Fibre Cabling Pros and Cons

Unlike copper cabling which carries an electrical signal, fibre uses light pulses to transmit data from point to point. The losses, or attenuation, of the signal is much less than that of electrical signals and therefore the distance covered before amplification is required can be much greater.

Fibre makes use of a phenomenon known as Total Internal Reflection. The fibre acts as a waveguide where any light that hits the side of the cable reflects back in and so travels down the fibre, even round corners, and comes out at the end. This effect is commonly seen in novelty applications such fibre optic lighting, and in medical applications such as endoscopes.

A basic fibre optic system consists of a transmitter to generate a light signal, an optical cable to contain the light, and a receiving device. The fibre itself is a completely passive component in the system.

Two main technologies are used to produce the light. Light-emitting diodes (LED), and  Injection-Laser diodes (ILD). LED light sources are less precise in their frequency than ILD sources, but cost a lot less. The choice is dependent on the bandwidth required, the transmission distance and the type of fibre being used.

Data is encoded as a series of light pulses and injected into the fibre by the transmitter. The receiver picks up the pulses and converts them back to the original data stream.

In theory all the signal stays inside the fibre. Of course in the real world some signal does leak out of the cable and losses are caused by internal reflections and dispersion in the cable. However these losses are very low and so fibre can be used over extremely long distances. For very long haul applications repeaters can be used to regenerate the signal.

Advantages of Fibre

Fibre has many advantages over copper.

• The carrying capacity of fibre is much greater than copper (though advancing technology has allowed copper speeds to be vastly increased in recent years).
• Due to the low signal losses very long distance transmissions are possible.
• Light is not electrical so it is not affected by electrical interference, a major benefit when running cables near lift motors or lighting ballasts, or anywhere subject to electrical interference or 'noise'.
• On long fibre runs using a technique known as Wavelength Division Multiplexing (WDM) it is possible to send multiple signals at slightly differing frequencies down a single fibre at the same time, increasing the carrying capacity even further.
• Fibre is difficult to tap into making it a very secure form of data transmission, though it is possible and you would still be advised to encrypt any sensitive data.
• Fibre is thin and light and flexible so you can fit more into a given size of cable duct.

Disadvantages of Fibre

• Cost. Fibre is generally more expensive than copper. Network interface cards, hubs, fibre switches, routers and patch cables all cost a lot more than their copper equivalents. Oddly the cable itself isn't much more expensive, just all the peripherals.
• More difficult installation. It may not be a disadvantage for you as you can charge more for your time, but it is indisputably harder to make good connections in fibre. For copper connections an electrical connection is sufficient, for fibre you need a good mechanical connection and a good optical contact. Not only must the ends of the join be a good close fit, but the alignment must be very accurate too.
• Specialist Tools. You will need specialist cleaving (cutting) polishing and joining equipment, in fact you may need more than one set of tools depending on the types of connectors that you are using.
• Two fibres per link. You will need two fibres for every connection, one for sending, one for receiving. It's very important that you label things correctly as connecting them the wrong way round will cause the connection to fail. This problem is further compounded by the range of connector types available, some are easier to mix up than others. A label printer is a wise investment.
• Individual fibre diameters are very small and potentially dangerous. Tiny shards of fibre left over from splicing can easily get into your skin, or worse into your blood stream, where they can be extremely hazardous. It is particularly important to protect your eyes from small fragments of fibre.
• Laser light is dangerous. Never look down a fibre, you won't see anything and it may damage your eyesight. There is never a good reason to look down a fibre. Remember that the frequencies used are infrared and therefore not visible to the eye. Take particular care when using a microscope to make sure that there is no power on the fibre. If you are using a Visible Fault Locator (VFL) red light will spill out of any breaks or faults in the fibre, but even here you don't need to look down the fibre to see the light.

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Types of Fibre Cabling

Fibres were originally made from very high purity glass, any impurities or irregularities will scatter the light and increase signal loss, reducing the bandwidth and distance. Today newer and cheaper plastic technologies are beginning to appear but for high bandwidth and long distance application glass remains by far the most commonly used material.

