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Data Centre

A Guide on Fibre Optic Cable

Fibre Optic Cable

A Guide on Fibre Optic Cable

FS Official 2014-09-10


It is undeniable that our world today is built on the rapid transfer of massive amounts of information. Decades ago, data were transferred from a network device to another device through copper wires. But now, it has become apparent that fibre optic cables are steadily replacing copper wires as a better means for data transmission.

Inside a Fibre Optic Cable

The core of optical fibres can be plastic (used for very short distances), but most are made from glass. And glass optical cables are made from silica, which, in pure form, has a very low loss in infrared region of the optical spectrum. Designed for long distance, very high performance data networking and telecommunications, fibre optic cable uses light to transmit information while the copper wire uses electricity.

Internal Construction

A fibre optic cable is composed of the core, cladding, coating, strengthening fibres and cable jacket, with its core and cladding being the two main elements. The core is the light transmission area of the fibre. The cladding is the layer completely surrounding the core. Surrounding the cladding is usually another layer, called a coating. 

The Internal Construction of an Optical Fibre Figure 1: The Internal Construction of an Optical Fibre

Working Principle 

The principle of light transmission in an optical fibre is known as the total internal reflection, which states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in. Based on this principle, light can move easily down the fibre-optic line. When this principle is applied to the construction of the fibre-optic strand, it is possible to transmit information down fibre optic cable in the form of light pulses. The core must be a very clear and pure material for the light or in most cases near infrared light (850nm, 1300nm and 1500nm). That’s why glass and plastic are the main materials for optical fibres.

The Working Principle of an Optical Fibre Figure 2: The Working Principle of an Optical Fibre

Primary Types of Fibre and Various Parameters

Generally speaking, there are three types of optical fibre cables, namely single-mode fibre, multimode fibre and plastic optical fibre (POF). The first two are made of glass while the last one is made of plastic.

 Single-mode fibre cable is a single strand (most applications use 2 fibres) of glass fibre with a diameter of 8.3 to 10 microns. With a relatively smaller core and narrow diameter, it can contain the beam of light in a much tighter space, which allows it to provide a higher transmission rate and up to 50 times more distance than multimode fibre, but it also costs more.

The Core of a Single-mode Fibre Figure 3: The Core of a Single-mode Fibre

 Multimode fibre cable has a little bit bigger diameter and allows multiple modes of light to travel, which means it has more space to generate and collect light, thus leading to a lower cost. It is ideal for backbone applications in homes and businesses. However, in long cable runs (greater than 900 metres), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single-mode fibre in new applications using Gigabit and beyond.

The Core of a Multimode Fibre Figure 4: The Core of a Multimode Fibre

 POF is a large core step-index fibre with a typical diameter of 1 mm. This large size makes it easy to couple lots of light from sources and connectors and do not need to be high precision. As a result, typical connector costs are 10-20% as much as for glass fibres and termination may be as easy as cutting with a razor blade! Being plastic, it’s also rugged and easy to install without fear of damage. POF has always been "lurking in the background" in fibre optic; it's a specialty fibre useful for illumination and low speed short data links. There is now a greatly increased interest in POF, as R&D has given it higher performance to go along with its ease of installation and low cost.

The Installation of Fibre Optic Cable

With the features of high bandwidth, low power loss, light weight and so on, fibre optic cables have gained popularity among network service providers and are widely adopted for today’s network cabling. Although with so many advantages of fibre optic cable, it doesn’t mean that fibre cable installation needs no care. Attentions should also be paid when installing fibre cables at different places such as runs in the conduit, above ceilings and below floors, runs in cable trays and risers.

Conduit Cabling

It’s inflexible for conduit installation for a big difficulty to make moves, adds and changes. When pulling cable in conduit, all transition points should be kept smooth. Try to leave additional piece of conduit outside of the transition to keep the cable from resting on a sharp edge. Or you can use inner duct or flexible conduit to keep cables from damage. You also need pull boxes for fishing the run and looping the cable for the next length of conduit. To ease the installation, fish tapes or pull cords should always be placed in the conduit.

Note: The inside radius of conduit bends should be at least 10 times the diameter of the cable. Conduit runs should be limited to 100 feet, with no more than two 90-degree bends between pull points or boxes.

Dropped Ceilings and Raised Floors Cabling

Installation in drop ceilings or raised floors may be the easiest. Suspended ceilings consist of low-weight panels supported by a system of metal frames or grids attached to the ceiling. Usually these panels can be easily moved away from the grid when they are pushed up. Then more space can be left for other equipment installation. Cables in these spaces should be well supported and organized. You are suggested to use some tools for easy cable management like trays, J-hooks and cable ties.

Trays Cabling

Cables trays or ladder racks provide a convenient, safe, efficient location for optical fibre cable installation. Trays can be installed in ceilings, under floors and in riser shafts. Trays can provide good support and certain protection for cable. But you should still notice that cables may also suffer from stresses. When running the fibre cables in the trays, better avoid as much tension, crushing and bending as possible. Make sure the cabling routes are away from sharp turns, snags (sometimes from other cables) and rough surfaces. Try not to pull cable under or between heavier cable or multiple cables that may create huge stress. It’s also true to moves and adds of fibre cables. Do not forget to secure the cable to the tray with cables ties to avoid damage during changes, and do this every three feet.

Riser Cabling

For vertical cabling in risers, if possible, at least use the cable rated OFNR. When pulling cables vertically, use some device to support the cable without crushing the core. Cables should be supported with cable ties in wiring closet. And begin the installation from the top, making it easier comparing to pulling cable from the opposite direction. Maybe handling procedures of optical fibre cables produced by different manufacturers are different. So follow the manufacturer’s instructions at first.

Fibre Optic Cable Selection Information

Customers may be dazzled by the variety of cables in market. To choose the right fibre optic cable, you need to know the following:

 Which mode of fibre is required? 

As it has been mentioned before, single-mode fibre is used for long haul or extreme bandwidth applications, while mutimode fibre is idea for short-distance transmission. Generally speaking, choose multimode optical cables if transmission distance is within 2km, if it is more than 2km, then use single-mode optical cables.

 How many fibres to be installed?

Unless you are making patch cords or hooking up a simple link with two fibres, it’s highly recommended that you include a number of spare fibres. Corporate network backbones are often 48 fibres or more. Most backbone cables are hybrids—a mix of 62.5/125 multimode fibre for today’s networks and single-mode fibre for future networks. To install as many fibres as you can afford because fibre is really inexpensive, compared to the cost of installation.

 Simplex or Duplex?

Simplex vs. duplex is just the difference between one fibre or two; between one connector at each end of a cable, or two connectors at each end. That's all there is to it. Duplex patch cords are the most common type, because the way that most fibre electronics work is that they need two fibres to communicate. One is used to transmit data signals, and the other receives them. However, in some instances, only one fibre is required, so simplex patch cords may be necessary for certain applications. If you're not sure, you can always be on the safe side by ordering duplex patch cords, and only using one of the two fibres.

Summary

Just over twenty years ago, fibre was introduced and required experts from Bell Labs to install it while copper wire was easy to install. Today it is often the opposite. Because fibre is so powerful, there is no question that online communication via the medium of fibre optic cable is our future, and fibre optics will carry the majority of the traffic. Only fibre optic cable, with its long reach, reliability, space savings, immunity to interference and relatively low cost, can help both public and enterprise operators control their budgets and deliver high quality services at even faster speeds.


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