Cable testing standards / Test parameters / Time base parameters / Testing Optical Fiber

Cable testing standards (Core)
4.2.5 This page will describe the TIA/EIA-568-B standard. This standard specifies ten tests that a copper cable must pass if it will be used for modern, high-speed Ethernet LANs.


All cable links should be tested to the maximum rating that applies for the category of cable being installed.

The ten primary test parameters that must be verified for a cable link to meet TIA/EIA standards are:

• Wire map
• Insertion loss
• Near-end crosstalk (NEXT)
• Power sum near-end crosstalk (PSNEXT)
• Equal-level far-end crosstalk (ELFEXT)
• Power sum equal-level far-end crosstalk (PSELFEXT)
• Return loss
• Propagation delay
• Cable length
• Delay skew

The Ethernet standard specifies that each of the pins on an RJ-45 connector have a particular purpose. A NIC transmits signals on pins 1 and 2, and it receives signals on pins 3 and 6. The wires in UTP cable must be connected to the proper pins at each end of a cable. The wire map test insures that no open or short circuits exist on the cable. An open circuit occurs if the wire does not attach properly at the connector. A short circuit occurs if two wires are connected to each other.

The wire map test also verifies that all eight wires are connected to the correct pins on both ends of the cable. There are several different wiring faults that the wire map test can detect. The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector. If the white/orange wire is terminated on pin 1 and the orange wire is terminated on pin 2 at one end of a cable, but reversed at the other end, then the cable has a reversed-pair fault. This example is shown in the graphic.

A split-pair wiring fault occurs when one wire from one pair is switched with one wire from a different pair at both ends. Look carefully at the pin numbers in the graphic to detect the wiring fault. A split pair creates two transmit or receive pairs each with two wires that are not twisted together. This mixing hampers the cross-cancellation process and makes the cable more susceptible to crosstalk and interference. Contrast this with a reversed-pair, where the same pair of pins is used at both ends.

Other test parameters (Optional)
4.2.6 This page will explain how cables are tested for crosstalk and attenuation.


The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Insertion loss is measured in decibels at the far end of the cable. The TIA/EIA standard requires that a cable and its connectors pass an insertion loss test before the cable can be used as a communications link in a LAN.

Crosstalk is measured in four separate tests. A cable tester measures NEXT by applying a test signal to one cable pair and measuring the amplitude of the crosstalk signals received by the other cable pairs. The NEXT value, expressed in decibels, is computed as the difference in amplitude between the test signal and the crosstalk signal measured at the same end of the cable. Remember, because the number of decibels that the tester displays is a negative number, the larger the number, the lower the NEXT on the wire pair. As previously mentioned, the PSNEXT test is actually a calculation based on combined NEXT effects.

The equal-level far-end crosstalk (ELFEXT) test measures FEXT. Pair-to-pair ELFEXT is expressed in dB as the difference between the measured FEXT and the insertion loss of the wire pair whose signal is disturbed by the FEXT. ELFEXT is an important measurement in Ethernet networks using 1000BASE-T technologies. Power sum equal-level far-end crosstalk (PSELFEXT) is the combined effect of ELFEXT from all wire pairs.

Return loss is a measure in decibels of reflections that are caused by the impedance discontinuities at all locations along the link. Recall that the main impact of return loss is not on loss of signal strength. The significant problem is that signal echoes caused by the reflections from the impedance discontinuities will strike the receiver at different intervals causing signal jitter.

Time-based parameters (Optional)
4.2.7 This page will discuss propegation delay and how it is measured.


Propagation delay is a simple measurement of how long it takes for a signal to travel along the cable being tested. The delay in a wire pair depends on its length, twist rate, and electrical properties. Delays are measured in hundredths of nanoseconds. One nanosecond is one-billionth of a second, or 0.000000001 second. The TIA/EIA-568-B standard sets a limit for propagation delay for the various categories of UTP.

Propagation delay measurements are the basis of the cable length measurement. TIA/EIA-568-B.1 specifies that the physical length of the link shall be calculated using the wire pair with the shortest electrical delay. Testers measure the length of the wire based on the electrical delay as measured by a Time Domain Reflectometry (TDR) test, not by the physical length of the cable jacket. Since the wires inside the cable are twisted, signals actually travel farther than the physical length of the cable. When a cable tester makes a TDR measurement, it sends a pulse signal down a wire pair and measures the amount of time required for the pulse to return on the same wire pair.

The TDR test is used not only to determine length, but also to identify the distance to wiring faults such as shorts and opens. When the pulse encounters an open, short, or poor connection, all or part of the pulse energy is reflected back to the tester. This can be used to calculate the approximate distance to the wiring fault. The approximate distance can be helpful in locating a faulty connection point along a cable run, such as a wall jack.

The propagation delays of different wire pairs in a single cable can differ slightly because of differences in the number of twists and electrical properties of each wire pair. The delay difference between pairs is called delay skew. Delay skew is a critical parameter for high-speed networks in which data is simultaneously transmitted over multiple wire pairs, such as 1000BASE-T Ethernet. If the delay skew between the pairs is too great, the bits arrive at different times and the data cannot be properly reassembled. Even though a cable link may not be intended for this type of data transmission, testing for delay skew helps ensure that the link will support future upgrades to high-speed networks.

All cable links in a LAN must pass all of the tests previously mentioned as specified in the TIA/EIA-568-B standard in order to be considered standards compliant. A certification meter must be used to ensure that all of the tests are passed in order to be considered standards compliant. These tests ensure that the cable links will function reliably at high speeds and frequencies. Cable tests should be performed when the cable is installed and afterward on a regular basis to ensure that LAN cabling meets industry standards. High quality cable test instruments should be correctly used to ensure that the tests are accurate. Test results should also be carefully documented.

Testing optical fiber (Optional)
4.2.8 This page will explain how optical fiber is tested.


A fiber link consists of two separate glass fibers functioning as independent data pathways. One fiber carries transmitted signals in one direction, while the second carries signals in the opposite direction. Each glass fiber is surrounded by a sheath that light cannot pass through, so there are no crosstalk problems on fiber optic cable. External electromagnetic interference or noise has no affect on fiber cabling. Attenuation does occur on fiber links, but to a lesser extent than on copper cabling.

Fiber links are subject to the optical equivalent of UTP impedance discontinuities. When light encounters an optical discontinuity, like an impurity in the glass or a micro-fracture, some of the light signal is reflected back in the opposite direction. This means only a fraction of the original light signal will continue down the fiber towards the receiver. This results in a reduced amount of light energy arriving at the receiver, making signal recognition difficult. Just as with UTP cable, improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber.

Because noise is not an issue when transmitting on optical fiber, the main concern with a fiber link is the strength of the light signal that arrives at the receiver. If attenuation weakens the light signal at the receiver, then data errors will result. Testing fiber optic cable primarily involves shining a light down the fiber and measuring whether a sufficient amount of light reaches the receiver.

On a fiber optic link, the acceptable amount of signal power loss that can occur without dropping below the requirements of the receiver must be calculated. This calculation is referred to as the optical link loss budget. A fiber test instrument, known as a light source and power meter, checks whether the optical link loss budget has been exceeded. If the fiber fails the test, another cable test instrument can be used to indicate where the optical discontinuities occur along the length of the cable link. An optical TDR known as an OTDR is capable of locating these discontinuities. Usually, the problem is one or more improperly attached connectors. The OTDR will indicate the location of the faulty connections that must be replaced. When the faults are corrected, the cable must be retested.

The standards for testing are updated regularly. The next page will introduce a new standard.