Tuesday, March 29, 2011

Solid Conductors vs. Stranded Conductors

UTP cable used as horizontal cable (permanent cable or cable in the walls) has a solid conductor, as opposed to patch cable and cable that is run over short distances, which usually have stranded conductors. Stranded-conductor wire consists of many smaller wires interwoven together to form a single conductor.

Connector types (such as patch panels and modular jacks) for solid-conductor cable are different than those for stranded-conductor cable. Stranded-conductor cables will not work with insulated displacement connector (IDC)-style connectors found on patch panels and 66-style punch-down blocks.

Though stranded-conductor wire is more flexible, solid-conductor cable has much better electrical properties. Stranded-conductor wire is subject to as much as 20 percent more attenuation (loss of signal) due to a phenomenon called skin effect. At higher frequencies (the frequencies used in LAN cables), the signal current concentrates on the outer circumference of the overall conductor. Since stranded-conductor wire has a less-defined overall circumference (due to the multiple strands involved), attenuation is increased.
KEY TERM: core 
The core of the cable is anything found inside the sheath. The core is usually just the insulated twisted pairs, but it may also include a slitting cord and the shielding over individual twisted pairs in an STP cable. People incorrectly refer to the core of the cable when they mean the conductor (the element that conducts the electrical signal).

Most cabling standards recommend using solid-conductor wire in the horizontal or permanent portion of the link, but the standards allow for stranded-conductor wire in patch cables where flexibility is more important. We know of several UTP installations that have used stranded-conductor wires for their horizontal links. Although we consider this a poor practice, here are some important points to keep in mind if you choose to use a mixture of these cables:
  • Stranded-conductor wire requires different connectors.
  • Stranded-conductor wires don't work as well in punch-down blocks designed for solid-conductor cables.
  • You must account for reduced horizontal-link distances.

Friday, March 25, 2011

Cable Markings

Have you examined the outside jacket of a twisted-pair or fiber-optic cable? If so, you noticed many markings on the cable that may have made sense. UL has requirements on how their designations are applied and the FCC requires that the category be placed every foot. For cables manufactured for use in the United States and Canada, these markings may identify the following:

  • Cable manufacturer and manufacturer part number.
  • Category of cable (e.g., UTP).
  • NEC/UL flame tests and ratings.
  • CSA (Canadian Standards Association) flame tests.
  • Footage indicators. Sometimes these are "length-remaining markers" that count down from the package length to zero so you can see how many feet of cable remains on a spool or in a box. Superior Essex (www.superioressex.com) is one cable manufacturer that imprints length-remaining footage indicators.

Here is an example of one cable's markings:
000750 FT 4/24 (UL) c(UL) CMP/MPP VERIFIED (UL) CAT 5e
These markings identify the following information about the cable:
  • The 000750 FT is the footage indicator.
  • The 4/24 identifies the cable as having four pairs of 24 AWG wire.
  • The (UL) symbol indicates that the cable is UL listed. Listing is a legal requirement of the NEC.
  • The symbol c(UL) indicates that the cable is UL listed to Canadian requirements in addition to U.S. requirements. Listing is a legal requirement of the CSA.
  • The CMP/MPP code stands for communications plenum (CMP) and multipurpose plenum (MPP) and indicates that the cable can be used in plenum spaces. This is the NEC flame/smoke rating.
  • The term VERIFIED (UL) CAT 5e means that the cable has been verified by the UL as being Category 5e compliant (and TIA/EIA-568-B compliant). Verification to transmission properties is optional.
  • SUPERIOR ESSEX is the manufacturer of the cable.
  • COBRA is the cable brand (in this case, a Category 5e–plus cable, which means it exceeds the requirements for Category 5e).
  • The numbers 2319 indicate the date of manufacture in Julian format. In this case, it is the 231st day of 2009.
  • H indicates the Superior Essex manufacturing plant.
Some manufacturers may also include their "E-file" number instead of the company name. This number can be used when calling the listing agency (such as the UL) to trace the manufacturer of a cable. In the case of UL, you can look up the E-file numbers online at www.ul.com.
Cables marked with CMR (communications riser) and CMG (communications general) must not be used in the plenum spaces.

