Uncle Ted's Guide To
VDV (voice/data/video) Cabling
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Overview of Structured Cabling
 
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Testing
 
Wrapup, Training Programs and Equipment

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Testing Wiring
 
Since Cat 5e/6 cable is used to the fullest extent of its performance envelope, testing is very important. There are three basic tests that are called for as part of the EIA/TIA-568 specs for all UTP cables: wiremap, length and high speed performance. We'll take a look at each of them and equipment needed to test them.
 
Wiremapping
Wiremapping tests to make sure the cable is connected properly, according to the standard for connecting pin-to-pin. Basically, wiremapping is a continuity test using an inexpensive tester. Just make sure the wiremapper you buy tests "split pairs" as noted below.
 
 
What Is A "Certified" Cable?
 
Certification is one of the most overused and least understood words in our industry! Did the cable pass an exam and get a certificate? Well, sort of. This term has been used by vendors of testers to mean that the cable was tested and passed by one of the Cat 5e/6/6A "certification" testers which test all the standard's specified performance parameters. It means that the cabling meets the minimum specifications of EIA/TIA standards and should work with any network designed to operate on a Cat 5e/6A link.
 
What is "Verification"?
 
Alternatively, cable may be tested to determine if it will carry the network signals intended for use on the cabling systems. These testers run bit error rate tests (BERT) over the cable as well as checking wiremaps and length. A Verifier will guarnatee the cabling will support Ethernet, but does not test to the cabling standards, only a problem if some other usage, such as analog video, may be used.
 
Wiremap
 
Wiremapping simple means that each wire is hooked up correctly, with no opens or shorts. That's mostly very straightforward. Each pair must be connected to the correct pins at the plugs and jacks, with good contacts in the terminations.
 
 

Most of the failures are simple enough to understand, like reversed wires in a pair, crossed pairs, opens or shorts. One possible failure, crossed pairs, is caused when both wires of a pair are crossed at one termination. The usual cause of a crossed pair is a 568A termination on one end and a 568B on the other.
 
The most difficult wiremap problem is a split pair, when one wire on each pair is reversed on both ends. It causes the signal to be sent on one wire each of two pairs. The usual DC wiremap will pass but crosstalk will fail. It takes a more sophisticated wiremapper or Cat 5e/6 tester to find a split pair, as some wiremappers which use only DC tests do not check crosstalk. In our experience, a split pair is usually caused by someone using punchdown color codes on jacks which splits the pairs.
 
Length
 
Since 568 cables must be less than 90 meters (296 feet) in the link and 100 meters in the channel (328 feet), length must be tested. This is done with a "time domain reflectometer" which is a fancy term for cable "radar". The tester sends out a pulse, waits for an "echo" from the far end and measures the time it took for the trip. Knowing the speed in the cable, it calculates the length. All cable certification testers include a TDR to measure length.
 
If you have a short or open, the TDR will also tell you where the problem is, making it a great tool for troubleshooting problems.
 
Attenuation
 
The proper operation of a LAN on the cable plant requires the signal strength be high enough at the receiver end. Thus the attenuation of the cable is very important. Since LANs send high speed signals through the cable and the attenuation is variable with the frequency of the signal, certification testers test attenuation at several frequencies specified in the 568 specs.
 
 
This test requires a tester at each end of the cable, one to send and one to receive, then one of them will calculate the loss and record it. There are pass fail criteria for the cable at Cat 3, 4, 5, 5e, and 6 max frequencies.
 
 
Crosstalk (NEXT)
 
It's called NEXT for "near end cross talk" since it measures the crosstalk (signal coupled from one pair to another) at the end where one pair is transmitting (so the transmitted signal is largest causing the most crosstalk.) Crosstalk is minimized by the twists in the cable, with different twist rates causing each pair to be antennas sensitive to different frequencies and hopefully not picking up the signals from it's neighboring pairs. Remember what we've said repeatedly: you MUST keep the twists as close to the terminations as possible to minimize crosstalk.
 
