Understanding Fiber Optic Cables and Connectors in Modern Networks

This whitepaper takes a deeper look into the various fiber optic cable and connector types used in modern networks, their specifications, benefits and draw-backs. It details typical applications and use in data center settings.

1. Introduction

There are many types of cables used in data centers. Depending on what you are trying to accomplish, the type of data center build and what equipment you’re deploying, you have many options available.

A change of cable type may also be desired when executing moves, adds or changes in an existing data center, and as technology advances, new options to consider and integrate.

2. Understanding Fiber Optic Cables

2.1 Quick Reference Guide – Fiber Optic Cable Types and Attributes

2.2 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.

2.3 Simplex vs. Duplex Fiber Optic Cables

A simplex cable is used when information or data only needs to flow in one direction, not more than one. So for instance, if you had both a transmission and a return path, you would need two simplex cables. This type of cable is also not reversible – it’s like a one-way street. Information can travel from the transmitter to the receiver, but the moment they are reversed, the cable will no longer function properly.

On the other hand, a duplex cable has two strands and can carry information in two directions using a single cable. Both ends have both transmitters and receivers, and the flow of data can be reversed at any time.

There are two types of duplex cables, however. Although they can be reversed, half-duplex cables can only transmit data in one direction at a time. So while there can be transmitters and receivers at both ends, like the center carpool lanes in many large cities, the lane is only open to traffic in one direction at a time.

A full-duplex cable, however, can transmit data in both directions at one time. So what is the difference when it comes to practical applications? Well, that depends almost entirely on the systems you are using.

Some systems require a simplex and even a singlemode simplex cable. Others actually require a full-duplex cable for the data they will transmit. Essentially, you need to determine what the equipment needs are and cable accordingly.

There are SC, ST, LC and other connectors on simplex and duplex cables, and they can be used in a variety of applications as a result. But besides connectors and simplex vs. duplex, other things matter in the area of cable selection as well.

2.4 DAC and AOC Cables

DAC, or direct attach cables, carry electrical current directly from one component to another. Data rates support some applications and they are better than copper, in the neighborhood of 4Gbps – 10Gbps. Additionally, DAC cables don’t generate heat.

On the other hand, Active Optical Cables, or AOC cables, conduct light rather than electricity, and so are immune to electrical interference. They can handle even higher data rates, up to 40 Gbps. The other advantage to these thinner cables is that they have a higher bend radius for use in high-density data centers, and also due to this, there is more space for airflow.

Either cable has a place in modern data centers, again depending on your specific application and data center build.

2.5 Fiber Optic Cable Jacket Colors

Color codes are used in fiber optics to identify fibers, cables and connectors. When a tech opens a fiber optic cable to prepare it for splicing, they will find a colorful bundle of buffer tubes. Color codes are especially important when making connections by splicing. Each splice tray has 72 splices so the arrangement of the colored buffer tubes and the colored fibers is used to keep all the connections correct.

Perhaps nothing is more complex in fiber optics than maintaining polarity of fibers when using multi-fiber array connectors of the MPO type. In the TIA-568 standard that covers fiber polarity, MPO polarity takes almost 40 pages to explain (so we won’t go into it here).

Patch cords used with patch panels can easily get mixed up. Standards use color codes for fiber and connector types to make it easy to find the right patch cord.

There is a color code standard in TIA, TIA-598 that addresses fiber optic color codes, which most manufacturers adopt and reference, although there are many exceptions based on customer requirements or preferences. Here is what TIA-598 recommends:

Colored outer jackets and/or print may be used on Premises Distribution Cable, Premises Interconnect Cable or Interconnect Cord, or Premises Breakout Cable to identify the classification and fiber sizes of the fiber. (Outdoor cables are generally black for protection against UV light and markings are printed on the cable.)

When colored jackets are used to identify the type of fiber in cable containing only one fiber type, the colors shall be as indicated in Figure 1. Other colors may be used providing that the print on the outer jacket identifies fiber classifications. Such colors should be as agreed upon between manufacturer and user.

Unless otherwise specified, the outer jacket of premises cable containing more than one fiber type shall use a printed legend to identify the quantities and types of fibers within the cable. Figure 1 shows the preferred nomenclature for the various fiber types, for example “12 Fiber, 8 x 50/125, 4 x SM.” Some manufacturers use black as the jacket color for hybrid or composite cables. When the print on the outer jacket of premises cable is used to identify the types and classifications of the fiber, the nomenclature of Figure 1 is preferred for the various fiber types.

Figure 1: Fiber Optic Colors

According to the The Fiber Optic Association (FOA) users have been installing hybrid (MMF+SMF) cables in the backbone for years. With the premises fiber optic cabling now including several varieties of 50/125 fiber, 62.5/125 and singlemode fibers, managing the cable plant is more difficult. FOA report having seen instances of users and installers being confused and getting bad test results, as well as having problems with networks operating when connected over the wrong fiber type. Connector color codes may be used to identify fiber type also. If unsure about the fiber, core size can be determined by examining the connector ferrule with a fiber optic inspection microscope while illuminating the fiber with a white light (flashlight). For fuller details, read this document that defines the twelve TIA/EIA colors for fiber conductors.

Read the full Whitepaper on the AnD Cable Products Website - https://andcable.com/understanding-fiber-optic-cables-connectors-whitepaper/

Louis Chompff, Founder & Managing Director, AnD Cable Products

Louis established AnD Cable Products – Intelligently Designed Cable Management in 1989. Prior to this he enjoyed a 20+ year career with a leading global telecommunications company in a variety of senior data management positions. Louis is an enthusiastic inventor who designed, patented and brought to market his innovative Zero U cable management racks and Unitag cabel labels, both of which have become industry-leading network cable management products. AnD Cable Products only offer products that are intelligently designed, increase efficiency, are durable and reliable, re-usable, easy to use or reduce equipment costs. He is the principal author of the Cable Management Blog, where you can find network cable management ideas, server rack cabling techniques and space saving tips, data center trends, latest innovations and more.

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