Testing Cat. 5 twisted-pair cable in a residence for performance compliance helps you deliver the results your customers expect.
After more than a decade of conceptualization, the home of the future may be just around the corner. But unlike something out of the Jetsons, tomorrow's home will resemble something closer to an office. Companies ranging from modem makers to television manufacturers are busy sketching visions of houses with electronic nerve centers that control everything from video service, Internet access, and security systems to HVAC and electrical systems.
Now the question is: Should you install high-performance cables in residential structures using a network configuration? The answer is yes! As broadband services move into the home, there must be a way to efficiently distribute the content. Experts believe that the use of unshielded twisted pair (UTP) Cat. 5 copper cabling will grow exponentially (see sidebar on this page).
Let's assume you've seen the market potential in home networking, and you now install low-voltage cabling in residences. You also consider voice/data/security/control cabling to be a multi-billion dollar growth industry with unlimited potential. So, after installing $3,000 to $4,000 in cabling and other gear for a homeowner, how can you assure the wiring provides the performance the customer expects?
By using appropriate hand-held test equipment, you can do what's called Cat. 5 certification or compliance testing, and give your customer that assurance. These stringent test requirements are described in detail in TIA/EIA Standard TSB-67, Transmission Performance Specifications for Field-Testing of Unshielded Twisted-Pair Cabling Systems. TSB-95 is another standard that covers additional testing parameters. As with everything else in the telecom industry, these test units continue to evolve.
These hand-held Cat. 5 testers come equipped with a master and slave unit, but they differ significantly in their product features, test times, and information readouts. Each unit has an auto-test function that displays such results as pass/fail, including tests to determine the cause of a "fail" readout.
Generally, a field tester can store the results of up to 500 "auto-tests." You can download these to a PC or a printer. The printout is your proof of proper wiring installation and network function. You can also select other diagnostic tests, such as fault location and traffic monitoring - to name a few.
WHAT ARE THESE TESTS REALLY FOR? There are two basic Cat. 5 compliance tests: a link test and 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 attaching any equipment.
Additional TSB Level II compliant (minimum) tests include:
- The wire-map test ensures a proper connection for each of the eight conductors (4-pair) at either end of the cable (i.e. 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) connects to pin one at the other end of the cable run. A wire map test quickly indicates cross pinning and other wiring errors. If this test fails, all of the meter's test functions stop - and the test doesn't go forward.
- The length test is 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 (see Fig. 1, on this page). This difference between electrically and physically measured lengths is important. The increased number of pair twists per foot in high-performance cable will account for a significant difference between the two.
For example, an exact physical measurement of 100 ft of Cat. 5 cable could (electrically) test as 106 ft to 112 ft or more. This length is based on time domain reflectometry (TDR) technology, which measures a test pulse (launch, return/receive time and pulse orientation) and calculates the length based on nominal velocity of propagation (NVP).
- The attenuation test applies a signal (a series of frequencies up to 100 MHz) at one end of the link and measures the signal strength at the other (Fig. 2). Signal radiation, resistance of the wire, and insulation absorption cause attenuation. You measure this loss on each pair. In general, the higher the signal frequency, and/or the greater the length of cable, the greater the attenuation. You measure signal strength in decibels (dB). The worst pair attenuation must not exceed 22 dB at 100 MHz.
- The near-end crosstalk test (NEXT) 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 (Fig. 3). NEXT is tested from both ends of the cable, and the worst case is recorded. Since it's frequency dependent, you can also measure NEXT at multiple frequencies over a specified frequency range.
If the tester shows a failed cable, you should replace (or shorten) it. Therefore, it's a good idea to test the cable at the prewire stage of the installation and place connectors on cables at prewire and protect until trim out. Test cables again after trim out to make sure you didn't do any damage.
Three factors are fostering solid growth in low-voltage/network (integrated) wiring in homes and apartments. First, home networks are a mass-market reality - thanks to computer and entertainment networking applications. Secondly, improvements in cable and digital subscriber line (DSL) technologies bring the Internet into the home at increasingly faster speeds. Therefore, it's more economical to implement a home network in a multiple PC residence, rather than add an additional Internet service provider (ISP) account. Thirdly, the agreements between structured wiring manufacturers/distributors and homebuilders indicate more builders recognize the value of structured wiring.
There are other forces at work here as well. Home computer sales are staging an unexpected surge. A big reason for this is a major shift in how consumers use them. PC makers, who have tried vainly for years to promote their machines as alternatives to television and game players, now race to feed the growing new market for entertainment applications. Dell Computer Corp., Gateway Inc., and Packard Bell NEC Inc. are all tapping into growing interest in digital music with systems designed as recording studios. Packard Bell NEC's offering comes with software for downloading and organizing MP3 music files from the Web, and a "rewritable" compact-disk drive will transfer songs to CDs you can play in the car or stereo.
Market researcher International Data Corporation (IDC) expects U.S. sales for home PCs to surge 37% this year to 15.8 million machines. The projections exceed even the peak sales that followed the release of Microsoft's Windows 95 software. IDC attributes the growth to falling prices and rebates tied to Internet service contracts, as well as the boom in entertainment applications.
Another study by Parks Associates, Dallas, confirms these trends. The study reveals a growing penetration of multiple PCs, home networks, and high-entertainment products. And while the primary market for products and services is found in higher income households, the interest in residential technology reaches across all income levels. Lower income households have nearly as much technology receptivity as households with higher incomes. The question of technology adoption then becomes much more of a price issue.