Fiber-optic cabling: Frequently asked questions

From the July, 2012  Issue of Cabling Installation & Maintenance Magazine

The Fiber Optics LAN Section of the Telecommunications Industry Association addresses some of the most popular questions about why, and how, to deploy fiber cabling in data centers and other environments.

By The TIA Fiber Optics LAN Section

With data center bandwidth demands rising quickly, more network planners than ever have had to consider fiber-optic cabling. This naturally leads to some basic questions, several of which are answered here.

Q: Why do I need to install optical fiber?

A: Network managers choose to install optical fiber for several reasons, depending on their application. A few of the major reasons are listed below.

Longer link lengths: Because of its high bandwidth and low attenuation, fiber cable can support much longer link lengths when compared to the industry standard of 100 meters for unshielded twisted-pair (UTP) copper cabling. For example, with 10 Gigabit Ethernet (GbE) copper is limited to 100 meters, but Om4 multimode fiber can support at least 400 meters. The longer lengths that fiber can support allow designers much more flexibility for laying out their infrastructure and maximizing the use of their real estate.

Network infrastructure longevity: Today's multimode fibers offer users the ability to support their network needs well into the future. With laser-optimized multimode fibers (Om3 and Om4), companies can easily migrate to 40GbE or even 100GbE and higher in their backbones. These fibers offer enough "headroom" to support anticipated applications for at least 10 to 20 years.

EMI/RFI immunity: In some installations—particularly industrial applications and some schools and hospitals—electromagnetic interference (EMI) or radio frequency interference (RFI) from fluorescent lighting or industrial equipment can cause network problems. Because fiber is dielectric, it is immune to these problems. In addition, unlike copper facilities, all-dielectric fiber cabling systems do not conduct lightning strikes or electrical currents that can damage sensitive electronic transmission equipment.

Q: Are there any applications that require fiber?

A: Most applications are media agnostic; that is, they are supported by a variety of different cable types including copper and either multimode or singlemode fiber. While copper can be used for many high-speed protocols, the link distances supported by standards are often very short. For example, the latest 40/100GbE standards only incorporate a 7-meter link distance for copper twinax cable, and UTP is not a supported media. Multimode fiber, on the other hand, can support 40/100GbE links up to 150 meters, while singlemode fiber can support 10-km link lengths. The increasing need for security is also driving users to deploy fiber. In fact, it is required for many government applications.

Q: Is fiber more difficult to install than copper?

A: It depends on the comfort level and training of the technicians. Because fiber has been accepted as the standard choice for communications backbones for many years, today's installers are generally comfortable with the technology, but there is a learning curve for those just starting out. Of course, the same could be said of new generations of copper cabling. The new generation high-speed copper cables require more stringent and time-consuming installation techniques than were required in the past. Compared to newer grades of copper cable, fewer regulations exist on the methods by which optical cable is pulled and terminated. In addition, there is no need to worry about the location of EMI/RFI sources during installation. Also, with fiber cables, there are no requirements for mitigating techniques when migrating to 10GbE and higher data rates as there are with UTP copper media.

Q: Which cabling media are typically used in data center/storage area network (SAN) environments?

A: There are a variety of different types of cabling media deployed in the data center. Multimode and singlemode fiber, direct attach connection (DAC) cables, CX4 copper cables, and Category 6A twisted-pair all have a place.

The cabling type that is deployed is typically based on port type, cost, and distance. Distance is dictated by the architecture of the data center, which can be centralized/direct connect, distribution/top-of-rack switching, zoned distribution, or a combination of these.

Fiber is often deployed to connect top-of-rack switches to an aggregation switch at the end of the row or in another location, in centralized architectures for the "home runs," and in zoned distribution architectures. Multimode fiber supports all distances in the typical data center, such as connecting top-of-rack switches within rows back to an aggregation or core layer, or connecting servers to end-of-row switches. For larger data centers, where multimode may not suffice, singlemode fiber can enable much longer distances. Singlemode fiber can also be deployed within the row as a strategy for future applications that might use multiple-wavelength technologies. Another alternative to traditional cable and connector deployments for connectivity between servers and switches within the rack is to use direct attached cables connecting to SFP+ ports.

Q: Can the same fiber-optic transceivers that are used with Om3 fiber, like SFP+ 10-Gbit/sec pluggable modules, be used with Om4 fiber or are there new transceiver types that need to be used?

A: Yes, you can use the same fiber-optic transceivers for both Om3 and Om4 fibers because the two fiber types are basically the same except that Om4 fiber has higher bandwidth. The IEEE 10-Gbit/sec Ethernet standard states that 300-meter Om3 and 400-meter Om4 link lengths are supported with 10GBase-S-compliant transceivers.

Q: Because fiber is made of glass, will it survive harsh conditions?

A: Optical fiber is not your typical kind of glass. Made of ultra-pure silica, it is an extremely strong material that has the ability to handle exposure to temperature and pressure extremes. In fact, tensile strength (resistance to pulling) of optical fiber exceeds 600,000 pounds per square inch—making it stronger than copper or steel strands of the same diameter and easily surpassing the strength requirements of today's communications applications. When cabled, glass fiber is protected and further strengthened by aramid or fiberglass yarns, a fiberglass rod, and/or an outer jacket constructed of non-conductive materials.

Editor's note: Thanks to our sister publication Lightwave for permission to publish this information. It originally was published, and can be found, as one of three articles in a document entitled "Connecting the data center." The other articles in the document are entitled "Data center considerations: Architecture choices," and "What's ahead for multimode fiber communications systems?" The document can be downloaded from www.lightwaveonline.com. ::

The Fiber Optics LAN Section (FOLS) of TIA represents technology leaders committed to providing the most current, reliable, and vendor neutral information about fiber optics and related technologies for advancing new and better communications solutions. For answers to more questions about the use of fiber optics for data centers and elsewhere in campus or enterprise networks, visit www.fols.org.

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