Untangle the Cable Selection Process

Choosing the right datacom cable for your next network installation can be a twisted mess if you don’t know what you’re doing. Facilities demand reliable high-speed, high-bandwidth transmission of business information. As such, the demand for communications networks that operate at the highest performance levels has never been greater. At the heart of these networks is the structured cabling system

Choosing the right datacom cable for your next network installation can be a twisted mess if you don’t know what you’re doing.

Facilities demand reliable high-speed, high-bandwidth transmission of business information. As such, the demand for communications networks that operate at the highest performance levels has never been greater. At the heart of these networks is the structured cabling system — a concept developed in the '80s and '90s and defined in a series of industry standards that ensure cabling equipment and network electronic equipment from a variety of manufacturers can work together effectively.

But how do you determine the best cable for your network? It helps to know what you're looking for, but that's only the first step. There is little doubt fiber optic cable is making inroads with network designers, but the market for copper is still strong. Factor in recent changes to cabling standards, and the decision process only becomes more muddled, but the following discussion should set you on the right course.

Selecting the proper copper cabling.

Although fiber optic cable's hold on the network wiring market is improving, there are still many uses for copper. Today's network operations require copper cabling with performance characteristics superior to those specified in previous Telecommunications Industry/Electronic Industry Association (TIA/EIA) standards, particularly TIA/EIA-568-B. New electronics (100BaseT and 1000BaseT) not only run faster (more MHz) than the electronics used for 10BasedT, but they also run “smarter” (delivering more bits per MHz). They have gone from providing signals with two voltage levels (NRZI for 10Base-T) to signals with three voltage levels (MLT3 for 100Base-T) and as many as five (PAM5 for 1000Base-T). However, five signal levels can generate a high level of noise and prevent the cable from achieving an adequate level of data throughput.

In addition, the protocols, or systems, have moved from using separate pairs for sending and receiving data to using each of the four pairs for sending and receiving data simultaneously. These new encoding schemes require higher pair-to-pair crosstalk performance, less attenuation, and more stringent cable specifications. Parallel signal timing is critical for 4-pair technology, so improved cabling performance is necessary.

Copper cabling is the preferred medium for wiring a horizontal run that extends from a communications room to the wall outlet serving a nearby workstation. Under this arrangement, a dedicated 4-pair, unshielded, twisted-pair (UTP) copper cable terminated at an intermediate cross-connection point within the telecommunications room serves each workstation.

UTP cabling is the most common medium for bringing voice and data signals to the desktop. It is also useful for low-grade video transmission. In any application that does not require full-motion video, UTP cabling is adequate.

As a result, UTP has an advantage in remote security monitoring and other low-bandwidth uses where the signal might be transmitted a very long distance or viewed from a remote site. The rash of school shootings makes security a major market for UTP video links, in which as many as 100 security cameras may be deployed at a facility.

Despite some restrictions, Cat. 5 UTP can still be specified to wire a school, hospital, or other facility. However, the next generation of cable, Cat. 6, handles 300 MHz and beyond. Although a standard for Cat. 6 cabling is still under development, it is available from several manufacturers and has already been used in a wide range of installations. The advantages of this total systems approach go beyond the products to include the cables, connectors, hardware, and electronics.

Selecting the proper optical-fiber cable.

The growth in fiber optic cabling installations can be attributed to the need for greatly enhanced network performance. Faster network protocols, more demanding applications, and more powerful computing resources are important reasons for selecting fiber optic cabling over copper for your network.

Higher bandwidth capabilities in a small cross-section cable, longer transmission distances, immunity to radio frequency interference (RFI) and electromagnetic interference (EMI), higher reliability and security, and lower maintenance are some of fiber's advantage over UTP copper cabling. In many respects, optical fiber is becoming more cost-effective than copper. In fact, some fiber- and copper-cable costs are now roughly equivalent, although optical-fiber connector and installation costs are still higher than copper.