 
 
Cat 5e /6 testers measure crosstalk from one pair to all three other pairs for each pair and compare it to the 568 specs, giving a pass/fail result. Some also calculate "ACR" or attenuation/crosstalk ratio, as it is a measure of how big the crosstalk signal is to the attenuated signal at the receiver. You want this number as big as possible, as it is an indication of the signal to noise ratio.
 
 
More Tests for Gigabit Ethernet
 
The latest generation of test specs for Category 5e and 6 includes a number of new tests to insure higher performance from the cable. These tests relate to higher bandwidth usage of the cable and simultaneous use of all four pairs, even in both directions at once.
 
Powersum NEXT is the NEXT on one pair when all three others are carrying signals. This is realistic with 1000Base-T Gigabit Ethernet where all pairs carry signals simultaneously.
 
Far end crosstalk, looking at the effect of the coupling from one pair to another over the entire length, measured at the far end. As tested, it's ELFEXT or equal level FEXT, or the ratio of FEXT to attenuation, sort of like ACR.
 
Delay Skew measures how much simultaneous pulses on all 4 pairs spread out at the far end. This measures the speed on each pair, which may be different due to the variations in number of twists (more twists means longer wires) or insulation. Since 1000Base-T Gigabit Ethernet uses all 4 pairs with the signals split into 4 separate signals, it's necesary to have all arrive simultaneously. Testers measure Propogation Delay, the actual transit time on the pairs to calculate Delay Skew.
 
 
Return Loss is a measure of the reflections from the cable due to variations in the impedance. These reflections can cause signal degradation, especially if the pairs are used in a full-duplex (bidirectional) mode. With 1000Base-T Gigabit Ethernet transmitting in both directions on each pair, return loss can cause big problems.
The "augmented" Cat 6 spec will have reference to "alien crosstalk" or the signal coupling from one pair in a cable to the same pair in another cable, a consequence of higher frequencies and the consistency of twists.

Testers
 
Wiremappers test the connections and Cat 5e/6 certification testers test the performance at high frequencies. Cable Certification testers are mostly automated, "push a button get a pass/fail" simple. In fact, certification testers test everything, wiremap, length, attenuation and crosstalk in one connection, give you a pass/fail result, help on troubleshooting and store the result for printing reports for the customer.
 
Some installers use the certification tester for all testing, after the cable is installed. But it's a very expensive unit that needs a trained operator and many failures are simply wire map problems. Others have each crew use an inexpensive wiremapper to make sure all connections are correct before the certification tester is brought in. By having each crew find and fix their own wiremap problems, testing and corrections are done as the cable is installed and the cost of the certification tester is not wasted on simple problems. It's just provides the high frequency tests and documentation required by most users.
 
 
Permanent Link Adapters
 
The tester's adapter interface cable may be the weakest link when testing. Conventional adapter cords may be the cause for many false failures in the field. Susceptable to the daily wear and tear associated with rough field conditions, they degrade with time and contribute to return loss.
 
Until now, each tester used personality modules specific to each manufacturers Cat 6 cabling for testing. The personallity modules insured that the connection between the adapter and the link under test yield optimum performance and more passes. The need for separate adapters for each manufacturer's cabling system was defacto acknowledgement of the incompatibility of various Cat 6 products.
 
 
A change in the definition of the "link" has been implemented in EIA/TIA568 B and ISO 11801 AM2 and it is now called the "permanent link." The permanent link moves the test reference point to the end of the test cable at the wall outlet or patch panel jack, including only the connector on the end of the tester interface cable. Compliance with this standard could speed compatibility among Cat 6 cabling systems.
 
Upgrading Testers
 
Category 5 testers generally cannot be upgraded to 5e or 6 but Category 5e testers can often be upgraded to Cat 6. Upgrades to Cat 6A testers are not usually available.
 

 

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