Common Abbreviations

So that you can better decipher the markings on cables, here is a list of common acronyms and what they mean:
  • NFPA The National Fire Protection Association
  • NEC The National Electrical Code that is published by the NFPA once every three years
  • UL The Underwriters Laboratories
  • CSA The Canadian Standards Association
  • PC The Premise Communication Cord standards for physical wire tests defined by the CSA
Often, you will see cables marked with NFPA 262, FT-4, or FT-6. The NFPA 262 (formerly UL-910) is the test used for plenum cables. The FT-4 is the CSA equivalent of UL 1666 or the riser test, and FT-6 is the CSA equivalent of NFPA 262.

Monday, March 21, 2011

Cable Jackets

The best place to start looking at cable design is on the outside. Each type of cable (twisted-pair, fiber optic, or coaxial) will have a different design with respect to the cable covering or the jacket.

KEY TERM: jacket and sheath 
The cable's jacket is the plastic outer covering of the cable. Sheath is sometimes synonymous with jacket but not always. The sheath includes not only the jacket of the cable but also any outside shielding (such as braided copper or foil) that may surround the inner wire pairs. With UTP and most fiber-optic cables, the sheath and the jacket are the same. With ScTP and STP cables, the sheath includes the outer layer of shielding on the inner wires.

One of the most common materials used for the cable jacket is polyvinyl chloride (PVC); UTP cables in the United States are almost exclusively jacketed with PVC, regardless of the flame rating of the cable. PVC was commonly used in early LAN cables (Category 3 and lower) as an insulation and as material for jackets, but the dielectric properties of PVC are not as desirable as those of other thermoplastics, such as FEP or PP, that can be used for higher-frequency transmission. Figure 1 shows a cutaway drawing of a UTP cable.

Figure 1: Cutaway drawing of a UTP cable showing insulated wire pairs, slitting cord, and jacket

Other substances commonly used in cable jackets of indoor cables include ECTFE (HALAR), PVDF (KYNAR), and FEP (Teflon or NEOFLON). These materials have enhanced flame-retardant qualities as compared to PVC but are much more costly. Where PVC can do the job, it's the jacket material of choice.
KEY TERM: rip cord 
Inside some UTP cable jackets is a polyester or nylon string called the rip cord, also known as the slitting cord or slitting string. The purpose of this cord is to assist with slicing the jacket open when more than an inch or two of jacket needs to be removed. Some cable installers love them; many find them a nuisance, as they get in the way during termination.
No standard exists for the jacket color, so manufacturers can make the jacket any color they care to. You can order Category 5e or 6 cables in at least a dozen different colors. Colors like hot pink and bright yellow don't function any differently than plain gray cables, but they sure are easier to spot when you are in the ceiling! Many cable installers will pick a different color cable based on which jack position or patch panel the cable is going to so that it is easier to identify quickly.

Wednesday, March 16, 2011

Fiber-Optic Cable

As late as 1993, it seemed that in order to move toward the future of desktop computing, businesses would have to install fiber-optic cabling directly to the desktop. It's surprising that copper cable (UTP) performance continues to be better than expected. 

Fiber versus fibre: Are these the same? Yes, just as color (U.S. spelling) and colour (British spelling) are the same. Your U.S. English spell checker will probably question your use of fibre, however.

