The industry is instinctually familiar with power wiring. Now, it's time for electrical contractors to learn as much about data wiring as they already know about power wiring.
It's no secret data wiring is nearly as important to the electrical construction industry as power wiring. While the datacom market is a great boon to electrical contractors, it's also brought a bit of a problem: Most understand power wiring almost instinctually, however, the industry has no similar base for data wiring. Sure, it's not too difficult to follow wiring diagrams and specifications, but most contractors don't understand the fundamental issues of data wiring.
You should understand data wiring the same way you understand power wiring; but of course, you must first master most of the electrical basics (Ohm's law, series, and parallel circuits, etc.). Then, you can easily obtain a fundamental understanding of data wiring. Covering copper data wiring only, the following can help you begin developing a gut-level understanding.
A common concern — the amount of power. When installing power wiring, the amount of power installers feed to the outlet is a primary consideration. Wishing to avoid overcurrent (too much power), they also want to avoid excessive voltage drop (too little power). You should have similar concerns with data cabling. If not enough signal passes from one end of the cable to the other, you can't read the data, and the transmission becomes useless.
Since all datacom circuits have limited power levels, don't worry about too much current flowing to the datacom devices. However, do be concerned with too little current.
As a signal propagates down a length of data cable, it loses some of its energy. Therefore, a signal that starts out with a certain level of voltage arrives at the load with a reduced-voltage level. The amount of signal loss is attenuation, 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. Then, the transmission is useless. This is the same thing as voltage drop; it's a loss of power.
The datacom concern — quality of signal. With power wiring, most installers are not usually concerned with the quality of the sine wave running through the conductors. Power companies keep their power as close to 60 Hz as they can, and voltage rarely varies from within preset limits. When troubleshooting a malfunctioning light fixture, the shape of the sine wave is not an immediate concern.
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 figure (in original article) shows what can happen to data signals. At the left of the drawing is a clean square-wave signal entering a data cable. Each segment of the signal represents either a zero (a lack of voltage) or a one (the presence of voltage). The signal pulses coming out at the right end of the long cable have spread out noticeably. When this happens, the electronic communication circuits can't distinguish between a zero and a one, making the signal useless.
This pulse spreading isn't a loss of power. A 5V signal enters the cable, and a 5V signal exits the cable; but the shape of the signal is vastly different. This drastically affects its usefulness.
Here are two concerns for data transmission:
Losing too much power (attenuation), and
Not maintaining the quality of the signal (distortion).
Both are critical concerns, and either 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 crosstalk) or capacitance of a data cable.
Installation. Like power wiring, the installation of data cabling consists of two primary phases, roughing in the wiring and trimming it later.
During the rough-in phase, remember to install all the cables in the proper places, taking care not to bend too tightly, pull too hard, skin, or otherwise damage the cables. Consider the routing of the cables, especially if they are unshielded.
Never place unshielded-copper cables too closely to sources of electromagnetism, such as motor windings, transformers, ballasts, or the like. Note where the fire barriers are within the structure, making proper allowances for crossing them.
Roughing data cable differs from roughing-in power wiring when it comes to protection during the construction process. When it's a long time between the cable installation and installing the jacks, it's up to you to protect your cables. If you don't protect them, you may pull and twist them by accident, damaging the cables.
In other situations, such as when using a complete raceway system, you may have little time between the cable installation and wiring the jacks.
As with power wiring, it's important to leave enough extra cable at each outlet point. Recommended lengths are a minimum of 3 m in the telecommunications closet for twisted-pair and fiber cable, and 30 cm for twisted-pair cable at the outlet. (Notice when you move from power wiring to data cabling, the units of measurement switch from English to metric.) Remember to check your specifications for requirements on extra cable.
Trimming data cabling is much the same as trimming power wiring (strip the cables, install the devices and plates, etc.). However, when trimming power wiring, installers usually test it by flipping a switch or hitting the outlet with a Wiggy; power is either present, or it's not.
Testing data cabling is not so simple. Remember, you should test not only for the presence of the signal, but also for the quality of the signal. You should also spend more time testing your cables and documenting those test results. This is simply part of the datacom business; there's no avoiding it.
Cable layouts. The computer industry terms data cabling layouts as topology, or sometimes architecture. These terms simply refer to the connection pattern of the data equipment and systems. Topology defines how your cables run. In most cases, this is a star pattern, meaning every data outlet gets its own home run. (This is the routing for the EIA/TIA 568 standard, which most new computer networks follow.)
Under this system, a Cat. 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. This can cause spacing problems, especially in preexisting buildings. If the closets are overcrowded, do something about this before estimating and planning.
You should use patchpanels and punch-down blocks (see the sidebar, "Punch-Down Blocks," on page 64D) to facilitate testing, additions to the system, and modifications to the cable plant (cabling system).
>From the punch-down block, short patch cables run to a patchpanel, 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 of the hubs together. Some installers refer to hubs as concentrators. It's important to place each item correctly and connect the data cables according to design.
The data outlet is almost always one or two RJ-45 jacks, mounted on a single-gang plate. (The RJ-45 is the 8-pin modular phone plug. Most companies use it for data networks.)
Other topics. Upcoming articles will cover topics such as decibels, testing and test equipment, characteristic impedance, and making money in the telecommunications business, since you now have a better idea of how this stuff really works.