Fibre Guidelines for choosing and testing
Fibre versus copper.
When planning a new or upgraded cabling infrastructure, you have two basic choices: fibre or copper. Both offer superior data transmission. The decision on which one to use may be difficult. It will often depend on your current network, your future networking needs, and your particular application, including bandwidth, distances, environment, cost, and more. In some cases, copper may be a better choice; in other situations, fibre offers advantages.
Although copper cable is currently more popular and much more predominant in structured cabling systems and networks, fibre is quickly gaining fans.
Fibre optic cable is becoming one of the fastest-growing transmission mediums for both new cabling installations and upgrades, including backbone, horizontal, and even desktop applications. Fibre optic cable is favored for applications that need high bandwidth, long distances, and complete immunity to electrical interference. It's ideal for high data-rate systems such as Gigabit Ethernet, FDDI, multimedia, ATM, SONET, Fibre Channel, or any other network that requires the transfer of large, bandwidth-consuming data files, particularly over long distances. A common application for fibre optic cable is as a network backbone, where huge amounts of data are transmitted. To help you decide if fibre is right for your new network or if you want to migrate to fibre, take a look at the following:
|Fibre vs. Copper
||10-Gigabit and beyond
||Evolving towards the desktop
||CAT7 under development
||40km+ @ 10Gbps
||100m @ 1000Mbps
||Susceptible to EMI/RFI interference crosstalk, and voltage surges
||Almost impossible to tap
||Lightweight, thin diameter.
Strong pulling strength.
|Heavy, thicker diameter.
Strict pulling specifications
The advantages of fibre.
Greater bandwidth-Because fibre provides far greater bandwidth than copper and has proven performance at rates up to 10 Gbps, it gives network designers future-proofing capabilities as network speeds and requirements increase. Also, fibre optic cable can carry more information with greater fidelity than copper wire. That's why the telephone networks use fibre, and many CATV companies are converting to fibre.
Low attenuation and greater distance—Because the fibre optic signal is made of light, very little signal loss occurs during transmission so data can move at higher speeds and greater distances. Fibre does not have the 100-meter (304.8-ft.) distance limitation of unshielded twisted-pair copper (without a booster). Fibre distances can range from 300 meters to 40 kilometers, depending on the style of cable, wavelength, and network. (Fibre distances are typically measured in metric units.) Because fibre signals need less boosting than copper ones do, the cable performs better.
Fibre networks also enable you to put all your electronics and hardware in one central location, instead of having wiring closets with equipment throughout the building.
Security—Your data is safe with fibre cable. It does not radiate signals and is extremely difficult to tap. If the cable is tapped, it's very easy to monitor because the cable leaks light, causing the entire system to fail. If an attempt is made to break the security of your fibre system, you'll know it.
Immunity and reliability—Fibre provides extremely reliable data transmission. It's completely immune to many environmental factors that affect copper cable. The fibre is made of glass, which is an insulator, so no electric current can flow through. It is immune to electromagnetic interference and radio-frequency interference (EMI/RFI), crosstalk, impedance problems, and more. You can run fibre cable next to industrial equipment without worry. Fibre is also less susceptible to temperature fluctuations than copper is and can be submerged in water.
Design—Fibre is lightweight, thin, and more durable than copper cable. And, contrary to what you might think, fibre optic cable has pulling specifications that are up to ten times greater than copper cable's. Its small size makes it easier to handle, and it takes up much less space in cabling ducts. Although fibre is still more difficult to terminate than copper is, advancements in connectors are making temination easier. In addition, fibre is actually easier to test than copper cable.
Migration—The proliferation and lower costs of media converters are making copper to fibre migration much easier. The converters provide seamless links and enable the use of existing hardware. Fibre can be incorporated into networks in planned upgrades.
Standards—New TIA/EIA standards are bringing fibre closer to the desktop. TIA/EIA-785, ratified in 2001, provides a cost-effective migration path from 10-Mbps Ethernet to 100-Mbps Fast Ethernet over fibre (100BASE-SX). A recent addendum to the standard eliminates limitations in transceiver designs. In addition, in June 2002, the IEEE approved a 10-Gigabit Ethernet standard.
Costs—The cost for fibre cable, components, and hardware is steadily decreasing. Installation costs for fibre are higher than copper because of the skill needed for terminations. Overall, fibre is more expensive than copper in the short run, but it may actually be less expensive in the long run. Fibre typically costs less to maintain, has much less downtime, and requires less networking hardware. And fibre eliminates the need to recable for higher network performance.
Multimode or single-mode, duplex or simplex?
Multimode-Multimode fibre optic cable can be used for most general fibre applications. Use multimode fibre for bringing fibre to the desktop, for adding segments to your existing network, or in smaller applications such as alarm systems. Multimode cable comes with two different core sizes: 50 micron or 62.5 micron.
