Verifying Your LAN Cables

For reliability and speed, the LAN cables in your network must form an unbroken, noise-free link between workstations and file servers. Electrical noise or other line transmission problems can cause your LAN to fail or, more often, mysteriously and insidiously slow down. When you're wiring a new LAN, modifying an existing system, or troubleshooting poor performance, a cable tester is an indispensable tool.

A cable tester contains a TDR (Time Domain Reflectometer) and perhaps additional test circuits. A TDR works by sending radar-like pulses through the LAN cable. It detects pulse reflections, analyzes them, and displays its findings. A cable tester typically can tell you the length of a cable, whether the cable is correctly wired internally (i.e., pin-to-pin wire mapping), whether it contains a short circuit (i.e., wires touching each other t hrough damaged or missing insulation), whether it contains a broken wire (or an open), and whether it suffers from electrical cross talk (i.e., interference between wires).

When noise blocks the data signal flowing through LAN cables, the network responds by retransmitting the data signal. If the noise happens infrequently, the network's retry mechanism will hide the problem and you won't even know it exists. If the noise happens with slight to moderate frequency, you'll scratch your head wondering why the network has slowed down; the retries keep the network from running at normal speed.

Two important criteria by which cable-testing hardware can judge the cable quality are signal attenuation and impedance. Attenuation is the decrease in a signal's strength over the length of a cable. A cable tester measures attenuation in decibels, with 0 dB signifying no signal loss. As an example of a typical value, the maximum attenuation allowed for 10Base-T UTP cabling is 11.5 dB.

Measurement of cha racteristic impedance, measured in ohms, is more important for coaxial cables than for twisted pair. The thickness (i.e., gauge) of the copper conductors, the distance between the conductors, and the properties of the insulation (i.e., the dialectric material) in the cable all influence impedance. The characteristic impedance of both thick and thin Ethernet cable is 50 ohms; ARCnet cable has a characteristic impedance of 93 ohms.

The method that a cable tester uses to detect discontinuities within a cable relies indirectly on cable impedance. A change in the characteristic impedance of a wire somewhere along its length causes reflections inside the cable. Broken wires, short circuits, and mixed wire types cause such unwanted signal reflections because they create impedance boundaries. By measuring signal reflections, a TDR locates discontinuities.

Cross talk is electrical interference between wire pairs in twisted-pair cable. Near-end cross talk (abbreviated NEXT) is the interference that occur s in the cable adjacent to a connector at either end. The tester measures NEXT by transmitting a signal through one wire pair and detecting the resulting spillover of current into the other wire pair.

Short circuits and open connections can occur months or years after installation, especially if cheap insulation dries out, becomes brittle, and cracks. Similarly, a water-soaked cable won't carry LAN traffic very well. Sometimes during installation a person will pull a wire around a corner and part of the insulation will scrape off (this is a shiner). The cable problem may not manifest itself until months later when it causes a network outage.

Some wiring problems happen during cable manufacture or during connection. Once in a great while, the factory or the installer will put connectors on the cable with the wrong wire leading to the wrong pin, and the new cable won't work at all. Or the person may mix up the wire pairs by attaching connectors in a way that causes one of the wires to carry a sign al that the other wire pair should carry (a condition known as reversed pairs).

Even with perfectly manufactured, carefully connected wire, you can still cause cable-related problems if you overlook the published limitations of the wiring specification in your planning of a network installation or enhancement. The result can be a LAN segment with cables that are too long or that have too many nodes in a segment. It's easy to overlook distance and number-of-nodes limitations when you're concentrating on giving people access to the network.

You should use a cable tester to check the installation of new LAN cables. When you build a new LAN or add a new cable segment to an existing LAN, you'll want to know that the new wires can carry noise-free LAN signals before you try to log on to a file server. If you have a contractor install and maintain your LAN wiring, insist that the contractor perform cable tests during the installation. If you install your own wiring, use a cable tester to check your wor k.