength increases, which explains why single-mode fiber (at 1550 nm) introduces far less attenuation and ultimately better signal quality at long distances. 

All of these explanations come down to one key theme—how much light are you losing and how does it affect your network?

Considering Fiber Light Loss in a Passive Optical Network

Every passive optical network (PON) has a loss budget—the amount of loss the network should theoretically have. This gives you a good idea of which cables and links to use together and also provides a baseline for ensuring you’ve installed your cables properly.

You have to be careful with budget light loss because there is no industry standard for measuring it, giving manufacturers free rein to pad their own products. However, there are three key areas you should measure fiber optic light loss: 

1. Loss measured through splitters
2. Loss number through splitters plus the loss number through one mated pair
3. Loss number through splitters plus the loss number through two mated pairs

At Garland we publish our loss using methodology #2, for additional details on budget light loss, read Jerry Dillard’s Loss Through Fiber Optic TAPs technical paper.

We’re Only Scratching the Surface of Network Connectivity

These three topics are key fiber facts that every modern network architect should understand. However, there is much more to the network connectivity discussion than we’ve detailed here. We’ve discussed other network connectivity topics in the past that compliment these fiber facts:

• Media Conversion—What the SX?
• Not Ready for the 10G to 40G Transition? Consider Cisco’s EtherChannel
• Avoiding the Critical OM3/OM4 Fiber Piftall—Micron Mismatching
• How to Make 40G Fiber More Accessible—A Primer on Cisco BiDi Technology

All of these blogs are helpful, but a comprehensive guide to networking would be more convenient.

Looking to add fiber TAPs to your next deployment, but not sure where to start? Join us for a brief network Design-IT consultation or demo. No obligation - it’s what we love to do!