Although for most of us fiber to the desktop is not yet cost-effective for traditional LAN networks, fiber-optic cable is touted as the ultimate answer to all our voice, video, and data transmission needs since it has virtually unlimited bandwidth and continues to make inroads in the LAN market. Some distinct advantages of fiber-optic cable include:
  • Transmission distances are much greater than with copper cable.
  • Bandwidth is dramatically higher than with copper.
  • Fiber optic is not susceptible to outside EMI or crosstalk interference, nor does it generate EMI or crosstalk.
  • Fiber-optic cable is much more secure than copper cable because it is extremely difficult to monitor, "eavesdrop on," or tap a fiber cable.
Fiber-optic cable can easily handle data at speeds above 10Gbps; in fact, it has been demonstrated to handle data rates exceeding 500Gbps!

Since the late 1980s, LAN solutions have used fiber-optic cable in some capacity. Recently, a number of ingenious solutions that allow voice, data, and video to use the same fiber-optic cable have emerged.

Fiber-optic cable uses a strand of glass or plastic to transmit data signals using light; the data is carried in light pulses. Unlike the transmission techniques used by its copper cousins, optical fibers are not electrical in nature.
Plastic-core cable is easier to install than traditional glass core, but plastic cannot carry data as far as glass. In addition, graded-index plastic optical fiber (POF) has yet to make a widespread appearance on the market, and the cost-to-bandwidth value proposition for POF is poor and may doom it to obscurity.

Light is transmitted through a fiber-optic cable by light-emitting diodes (LEDs) or lasers. With newer LAN equipment designed to operate over longer distances, such as with 1000Base-LX, lasers are commonly being used.

A fiber-optic cable (shown in Figure 1) consists of a jacket (sheath), protective material, and the optical-fiber portion of the cable. The optical fiber consists of a core (8.3, 50, or 62.5 microns in diameter, depending on the type) that is smaller than a human hair, which is surrounded by a cladding. The cladding (typically 125 micrometers in diameter) is surrounded by a coating, buffering material, and, finally, a jacket. The cladding provides a lower refractive index to cause reflection within the core so that light waves can be transmitted through the fiber.

Figure 1: A dual fiber-optic cable

Fiber-Optic Cabling Comes of Age Affordably

Fiber-optic cable used to be much harder to install than copper cable, requiring precise installation practices. However, in the past few years, the cost of an installed fiber-optic link (just the cable and connectors) has dropped and is now often the same as the cost of a UTP link. Better fiber-optic connectors and installation techniques have made fiber-optic systems easier to install. In fact, some installers who are experienced with both fiber-optic systems and copper systems will tell you that with the newest fiber-optic connectors and installation techniques, fiber-optic cable is easier to install than UTP.

The main hindrance to using fiber optics all the way to the desktop in lieu of UTP or ScTP is that the electronics (workstation network interface cards and hubs) are still significantly more expensive, and the total cost of a full to-the-desktop FO installation (Centralized Cabling, per ANSI/TIA-568-C) is estimated at 30 percent greater than UTP. However, the Fiber-to-the-Telecommunications-Enclosure topology can bring fiber closer to the desk, while still using UTP for the final connection, and actually lower the cost compared to traditional topologies

Two varieties of fiber-optic cable are commonly used in LANs and WANs today: single-mode and multimode. The mode can be thought of as bundles of light rays entering the fiber; these light rays enter at certain angles.
KEY TERM: dark fiber 
No, dark fiber is not a special, new type of fiber cable. When telecommunications companies and private businesses run fiber-optic cable, they never run the exact number of strands of fiber they need. That would be foolish. Instead, they run two or three times the amount of fiber they require. The spare strands of fiber are often called dark fiber because they are not then in use—that is, they don't have light passing through them. Telecommunications companies often lease out these extra strands to other companies.