There are three types of fibre optic cable: single mode, multimode and plastic optical fiber (POF). If a cable has a (relatively) wide diameter the light can travel along several different paths, known as modes. For example a beam can travel straight down the centre of the fibre, or can bounce repeatedly off the sides. The narrower the fibre and the more narrowly focused the beam of light the more limited will be the number of possible modes. In a perfect world you would want all the light to travel in a single mode down the centre of the fibre. In the real world fibre is constructed of either Step Index or Graded Index glass. They use different technologies to keep the signal in the fibre.

Step Index fibre consists of two types of glass with different refractive indexes. The glasses run coaxially along the length of the fibre. A beam of light hitting the boundary between the layers is refracted back into the core. Core diameter tends to be small, typically 10 microns. Step index fibres are expensive but losses are very low, typically 2dB per kilometre or less.

Graded glass fibre has a refractive index that varies progressively from the centre to the edge so stray beams are again directed back to the centre. Typically graded glass fibres are larger diameter, 62 or 50 microns.

• Single Mode is the smallest diameter, most expensive, but also the fastest and lowest loss type of fibre. The thinness of the fibre constrains the light to follow a single path or mode, or a very small number of paths, resulting in lower distortion and less spreading of the signal over longer distances. Typically around 8.3 to 10 microns wide, single mode fibre also requires a light source with a very narrow spectral range. Typically frequencies, usually 1310 and 1550 nanometers are used, selected because they produce the lowest losses in single mode fibre materials. Joins and splices in the fibre need to be highly accurate and to a very fine tolerance to minimize losses.
• Multimode fibre is a thicker type of cable, typically between 50 and 100 microns diameter, common sizes are 50, 62.5 and 100 microns. Multimode fibre allows multiple propagation modes, up to 1000 light paths, and yet permits high data rates, up to 10Gigabit and beyond over medium distances. The actual number of modes depends on the characteristics of the fibre, chiefly the diameter and the frequency of the injected light. Typical frequencies used are 850 and 1300 nanometres, selected because they produce the lowest losses in multimode fibre materials. Since the various paths have different lengths the original signal becomes spread out over longer distances, say 1km, causing distortion and degradation of the signal.
• Plastic Optical Fibre (POF) is a low cost fibre suitable for short cable runs. It supports a data transfer speed of 2.5GB/s.

Cable Buffer Types

Clearly with such thin fibres it would be impossible to pull or bend them if they were unprotected. The naked fibre is enclosed in a tougher casing, known as a buffer. The two main types are tight tube buffer and loose tube buffer. Tight tube buffer is usually a protective plastic moulded over each fibre, making them easier to handle and to terminate. Used mostly in indoor applications

In loose tube buffer a bundle of coated fibres is run inside a larger diameter tube, usually filled with waterproof gel to protect from water and to cushion shocks to the cable. Used mostly in harsh outdoor applications.

Being so small it is common for a number of fibres to be bundled into a larger diameter cable and for the cable to include strength members, usually of materials such as Kevlar. These allow you to pull the cable without stressing the individual fibres.

For outdoor cables the whole cable may be armoured with a protective metal sheath.

All cables are finished with a durable plastic jacket, available in a number of colours.

Cable Designations

When specifying cables you will need to understand the various ratings and suitability for your applications. Typically two numbers are used to specify the ratio of core size to cladding size. The first figure is the core diameter and the second is the cladding diameter, both are quoted in microns. So a cable marked 50/125 has a core of 50 microns and a cladding diameter of 125 microns. Sometimes a third digit is specified, this indicates the outside diameter of the whole cable. Common sizes, and typical applications are as follows:

• 8/125 Single Mode fibre for very high speed applications and for long cable runs
• 50/125 Multimode fibre for structured wiring applications, modern replacement for the older 62.5/125 standard
• 62.5/125 the original Multimode cable size, still very commonly encountered in the field

In addition cables are available with a single fibre, a duplex fibre, i.e. a pair of fibres inside a common jacket, and cables containing large bundles of fibres, sometimes hundreds. In WAN/LAN applications duplex or cables with an even number of fibres are most common, since the network needs a cable for sending and one for receiving data.

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Frequencies and Applications

Although it is possible to find just about any cable type used on any network there are some common combinations.

