The Fiber Optic Association

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Reference Guide To Fiber Optics


Topic: Testing UTP Premises Cabling  Table of Contents: The FOA Reference Guide To Fiber Optics


Testing UTP Cabling
 
Since Cat 5e/6/6A UTP cable is used to the fullest extent of its performance envelope, comprehensive performance 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.
 
 
What Is A "Certified" Cable?
 
Certification 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/6 link.
Thus, a "certification tester" or "certifier" is an instrument that tests the cabling and compares it to the TIA-568 standards, certifying that the cable meets the minimum performance specifications  required by the standard.
 
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 network bit error rate tests (BERT) over the cable as well as checking wiremaps and length. A "cable verifier" will guarnatee the cabling will support Gigabit Ethernet, for example, but does not test to the TIA cabling standards, only a problem if some other usage, such as analog video, may be used.
 
Wiremap
 
Wiremapping is a simple test that confirms that each wire is hooked up correctly, with no opens or shorts. UTP intended only for POTS (plain old telephone service) voice applications actually only needs to be tested for wiremap. Wiremapping is very straightforward. Structured cabling standards do not consider simple voice grade cable, only cable of Categroy 3 or above, so most cable testing will require more than just wiremapping. Each pair must be connected to the correct pins at the plugs and jacks, with good contacts in the terminations. A "wiremapper" is basically a continuity checker that determines if pins are correctly connected.
  
wiremap
 

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/6/6a 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.
More information on wiremap testing and troubleshooting.
 
Length
 
Since 568 cables must be less than 90 meters (296 feet) in the permanent link and 100 meters in the channel (328 feet), cable length must be tested. This is done with a "time domain reflectometer" (TDR) which is a cable "radar". The tester sends out a pulse, waits for a reflection from the far end and measures the time it took for the trip. Knowing the speed of the pulse in the cable (calibrated for various cable types), it calculates the length. All cable certification or verification testers include a TDR to measure length.

TDR time domain reflectometer
 
Besides measuring length, the TDR looks at the polarity of reflected pulses to find shorts or opens. If you have a short or open, the TDR will tell you what and where the problem is by looking at the return pulse, making it a great tool for troubleshooting problems. If the return pulse is the same polarity, the cable is open. If the pulse is of opposite polarity, the cable is shorted. If no return pulse is seen, the cable is terminated at its characteristic impedance.

Performance Testing For Certification

Performance testing for attenuation, crosstalk, etc. requires testing over the full frequency range of the cable. The frequency range for each cable type is:

Cat 3: 16 MHz
Cat 5/5e: 100 MHz
Cat 6: 250 Mhz
Cat 6A: 500 MHz
 
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 of the cable is variable with the frequency of the signal, certification testers test attenuation at many frequencies specified in the 568 specs.
UTP Attenuation
 
 
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, 6 and 6A max frequencies. Here is how a typical cable attenuation changes with frequency.
Cat 6A attenuation
 
 
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.

UTP 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.
 
 
Tests on Cat 5e/6 for Gigabit  Ethernet
 
The additional test specs for Category 5e and 6 includes a number of new tests to insure higher performance from the cable to make it compatible with Gigabit Ethernet. These tests relate to higher bandwidth usage of the cable and simultaneous use of all four pairs in both directions at once.
 
Powersum Crosstalk (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, similar to 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.

UTP 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.
UTP return loss


Requirements For Testing Cat 6/6A For 10Gigabit Ethernet

The development of augmented Cat 6 (Cat 6a) cable for use on 10 Gigabit Ethernet links added a new test. The cable is so precisely made, especially the rate of twist in the pairs, that cable pairs can interfere with the same pair in other cables nearby. This added a new  test for Cat 6A which is called "Alien Crosstalk."

alien crosstalk

Performing this test is time consuming and is highly dependent on the physical location of cables. Some controversy regarding the relevance of this test exists in the industry, with some cabling vendors not requiring it.

Cable Testers
 
Wiremappers test the connections and Cat 5e/6 certification testers test the performance at high frequencies. Cable Certifiers test the cable according to TIA-568 standards. Cable Verifiers test the cable to see if it will transmit Ethernet signals without errors.
Cable Certification testers are mostly automated, "push a button get a pass/fail" simple. 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.

Cable Verifiers are a new class of testers that use the Ethernet communications protocols to ensure the cable supports the system intended for use on it, generally a LAN or connection to a wireless access point.

Since some UTP cables are used for non-Ethernet applications like CCTV, security or building management systems that are designed to operate on TIA-568 standardized cabling, a certification tester may be a better choice for them.
 
 
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, crosstalk and attenuation.
 
Until now, each tester used personality modules specific to each manufacturer's Cat 6/6a cabling for testing. The personallity modules insured that the connection between the adapter and the link under test yield optimum performance and more valid tests. 
 
 
A change in the definition of the "link" was 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. 
 
 Test your comprehension with the section quiz.

 

Table of Contents: The FOA Reference Guide To Fiber Optics



 


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