Friday, March 11, 2011

Screened Twisted-Pair (ScTP)

A recognized cable type in the ANSI/TIA-568-C standard is screened twisted-pair (ScTP) cabling, a hybrid of STP and UTP cable. ScTP cable contains four pairs of unshielded 24 AWG, 100 ohm wire (see Figure 1) surrounded by a foil shield or wrapper and a drain wire for grounding purposes. Therefore, ScTP is also sometimes called foil twisted-pair (FTP) cable because the foil shield surrounds all four conductors. This foil shield is not as large as the woven copper-braided jacket used by some STP cabling systems, such as IBM types 1 and 1A. ScTP cable is essentially STP cabling that does not shield the individual pairs; the shield may also be smaller than some varieties of STP cabling.
Figure 1: ScTP cable

The foil shield is the reason ScTP is less susceptible to noise. To implement a completely effective ScTP system, however, the shield continuity must be maintained throughout the entire channel—including patch panels, wall plates, and patch cords. Yes, you read this correctly; the continuity of not only the wires but also the shield must be maintained through connections. Like STP cabling, the entire system must be bonded to ground at both ends of each cable run, or you will have created a massive antenna, the frequencies of which are inversely proportional to the length of the shield. The net effect is that the noise is out of band.

Standard eight-position modular jacks (commonly called RJ-45s) do not have the ability to ensure a proper ground through the cable shield. So special mating hardware, jacks, patch panels, and even tools must be used to install an ScTP cabling system. Many manufacturers of ScTP cable and components exist—just be sure to follow all installation guidelines.

ScTP is recommended for use in environments that have abnormally high ambient electromagnetic interference, such as industrial work spaces, hospitals, airports, and government/military communications centers. For example, ScTP is used in fast-food restaurants that use wireless headsets for their drive-through-window workers; some wireless frequencies can interfere with Ethernet over copper. The value of an ScTP system in relation to its additional cost is sometimes questioned, as some tests indicate that UTP noise immunity and emissions characteristics are comparable with ScTP cabling systems. Often, the decision to use ScTP simply boils down to whether you want the warm and fuzzy feeling of knowing an extra shield is in place.

Sunday, March 6, 2011

Unshielded Twisted-Pair (UTP)

Though it has been used for many years for telephone systems, unshielded twisted-pair (UTP) for LANs first became common in the late 1980s with the advent of Ethernet over twisted-pair wiring and the 10Base-T standard. UTP is cost effective and simple to install, and its bandwidth capabilities are continually being improved.
An interesting historical note: Alexander Graham Bell invented and patented twisted-pair cabling and an optical telephone in the 1880s. During that time, Bell offered to sell his company to Western Union for $100,000, but it refused to buy.

UTP cabling typically has only an outer covering (jacket) consisting of some type of nonconducting material. This jacket covers one or more pairs of wire that are twisted together. In this chapter, as well as throughout much of the rest of the book, you should assume unless specified otherwise that UTP cable is a four-pair cable. Four-pair cable is the most commonly used horizontal cable in network installations today. The characteristic impedance of UTP cable is 100 \ohms plus or minus 15 percent, though 120-ohm UTP cable is sometimes used in Europe and is allowed by the ISO/IEC 11801 Ed. 2 cabling standard.

A typical UTP cable is shown in Figure 1. This simple cable consists of a jacket that surrounds four twisted pairs. Each wire is covered by an insulation material with good dielectric properties. For data cables, this means that in addition to being electrically nonconductive, it must also have certain properties that allow good signal propagation.

Figure 1: UTP cable

UTP cabling seems to generate the lowest expectations of twisted-pair cable. Its great popularity is mostly due to the low cost and ease of installation. With every new generation of UTP cable, network engineers think they have reached the limits of the UTP cable's bandwidth and capabilities. However, cable manufacturers continue to extend its capabilities. During the development of 10Base-T and a number of pre–10Base-T proprietary UTP Ethernet systems, critics said that UTP would never support data speeds of 10Mbps. Later, the skeptics said that UTP would never support data rates at 100Mbps. After that, the IEEE approved the 1000Base-T (1 Gb/s) standard in July 1999, which allows Gigabit Ethernet to run over Category 5 cable. Just when we thought this was the end of copper UTP-based applications, in 2006 the IEEE approved the 10GBase-T standard, which allows 10 Gigabit Ethernet over unshielded Category 6 and 6A cable!