Comparing computer with power wiring

The datacom market has been a great boon to electrical contractors, and CEE News is adding a new datacom section as a result. This article, which covers the basics of copper data wiring, is the first of many installments by contracting guru Paul Rosenberg, a datacom expert who's written and lectured extensively about low-voltage installations.Upcoming articles will cover such topics as decibels, testing

The datacom market has been a great boon to electrical contractors, and CEE News is adding a new datacom section as a result. This article, which covers the basics of copper data wiring, is the first of many installments by contracting guru Paul Rosenberg, a datacom expert who's written and lectured extensively about low-voltage installations.

Upcoming articles will cover such topics as decibels, testing and test equipment, characteristic impedance, and making money in the telecom business. For now, this first installment should give you the fundamentals of how this data communications stuff works. -The Editors

Gaining datacom knowledge Data wiring is becoming as important to the electrical construction industry as power wiring, but it has also added problems for contractors. Most of us were so well trained in power wiring that we understand power wiring almost instinctually. Electricians understand the fundamental issues involved and can extrapolate from that base whenever the need arises, but they have no similar base for data wiring. Sure we can follow wiring diagrams and specifications, but we do not understand data wiring on a deep, instinctual level.

Electricians can and should understand data wiring the same way they understand power wiring. Luckily, because electricians have already mastered most of the electrical basics (Ohm's law, series and parallel circuits, etc.), getting a fundamental understanding of data wiring should be fairly easy.

The common concern-amount of power When installing power wiring, the primary concern is with the amount of power fed to the outlet in order to avoid overcurrent (too much power), and excessive voltage drop (too little power). Installers have the same concern with data cabling. If not enough signal passes from one end of the cable to the other, the data cannot be read, and the transmission is useless.

Because all datacom circuits have limited power levels, there is no concern with too much current flowing to the datacom devices; however, there is a concern with too little current. As a signal propagates down a length of data cable, it loses some of its energy. A signal that starts out with a certain level of voltage will arrive at the load with a reduced voltage level. The amount of signal loss is known as attenuation, and it is measured in decibels (dB). If the voltage of a datacom signal drops too much, the signal will no longer register at the other end of the line, and the transmission will be useless. This is the same thing as voltage drop-a loss of power.

The datacom concern-quality of signal With power wiring the quality of the sine wave running through the installed conductors is not usually a concern. Power companies keep power as close to 60 Hz as measurable, and voltage rarely varies from within pre-set limits. When troubleshooting a malfunctioning light fixture, the shape of the sine wave is not an immediate concern.

But with datacom work, the quality of the signal is a primary concern. The readability of a data transmission depends on very careful timing and properly shaped pulses of electricity.

The diagram on the opposite page shows what can happen to data signals. A clean square-wave signal entering a data cable is shown at the left of the figure. Each segment of the signal represents either a zero (a lack of voltage) or a one (the presence of voltage). At the right of the figure, the signal is shown coming out the end of a long cable. The signal pulses have spread-out noticeably. When the pulses are spread this way, the electronic communication circuits cannot distinguish between a zero and a one, which makes the entire signal useless.

This pulse-spreading is not a loss of power; a 5-V signal enters the cable, and a 5-V signal exits the cable. But the shape of the signal is vastly different, and this drastically affects its usefulness.

For data transmission, there are two concerns: 1. Losing too much power (attenuation). 2. Not maintaining the quality of the signal (distortion). Both of these are critical concerns, and either one (or both) can disrupt a data transmission. The distortion of a data signal is generally due to either the excessive inductance (including inductance between circuits, also known as cross-talk) or capacitance of a data cable.

Installation Like power wiring, installing data cabling consists of two primary phases: roughing in the wiring, and trimming it later.

During the rough-in phase, it's important to put all the cables in the proper places and to install them carefully (not bent too tightly, pulled too hard, skinned, or otherwise damaged). It is also important to consider the routing of the cables during rough-in, especially if they are unshielded. Unshielded copper cables should never be placed too closely to sources of electromagnetism, such as motor windings, transformers, ballasts, or the like. It is also important to consider fire stopping. Note the location of fire barriers in the structure and make proper allowances for crossing them.

Unlike power wiring, data cables must be protected during the construction process. This is critical when there will be a long time between cable installation and jack installation. Protect the cables any way that will work. If the cables are not protected, they may be pulled and twisted by accident. This will damage the cables, even though the damage won't show-up until later, when tested.

In other situations, such as when a complete raceway system is used, there may be very little time between the cable installation and the wiring of the jacks.

Also like power wiring, it is important to leave enough extra cable at each outlet point. The recommended lengths are a minimum of 3 meters in the telecommunications closet for both twisted-pair and fiber cable, and 30 centimeters for twisted-pair cable at the outlet. (The units of measurement switch from English to metric when one moves from power wiring to data cabling.) Also, remember to check specifications for requirements on extra cable.

Trimming data cabling is pretty much the same as trimming power wiring (strip the cables, install the devices and plates, etc.) except a lot more testing is required. When trimming power wiring, installers generally test it by flipping a switch or hitting the outlet with a Wiggy. Power is either present, or it is not. Testing data cabling is not so simple. Remember to test for both the presence of the signal and the quality of the signal. Serious time must be spent testing the cables and documenting those test results. This is simply part of the datacom business, and there is no way to avoid it.

Cable layouts The layout of data cables is termed topology by the computer industry, or it is also sometimes termed architecture. Both of these terms simply refer to the connection pattern of the data equipment and systems. Topology defines how the cables will be run. In most cases, the cables will be run in a star pattern, meaning that every data outlet gets its own home run. (This is the routing for the EIA/TIA 568 standard, which almost all new computer networks follow.)

Under this system, a Category 5 (or now, level 6 or 7) cable runs from the outlet to a communications closet. Usually, the communications closet is a telephone closet, with a little extra equipment added to it. This can make for space problems, especially in pre-existing buildings. During your estimating and planning, bear in mind that the closets may be over-crowded and do something about this beforehand if at all possible.

At the wiring closet, your cable home runs will connect to a punch-down block. (The standard method of connecting communications conductors is at a multi-terminal assembly of self-stripping, crimp connections. This component is called a punch-down block, 66 block, or 110 block. Punch-down block is the generic name, 66 blocks are designed especially for voice conductors, and 110 blocks are designed especially for data conductors.) Patch panels and punch-down blocks are used to facilitate testing, additions to the system, and modifications to the cable plant (cabling system).

From the punch-down block, short patch cables are run to a patch panel, and from there to a hub. A hub is an electronic device that takes the signals from each of the cables, and puts them into a backbone cable. A backbone cable is a "riser" for data circuits; it runs between floors of the building, and connects several hubs together. You may also hear hubs referred to as concentrators. In most cases, the operation of these electronic items is not important to you during their installation. What is important, is that each item is placed correctly, and that the data cables are connected according to design.

The data outlet itself will almost always be one or two RJ-45 jacks, mounted on a single-gang plate. (The RJ-45 is the eight-pin modular phone plug, and is nearly universally used for data networks.)

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.