In premise networks, the technological differences between copper and fiber are important. The biggest advantage of fiber optics is its distance capability. Cat. 5, Cat. 5e, and Cat. 6 have a 295-ft limitation, which includes the length of cable running up and down walls to avoid other equipment or obstructions. Optical fiber, however, can carry signals up to 6,500 ft over multimode fiber and up to 62 mi over single-mode fiber before any appreciable signal degradation occurs.

Until Gigabit Ethernet (GbE) was developed, all multimode fiber was optimized for fiber distributed data interface (FDDI), a 100Mbps network that uses a light-emitting diode (LED) source on 62.5 micron fiber. But GbE needs more bandwidth than was available from the LED source, which has an upper limit of about 300 Mbps. Two light sources can be used with the GbE standard: an 850-nm vertical-cavity, surface-emitting laser (VCSEL) and a 1,310 nm telecommunications laser used in single-mode networks.

Fiber cable manufacturers now offer 62.5/125 micron and 50/125 micron multimode optical-fiber cable with matched lasers to support GbE that will be able to run 10 GbE in the future. Both new fiber types are slightly more expensive than today's 62.5/125 singlemode fiber, and the 50/125 multimode fiber is also incompatible with most of the multimode fiber already installed in the United States.

The fiber you need will depend on whether you're using it for networks at Gigabit speeds or beyond. For a network that will not run higher than 100 Mbps, the higher-performance optical fiber would not be economical. However, if you install an optical-fiber system for a GbE network, choose a fiber that is matched for use with a laser source. If a current network uses an LED light source, the new fiber will still work with the LEDs, and an upgrade to higher speeds is still possible. The choice between the 50/125 and 62.5/125 multimode fiber depends on acceptable cost and compatibility with current cabling.

An optical fiber is fragile, so it must be protected against mechanical forces (such as compression and tension) and dirt and moisture, which can degrade its light-carrying capability. The first level of protection for an optical fiber is a buffer tube, with either a loose-tube or tight-tube design.

Loose-tube design allows the fiber to move freely inside the tube. Because different materials in a fiber optic cable expand and contract at varying rates with temperature changes, a loose tube allows the fiber to remain relatively unstressed at low temperatures. Used primarily in an outdoor-rated cable, the loose tube is filled with a gel that prevents water intrusion and hydraulically cushions the glass fiber. This type of cable is typically rated for ambient temperatures down to -40°C.

Installers typically dislike using the water-blocking gel because they need rags and solvents to clean the messy gel from the cable ends. Now, however, a new generation of dry-blocking cables that cost only slightly more than gel-filled cables is available.

A tight-tube design prevents the fiber from moving freely because the jacket is tightly bonded to the fiber. A tight-tube cable has high-impact strength, abrasion resistance, a relatively small diameter, and is mostly used indoors.

To provide additional protection, an optical-fiber cable is covered with additional outer layers or jackets, often with high-strength strands that accept the tension forces when the cable is pulled. An outdoor rated cable may have vapor barriers and special jacketing for direct burial. Cables designed for long-distance pulls have high tensile strength steel or plastic fiber pulling members. Cables are covered with a metallic armor when a high degree of mechanical protection is needed.

Cabling for indoor applications include general-use PVC-jacketed cables and riser cables that resist the spread of flames. Jacketed cables for use in air-handling plenums meet flammability and smoke requirements for the jacketing materials in that application. Non-conductive cables can be used in conduits with current-carrying conductors.

Specifications are increasingly calling for hybrid cables that combine single-mode and multimode fibers in anticipation of future needs.

Less well known is plastic optical fiber (POF). The makers of POF cable and components, after attempting to gain acceptance for POF as a viable medium for horizontal cabling use, may find that home networking, and particularly computer-electronics applications are an important market.

Currently two types of POF are available: the step-index type and the graded index type, which handles much higher bandwidth than the step-index type. Currently under development, a third type involves the use of fluoropolymer materials, which can operate over a very wide range of wavelengths, including 850 nm and 1,300 nm.

Regardless of whether your next project calls for copper or fiber optic cable, you need to know the basics of the selection process. Becoming familiar with the standards that govern installations cabling is important, but a familiarity with the system you're working on is critical.

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