Multimode Applications	Wavelength	Cable size
10M Ethernet	850nm	62.5/125 or 50/125micron
100M Fast Ethernet	1300nm	62.5/125 or 50/125micron
Gigabit Ethernet	850nm/1300nm	62.5/125 or 50/125micron
ATM 155Mbps	1300nm	62.5/125 or 50/125micron
FDDI	1300nm	62.5/125 or 50/125micron

Single Mode Applications	Wavelength	Cable size
10M Ethernet	1310nm*	8/125micron
100M Fast Ethernet	1310nm*	8/125micron
Gigabit Ethernet	1310*/1550nm	8/125micron
ATM 155Mbps	1310nm*	8/125micron
FDDI	1310nm*	8/125micron

*Note: in practice there is no real difference between 1300nm and 1310nm the different names were used to help differentiate between multimode and single mode systems.

Standards

Cabling needs to conform to ANSI/TIA/EIA-568 Standard for 62.5/125 multimode fibre

ANSI/TIA/EIA-568-B.3 Standard for 50/125 multimode fibre.

The tests you have to do will depend on the Standard you require for your network.

• TIA/EIA-455-171A FOTP-171 Single ended loss test Standard
• TIA-526-14-A - OFSTP-14 Double ended loss test Standard
• TIA-526-7 - OFSTP-7 Single mode cable Standard

Testing details are covered later in this document.

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Fiber Connectors

A wide range of connector types is available.

The ST (Straight Tip) is the most popular connector because it is cheap and easy to install. Single mode versions differ from multimode in that they are made to finer tolerances and are more accurately aligned.

The SC connector was specified by the old EIA/TIA 568A specification, but its higher cost and difficulty of installation (until recently) limited its popularity. However, newer SCs cost less and are easier to install, so their use has been growing.

The duplex FDDI, ESCON and SC connectors are used for patch cords to equipment and can be mated to ST or SC connectors at wall outlets. Single mode networks use FC or SC connectors.

EIA/TIA 568 B allows any fibre optic connector that has is compatible with the FOCIS (Fiber Optic Connector Intermateability Standard). This allows the use of several new "Small Form Factor" (SFF) connectors, including AT&T LC, the MT-RJ, the Panduit "Opti-Jack," 3M's Volition, and the E2000/LX-5.

ST Connector

ST is the most common multimode fibre connector type, but can also be used for single mode. It consists of a bayonet mount and a cylindrical ferrule made from ceramic, metal or plastic. Typical insertion loss is around 0.25dB, but it is wise to allow for a loss of 0.5dB per connector. ST connectors are rated for 500 mating cycles.

FC/PC single mode connectors

FC/PC single mode connectors, screw on fitting and a keyway that must be aligned before tightening. Often replaced by SCs and LCs. Typical insertion loss is around 0.25dB, but it is wise to allow for a loss of 0.5dB per connector.

SC Connector

SC, a snap-in connector that is widely used in single mode systems, but can also be used for multimode. It's a snap-in connector that latches with a simple push-pull motion. Also available in a duplex configuration with two connectors side by side. Typical insertion loss is around 0.25dB, but it is wise to allow for a loss of 0.5dB per connector.

Small Form Factor (SFF) connectors

Where space is at a premium smaller connectors may be used.

LC single mode connector

LC single mode connector using a 1.25 mm ferrule, half the size of the ST, but can also be used for multimode. Typical insertion loss is around 0.25dB, but it is wise to allow for a loss of 0.5dB per connector. LC connectors are rated for 500 mating cycles.

MT-RJ is a duplex multimode connector

MT-RJ is a duplex multimode connector with both fibres in a single ferrule. Available in male and female versions. Typical insertion loss is around 0.25dB for multimode fibre and 0.35dB for single mode fibre, but it is wise to allow for a loss of 0.5dB per connector. MT-RJ connectors are rated for 1000 mating cycles.

Opti-Jack

Opti-Jack consists of two ST-type ferrules side by side in a package the size of a RJ-45. Male and female versions are available.

E2000/LX-5

E2000/LX-5 similar to LC but with a shutter to protect the fibre.

