By Jason Chesla, T3 Innovation
Test products: Have you even been stumped when your network goes down and you can't find anything wrong? When this happens, there's a good chance that it's due to a cable fault. Here are some tips on measuring an Ethernet cable using a cable tester.
In order to find out what's wrong and where the fault is, there are several basic tests that you need to carry out. These will help you identify the reason for the faulty cable.
· Cable length: Ethernet cables should be limited in length between active devices to a maximum distance of 100 meters or 328 feet. The cable length can be measured by either a capacitive tester that's calibrated to the capacitive characteristics of the cable or with a Time Domain Reflectometer (TDR) tester. While both types can measure cable length, a TDR tester is more useful because it can measure the distance to a fault, a feature the capacitive tester lacks. It's also worth checking if the tester reports the length of each cable pair, as some testers only report the average length of the four pairs in an Ethernet cable.
· Cable and wiring faults: Often the reason for poor performance of an Ethernet cable is due to damage to the cable, which causes an open circuit or insulation damage that creates contact between two adjacent wires. Other issues are split pairs, reversed connections and crossed wires. These problems are hard to spot unless a sophisticated cable tester is used.
· Power over Ethernet (POE): POE is when a device receives power through the Ethernet cable, usually from a POE power supply or POE enabled switch. Problems can be caused by high circuit resistance, which means that the device does not receive the required voltage and current. There are two standards: a 12.5 watt supply and a 25 watt power supply with maximum voltages of 57 volts. A tester should be able to measure POE voltage and current and identify the class.
· Location: It's sometimes necessary to understand which port on a hub an Ethernet cable is connected to so as to be able to correctly trace cables. Many testers offer a link light facility that injects a signal at a user-specified frequency that causes the switch or hub light to flash. Another useful function is the ability to inject an audio signal into a cable, using a detector to follow the cable route and to identify the correct termination.
· Link speed: The link speed reflects the actual Internet speed that the link is capable of. This is also determined by other equipment in the circuit that should be compatible with the advertised connection speed.
These tests enable a technician to identify Ethernet cable faults and the characteristics of the cable.
Ethernet Cable Classes
There are several Ethernet cable classes that are intended for different applications. These range from Category 3 (Cat 3) cables, often used for telephone lines, right up to Cat 7, used for really fast Internet applications. The category of the cable should always be printed on the cable, making it relatively easy to identify what cable is used.
· Cat 3 cable: A Cat 3 cable consists of a single twisted pair and was commonly used for residential telephones lines. Sometimes the cable may contain two, three or even four pairs. It is capable of operating at 10 Mbit/s. It is hardly ever used in new installations, although is commonly found in older buildings.
· Cat 5 cable: Cat 5 cable comprises the backbone of most networks. It has eight wires formed into four twisted pairs. The degree of twisting in each pair differs significantly and is between 52 and 72 turns per meter. The reason for this is to reduce interference and crosstalk between the pairs. Because of this, the signal to noise ratio for balanced lines is good. Cat 5 cable may have stranded conductors that are used for flexible connections and solid conductors for fixed installations. It's important to be careful not to bend these cables too sharply; the minimum bend radius is four times the outer diameter of the cable, so in practice the bending radius should be around one inch. Cat 5 cables can operate at 100 MHz and are suitable for 10BASE-T, 100BASE-TX and Gigabyte Internet speeds. A higher specification cable, Cat 5e has tighter crosstalk specifications. Cat 5 cables are suitable for 25 watt POE.
· Cat 6 cable: Cat 6 cable is visually similar to Cat 5e cable but has a higher bandwidth (250 MHz) and more stringent specifications than Cat 5e. This cable is backwardly compatible to Cat 5, but with its higher bandwidth can operate at higher speeds, including 10GBASE-T Internet. When used at this speed, the maximum cable length that's possible is significantly reduced. A higher specification cable, Cat 6a, which can operate at 500 MHz, is able to operate over the normal 100 meters (328 foot) distance.
Ethernet Cable Measurements Issues
As Internet speeds have increased, so has the potential for problems. Apart from issues such as split pairs, crossed wires and reversed connections, external interference can cause significant noise. At higher network speeds, it's also important to be very careful to terminate cables properly and to avoid cutting the insulation back more than is necessary.
The quality of the cable may also cause problems. In particular, many cheaper cables sold as Cat 6 cables may exhibit excessive cross talk and return loss and, in fact, be inferior to Cat 5 cables. So, when experiencing difficulty operating at the advertised speed, consider the possibility that the cable itself may be at fault.
Cable Prowler™ from T3 Innovation
The proper measurement of Ethernet cables is not possible without a dedicated tester. One such tester is the Cable Prowler from T3 Innovation. This low cost unit has capabilities that exceed those of many more expensive units. It has a bright, high resolution, full-color screen that's easy to read. It's suitable for use on Cat 3, 5, 5e, 6, and 6a cables as well as on coaxial and telephone cables. It's able to show the advertised Internet speed for Gigabyte Internet and link to circuits operating at 100BASE-TX speed.
The screen graphically displays each cable pair in different colors and clearly indicates faulty connections. The actual length of each pair is accurately measured using TDR (Time Domain Reflectometry) so it's possible obtain the minimum and maximum length of each pair. POE can be measured and the power as well as minimum and maximum load voltages are displayed.
Single-ended cable tests as well as continuity tests using remote connecters supplied with the tester is possible. The unit can do a link status test, has a link light ability as well as a tone generator for tracing cables. Tests can be labeled, saved and printed.