Single-mode—Single-mode is used over distances longer than a few miles. Telcos use it for connections between switching offices. Single-mode cable features an 8.5-micron glass core.
Duplex—Use duplex multimode or single-mode fibre optic cable for applications that require simultaneous, bidirectional data transfer. Workstations, fibre switches and servers, fibre modems, and similar hardware require duplex cable. Duplex is available in single- and multimode.
Simplex—Because simplex fibre optic cable consists of only one fibre link, you should use it for applications that only require one-way data transfer. For instance, an interstate trucking scale that sends the weight of the truck to a monitoring station or an oil line monitor that sends data about oil flow to a central location. Simplex fibre comes in single- and multimode types.
50- vs. 62.5-micron cable.
Although 50-micron fibre cable features a smaller core, which is the light-carrying portion of the fibre, both 62.5- and 50-micron cable feature the same glass cladding diameter of 125 microns. You can use both in the same types of networks, although 50-micron cable is recommended for premise applications: backbone, horizontal, and intrabuilding connections, and should be considered especially for any new construction and installations. And both can use either LED or laser light sources.
The big difference between 50-micron and 62.5-micron cable is in bandwidth—50-micron cable features three times the bandwidth of standard 62.5-micron cable, particularly at 850 nm. The 850-nm wavelength is becoming more important as lasers are being used more frequently as a light source.
Other differences are distance and speed. 50-micron cable provides longer link lengths and/or higher speeds in the 850-nm wavelength. See the table below.
|50- vs. 62.5-micron fibre
||at 850 nm
||at 1310 nm
The ferrules: ceramic or composite?
As a general rule, use ceramic ferrules for critical network connections such as backbone cables or for connections that will be changed frequently, like those in wiring closets. Ceramic ferrules are more precisely molded and fit closer to the fibre, which gives the fibre optic cables a lower optical loss.
Use composite ferrules for connections that are less critical to the network's overall operation and less frequently changed. Like their ceramic counterparts, composite ferrules are characterized by low loss, good quality, and a long life. However, they are not as precisely molded and slightly easier to damage, so they aren't as well-suited for critical connections.
Testing and certifying fibre optic cable.
If you're accustomed to certifying copper cable, you'll be pleasantly surprised at how easy it is to certify fibre optic cable because it's immune to electrical interference. You only need to check a few measurements.
- Attenuation (or decibel loss)—Measured in decibels/kilometer (dB/km), this is the decrease of signal strength as it travels through the fibre cable. Generally, attenuation problems are more common on multimode fibre optic cables.
- Return loss—This is the amount of light reflected from the far end of the cable back to the source. The lower the number, the better. For example, a reading of -60 decibels is better than -20 decibels. Like attenuation, return loss is usually greater with multimode cable.
- Graded refractive index—This measures how the light is sent down the fibre. This is commonly measured at wavelengths of 850 and 1300 nanometers. Compared to other operating frequencies, these two ranges yield the lowest intrinsic power loss. (NOTE: This is valid for multimode fibre only.)
- Propagation delay—This is the time it takes a signal to travel from one point to another over a transmission channel.
- Optical time—domain reflectometry (OTDR)-This enables you to isolate cable faults by transmitting high-frequency pulses onto a cable and examining their reflections along the cable. With OTDR, you can also determine the length of a fibre optic cable because the OTDR value includes the distance the optic signal travels.
There are many fibre optic testers on the market today. Basic fibre optic testers function by shining a light down one end of the cable. At the other end, there's a receiver calibrated to the strength of the light source. With this test, you can measure how much light is going to the other end of the cable. Generally, these testers give you the results in dB lost, which you then compare to the loss budget. If the measured loss is less than the number calculated by your loss budget, your installation is good.
Newer fibre optic testers have a broad range of capabilities. They can test both 850- and 1300-nanometer signals at the same time and can even check your cable for compliance with specific standards.
A few properties particular to fibre optic cable can cause problems if you aren't careful during installation.
- Intrinsic power loss—As the optic signal travels through the fibre core, the signal inevitably loses some speed through absorption, reflection, and scattering. This problem is easy to manage by making sure your splices are good and your connections are clean.
- Microbending—Microbends are minute deviations in fibre caused by excessive bends, pinches, and kinks. Using cable with reinforcing fibres and other special manufacturing techniques minimizes this problem.
- Connector loss—Connector loss occurs when two fibre segments are misaligned. This problem is commonly caused by poor splicing. Scratches and dirt introduced during the splicing process can also cause connector loss.
- Coupling loss—Similar to connector loss, coupling loss results in reduced signal power and is from poorly terminated connector couplings.
Remember to be careful and use common sense when installing fibre cable. Use clean components. Keep dirt and dust to a minimum. Don't pull the cable excessively or bend it too sharply around any corners. That way, your fibre optic installation can serve you well for many years.