Connector Summary

	Simplex	Duplex
ST	√
SC	√	√
FC/PC	√
LC	√	√
MT-RJ		√
Opti-Jack		√
Fddi	√	√
ESCON		√
E2000/LX-5	√	√

ST/SC/FC/FDDI/ESCON connectors have a ferrule size of 2.5 mm. They can be matched using mating adapters.

For testing purposes you may find it useful to have a set of hybrid reference cables with connectors to suit your light sources, or a set of adapters for all these connectors.

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Fibre Performance Testing

A number of factors add up to the total losses in a fibre. Connectors poorly installed and stretched or excessively bent cables have the largest effects and will account for most of the losses.

Since the connections have to be both physical and optical there are a number of lesser factors that can affect the performance of the finished installation.

• Signal Attenuation
• Acceptance Angle
• Numerical Aperture (NA)
• Dispersion, Modal and Chromatic

Attenuation

Attenuation is a measure of the signal loss.

Attenuation in the fibre itself is usually extremely low. However every time you have to make a connection or splice significant losses will be introduced into the cable, reducing the effective length that you can run.

Attenuation is measured in decibels or decibels per kilometer (dB or dB/km). 3dB represents a 50% signal loss.

Acceptance Angle

In multimode fibre light can be introduced at a variety of angles and still be transmitted. The greater this angle the less perfectly aligned the connection has to be. The price you pay is in an increased level of Modal Dispersion, see the section below.

Numerical Aperture (NA)

Numerical Aperture is a decimal value between 0 and 1 that represents the ability of the fibre to accept light over a range of acceptance angles.

Lower values indicate that a fibre accepts light over a narrow range of angles. Single mode fibres have low NA values, so the light has to be very focused and enter at a very shallow angle, while multimode fibres have higher NA values and can accept less focused light from a wider range of angles.

This property explains why narrow band laser sources are commonly required on single mode fibres, whereas LED sources are acceptable for multimode fibres.

Modal and Chromatic Dispersion

In multimode fibre the multiple paths allow the signals to travel different distances and so degrade the signal in a way analogous to delay skew in twisted pair copper. This effect is known as Model Dispersion and gets worse the greater the difference in entrance angle of the light beams.

A similar effect, known as Chromatic Dispersion occurs when different frequencies travel at different speeds through a fibre again spreading and degrading the signal.

On short runs, such as those used on LANs these effects are likely to be small.

Tools

It's pretty clear from the above that you will need specialist tools to check the installation and performance of your cabling.

As a minimum you will need a light source and a power meter, reference cables and adapters. The light source will inject a known quantity of light at a specified frequency into the fibre and the power meter will measure the quantity and quality of the light received.

It is normal to test cables in both directions. Many installers find it useful to work in pairs and to supply each worker with a test kit so that both ends can be tested quickly without having to go back and forth swapping meter and source.

In addition it may be useful to have a Visual Fault Locator (VFL). If there are any breaks in the cable visible red light will spill out.

If you install long cable runs an Optical Time Domain Reflectometer (OTDR) may be useful. This device sends light pulses down the fibre and measures reflection throughput the entire length of the fibre. By measuring the time differences a plot is produced showing attenuation, length and losses at splices and connectors.

Bear in mind though that these are very expensive and a major investment, and not normally required for testing LANs. Investing in an OTDR is only really justified if you are involved in long cable runs, in a LAN environment you are just as likely to see reflections from connectors that will confuse the readings. Money spent on a good power meter and a dual light source is likely to be a wiser investment.

You will also need splicing and terminating tools, cleaning materials, polishers, and a microscope to check connections.

Remember that reference cables, plugs and connectors have a finite life and need to be replaced after a number of uses. Follow the manufacturer's recommendations or the readings may not be accurate.

Tests

Most of the testing you are likely to have to do will involve Local Area Networks (LANs). Obviously you do not need to perform any electrical tests, but you do need to test optical power and signal loss.

It is wise to test the cable on the reel for continuity. If the cable has been damaged you may as well find out now before you install it. A VFL is very quick way to perform this test, all you need is to check that light can travel the full length of the cable.

Power measurement

For a practical system you need enough power to make things work, but not so much that it overloads the receiver and causes problems.

You need a Power Meter and Source (of the correct frequency) and these need to be matched to known reference cables.

Optical power is measured in decibels (dB).

