Cable testing verifies you've completed an installation in accordance with all of the terms and conditions of the contract and industry standards.
When you're testing copper cabling, it's important to understand and follow three distinct phases: visual inspection; test measurements; and documentation. The visual verification step should include all pathways and spaces (where possible), telecommunications closets, and equipment rooms. This means inspecting the following elements at minimum: infrastructure; grounding and bonding; cable placement; cable termination; equipment and patch cords; and the proper labeling of all components.
Finding the right testers. After completing your visual inspection and correcting all discrepancies, it's time for testing. All field testers perform a specific function or a range of functions required for the testing and certification of a specific cable type. These instruments vary greatly in capability and price. Therefore, you must select the most cost-effective unit to perform the necessary tests for your application. To help you find the right tester for your specific installation, here's a brief overview of the types of cabling field test equipment on the market today.
Multimeter. The multimeter is probably the most basic and widely used field tester. Available in analog and digital varieties, multimeters measure voltage, current, and resistance in copper wires. By using a shorting device on one end of the pair, you can also test continuity. When testing coaxial cable, it's possible to calculate the length of the cable by determining the actual resistance of the loop. Since most coaxial cable LAN applications require only continuity and length verification, the multimeter is an ideal choice for this type of testing.
Induction amplifier/tone generator. Also known as a toner or cable tracer, this type of equipment enables installers to identify a specific pair by generating a tone on one end of the pair - with an inductive amplifier identifying it at the opposite end. Most units are now a combination of tone generator and continuity tester, commonly known as a wand and toner set. This solution is ideal for cable identification and troubleshooting.
Wire map testers. Wire map testers, also known as pair scanners, are low-cost cable testers that usually test for opens, shorts, crossed pairs, and miswires (such as reversed pairs in a 4- or 25-pair cable). Some testers in this category also test for split pairs. These devices are good for quick, basic tests; however, they lack the sophisticated diagnostic capabilities of more expensive testers. Most manufacturers design these units exclusively for UTP/ScTP.
Cable-end locator kit. Sometimes called an office locator kit, this is a set of numbered 8-pin modular plugs, which the cable test equipment can identify. With this option, you insert the plugs into outlets in the work area, then search with the tester until it finds the plug at the opposite end of the cable.
Certification field testers. Installers use certification field testers to verify a cabling system meets the transmission performance requirements as specified in TIA/EIA TSB-67. All variations of these units test a cabling system up to at least 100 MHz. In the autotest mode, they include length, attenuation, wire map, and near-end crosstalk (NEXT) tests. When operating in autotest mode, the field tester compares the actual measured values with required values for Cats. 3, 4, or 5, and displays pass or fail for the entire battery of tests. It also displays pass or fail and the actual tested values for each parameter.
These devices can also perform other measurements, including impedance, capacitance, resistance, delay, delay skew, equal level far-end crosstalk (ELFEXT), and attenuation-to-crosstalk ratio (ACR) calculations. In addition to the TIA/EIA TSB-67 standard, they include the classes of ISO/IEC and the pass/fail criteria in their database. Each certification field tester is configured to test continuity and length of coax. Several have the ability (with an add-on module) to perform the power meter test for attenuation of optical cable fiber. Certification testers can store test data and export it to a database or output it to a printer. Most field testers can also link to a PC and download and store data directly to the PC hard drive or floppy disk.
Certification test sets. Certification test units test a UTP and ScTP cabling system to at least 100 MHz, and measure/record the following parameters: wire map length; attenuation; NEXT, as well as return loss; ELFEXT; ACR; propagation delay; delay skew; power sum NEXT; power sum ACR; and power sum ELFEXT. We expect these additional parameters to be part of TIA/EIA TSB-95 and ANSI/TIA/ElA-568-A-5, "Enhanced Category 5E." The autotest feature compares the actual measured values with required values for Cats. 3, 4, or 5 performance and displays pass or fail for the entire battery of tests. The certification test equipment also displays pass or fail and the actual measured values for each test individually. These units can also test for impedance, capacitance, and loop resistance and store/export data to a database or output to a printer.
Time domain reflectometer (TDR). The TDR locates and tests all cable defects, splices, and connectors and gives loss values for each occurrence. Originally developed for use on coaxial networks, the TDR can measure the electrical length of a cable and is an excellent troubleshooting tool for UTP, ScTP, and STP-A. To measure the cable, you inject a high rise-time pulse into the cable, and then look for the reflections caused by impedance mismatches to return.
Telephone test set (butt set). We use a telephone test set to test voice circuits and the following functions:
- Simulate the user's telephone equipment,
- Identify circuits, and
- Circuit diagnostics and troubleshooting.
Test adapters, leads, and cables. The cabling installer must have proper adapters available to connect the test equipment to the cable under test. If your tester isn't performing as well as it should, see the table, right, to aid in your troubleshooting.
Necessary tests. Most of the testers outlined above use an autotest function to make measurements, which automatically completes a test series. The most important of these tests include:
NEXT. You can evaluate this condition across a frequency range, or compare it against a category or classification limit through a frequency range. You must make NEXT measurements at both ends of the cabling. The category or classification limit determines pass/fail of the cabling.
Delay. Evaluated at all frequencies from 1 MHz to the highest frequency for a given category or compared against a category limit for each frequency.
Attenuation to crosstalk ratio (ACR). ACR is a mathematical formula that calculates the ratio of attenuation to near-end crosstalk for each combination of cable pairs. An ACR test provides the following information: pass/fail (ISO/IEC 11801 only); worst-case ACR; worst-case frequency; and margin.
Return loss. Return loss measures the difference between the test signal's amplitude and the amplitude of signal reflections returned by the cable. The return loss test indicates how well the cable's characteristic impedance matches its rated impedance.
Delay skew (ANSI/TINEIA-568-A-1). Pairs within a 4-pair cable each use a different twist length to improve NEXT performance. This introduces a physical difference in length between pairs. Additionally, the velocity of propagation may vary between pairs if you're using different insulating materials for one or more pairs. We refer to these cables as 1`3, 2`2, and 3`1 cables. A number of field testers have settings to account for this difference, which should be within the operating limits of current cabling and applications standards.
Variations between pairs should not be a major source of concern when checking the quality of the installation. However, delay skew can become a significant factor in the performance of such recently introduced LAN technologies such as 100VGAnyLAN and 100Base-T2/T4. A value of 45 ns maximum delay skew has been established under Addenda 1, ANSI/TIA/EIA-568-A for cable.
Once your testing is complete, don't forget about documentation. This final phase is one of the most critical steps to proper installations. Documentation serves as the only detailed written record of your work. Without it, you may be left out in the cold. Guidelines for effective documentation depend on the installation. However, if not specified by the customer, you should follow the rules for documentation listed in ANSI/TIA/EIA-606, Administrative Standard for the Telecommunications Infrastructure of Commercial Buildings.