Get ready to test structured wiring in the home

After more than a decade of conceptualization, the home of the future may be just around the corner for many U.S. consumers. But unlike something out of the Jetsons, tomorrow's home will resemble something a little closer to the office.

Companies ranging from modem makers to television manufacturers are busy sketching out visions of houses with electronic nervecenters that control everything from video service, Internet access and security systems to HVAC and electrical systems.

As broadband services move into the home, people are looking for ways to distribute the content. Therefore, the market for homes using unshielded twisted-pair (UTP) Cat.5 copper cabling to make up at least part of their network will be growing exponentially. People talked about Cat. 5 cabling for data networks in the workplace all through the 1990s. And now, with increasing performance demands, people are talking about Cat. 5e and Cat. 6 cabling for office applications. But what about residential applications? Should high-performance cables be installed in homes using a network configuration? The answer is...Yes!

If good quality cable is used, connectors are applied correctly, and the cable tests out correctly regarding continuity, isn't the network system going to be able to carry all of the neat high-speed data applications coming down the pike? Nobody really knows yet.

Let's assume that you have seen the market potential in home networking and that you now install low-voltage cabling in residences. You consider voice/data/security/control cabling a multi-billion dollar growth industry with unlimited potential. So, after you have installed $3,000 to $4,000 in cabling and other gear for a homeowner, can you now assure your customer that the wiring provides the expected performance?

Hand-held testers By using appropriate hand-held test equipment, you can do what is called Cat. 5 certification, or compliance testing, and give your customer that assurance. Test requirements are stringent, and they are described in detail in TIA/EIA TSB-67, Transmission Performance Specifications for Field Testing of Unshielded Twisted- Pair Cabling Systems. TSB-95 covers additional testing parameters. As with everything else in the telecom industry, these test units are still evolving.

All of the hand-held Cat. 5 testers have a master and a slave unit, but they differ significantly in their product features, test times and information readouts. Each unit also has an auto-test function that displays the results as pass or fail, as well as a variety of additional test options that can be done to determine what caused a "fail." You can also select other diagnostic tests such as fault location or traffic monitoring.

After studying the product literature, it is easy to see the differences among the testers. In addition, operating features that are beyond those required by TSB-67 are worth comparing before making a selection. You might want to rent one or more testers for your own field use before picking a single brand as your favorite.

Generally, a field tester can store the results of up to 500 autotests, and these results can be downloaded to a PC or a printer. The printout is proof that the wiring was done correctly and that the network will work properly when it's up and running.

Most current models are referred to as TSB-67 Level II, which means the unit has increased dynamic range and accuracy and can make measurements well beyond the basic Cat. 5 link test.TSB Level II compliant (minimum) tests are:

The wire map test ensures that each of the eight conductors (four-pair) is connected properly at either end of the cable (i.e., that each pin "maps" to the same pin at the opposite end). For example, pin one of the eight-position, eight-conductor connector (jack, work area outlet or patch panel) travels straight through to pin one at the other end of the cable run. And the same applies to pins two through eight. A wire map test will quickly indicate cross-pinning and other faults. If this test fails, all of the meter's test functions stop.

The length test is actually a measurement of the electrical length of the circuit under study. A Cat. 5 link cannot exceed 295 ft and a channel cannot exceed 328 ft. It's important to note the difference between electrically and physically measured lengths. The increased number of pair twists per foot in high-performance cable will account for a significant difference betweenthe two. For instance, an exact physical measurement of 100 ft of Cat. 5 cable could (electrically) test as 106 ft to 112 ft or more. This electrical length is based on TDR (time domain reflectometry) technology, which measures a test pulse (launch, return/receive time and pulse orientation), and calculates the length based on NVP (nominal velocity of propagation). You can't get a valid auto-test pass if the link or channel is longer than 295 ft or 328 ft, respectively.

The attenuation test applies a signal (a series of frequencies up to 100MHz) at one end of the link and measures the signal strength at the other end. Attenuation is caused by signal radiation, resistance of the wire and insulation absorption, and this loss is measured on each pair. In general, the higher the signal frequency, and/or the greater the length of cable, the greater the attenuation. Signal strength is measured in decibels (dB). The worst pair attenuation must not exceed 22dB at 100MHz.

The NEXT (near-end crosstalk) test measures the signal picked up on one pair of conductors from all other pairs. The tester measures crosstalk by applying a test signal to one pair at a series of frequencies and measuring the amplitude of the signal the other cable pairs receive. NEXT is tested from both ends of the cable and the worst case is used. Because it is frequency dependent, NEXT is also measured at multiple frequencies (steps) over a specified frequency range.

When and where to test What part of the cabling network should you test? There are two basic Cat. 5 compliance tests: a link test and a channel test. A link is the path from the connector at the distribution device (call it a distribution panel) to the connector at the outlet. A channel is the path from the attached device connector at the one end to any attached device at the other end. Thus, a channel is the link plus the connecting cords and plugs. Basically, a link is that installed length of cable, before any equipment is attached. Thus the length of a cable link should not exceed 90 meters, and the length of a cable channel should not exceed 100 meters.

If the tester shows that a cable has failed, it should be replaced (or shortened). Testing should be done at the prewire stage of the installation. Thus, connectors should be placed on cables at prewire and the connectors protected until trim-out. Cables should be tested again after trim-out to make sure nothing was damaged.

Most likely in the future, residential and business installations and/or testing techniques and practices eventually will be the same. So, logically, Cat. 5 standards don't care whether the network is physically located within a home or an office. Instead it is based on performance characteristics and other criteria.