Multimode networks normally need to be tested at 850nm and 1300nm, a dual source is a wise investment as it allows you to run the two tests automatically saving time.

Single mode installations are usually tested at 1300nm. If you know the fibre will be used with Wavelength Division Multiplexing (WDM) you should also test at 1550nm.

Loss tests

The principle of measuring losses is simple, it's the difference between what you inject into the cable at the transmitter and what arrives at the receiver. Remember that every splice and connector (and usually to a much lesser extent the cable itself) all contribute to the losses.

For this test you need to inject a measured amount of light from a calibrated source into the cable and measure how much reaches the other end.

It is usual for the connector on the light Source to be fixed, but the power meter can accept different connectors. Note that reference cables do not last forever and need to be replaced after a number of uses.

Always keep the ends of reference cables protected to prevent dirt and scratches. Any damage to the ends of the cables will cause errors in the results.

Power loss is measured in dBm or microwatts.

Your tests should mimic as closely as possible the technology that will be used on the customer's LAN. So for example if LAN applications will be using LED sources then your tests should also use an LED source, if the LAN uses laser sources then you should use the same in your tests.

Typical values for losses are between 0.5 and 0.75 dB per connector, 0.2 to 0.3 dB per splice.

Cable losses for Multimode are 3 to 3.75dB/Km at 850nm, and 1 to 1.5dB/Km at 1300nm.

Single mode cable losses are 0.4dB/Km at 1310nm and 0.3dB/Km at 1550nm. Check these values against the specification supplied by the manufacturer.

In order to measure the losses you need establish a baseline reference. Do this by attaching a launch cable between the source and the power meter. This measurement is your baseline. Using the same launch cable connect it to the cable you wish to measure and connect the power meter to the other end. Take a second reading. By comparing it to the first reading you can ascertain the loss. You should measure at 850nm and 1300nm for multimode systems.

The maximum loss permitted for the network and application is called the Optical Loss Budget (OLB). Your total losses should be less than the permitted maximum. To work out the total losses for a cable you need to add all the losses in connectors splices etc. Use the formula:

OLB = (number of connectors x connector loss) + (number of splices x splice loss) + (cable length x cable coefficient in dB/Km)

With some power meters you can save time by setting a zero loss reference with the launch cable attached, then all you have to do is connect to the cable to be tested and measure the loss in dB directly.

You can also perform a test with a receive cable attached at the other end of the cable run. This called a double ended loss test.

The tests you have to do will depend on the Standard you require for your network.

• TIA/EIA-455-171A FOTP-171 Single ended loss test Standard
• TIA-526-14-A - OFSTP-14 Double ended loss test Standard
• TIA-526-7 - OFSTP-7 Single mode cable Standard

Results

Many manufacturers of test equipment imply that all you have to do is press the button and accept the results as shown. However you need to be somewhat more careful in the real world. Are you testing against the correct standard for the cable and network? Even if the standard is fine are the parameters set at the correct level? If you happen to select a set of parameters that are not appropriate you may get a pass, or indeed a failure, where it is not warranted.

Interpretation of the results requires you to know what has actually been tested, only then can you decide to accept the results or perform a retest.

You should always record the results, either on paper or in the meter itself. These should become a standard part of your installation process and kept as a record of the job both for yourself and for your customer. Remember the quicker you can present a finished report the sooner you can present your bill so the additional cost of a meter with built-in memory may be quickly recouped.

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Summary

Fibre is smaller, faster, higher capacity and more secure than copper. Fibre is electrically safe and can run extremely long distances with minimal signal loss and degradation.

Two fibres per connection are required. Fibre systems need careful installation and peripherals are more expensive.

You will need specialist tools to splice and connect fibres and to check your work for mechanical and optical alignment.

You will need to test the installation for mechanical damage, optical power and signal loss.

You will need to measure not just connectivity, but also the quality of the connections and you should test against the standard specified for the finished network.

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References & Links

Cabling: The Complete Guide to Network Wiring, Third Edition. Barnett, Groth, McBee, Sybex 2004.

http://www.arcelect.com/fibercable.htm

http://retail.ihs.com/abstracts/tia/tia-526-7.jsp

Thanks to Servicepower Ltd for the use of their fibre connector images.

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