If 90% of all problems in a network are on the Physical Layer, then it isn’t any surprise that cable testing has become quite important in the industry. And although it may sound simple to test a pair of cables for faults, actually troubleshooting wire maps, crosstalk, propagation delay, or insertion loss can be more than headache-worthy. And thus, understanding how to troubleshoot these problems saves time, yet also supplies good information to prevent common mistakes in the future.
How to Avoid Crosstalk
You’ll come to find that the most common mistakes are usually the simplest to avoid. For instance, crosstalk is very commonly created when connectors are not installed properly at both ends of the cable. Crosstalk is the effect we get when electromagnetic energy from one cable leaves an imprint on adjacent cables. (You’ll often see this referred to as “noise.”)This usually isn’t a problem, however, since we twist wires inside Ethernet cable to cancel out this effect. So how, then, does crosstalk become such a problem?
You’ll notice that the Ethernet cable on the right has too much wire left over- we actually need to crimp the connector to the point where no internal wiring is visible. It’s important to note that while the wires do need to be separated to properly install a connector, they should only be separated as little as possible. Otherwise the lack of cancellation will create crosstalk- and possibly cause hard-to-track failures in a network.
How to Avoid Signal Attenuation
Attenuation is the decrease in signal amplitude. If we have a small signal, it becomes increasingly harder to decipher the signal. Much like a yell is easier to understand that a faint whisper, computers appreciate healthy signal amplitudes.
You’ll notice in the above diagram that as distance increases, amplitude decreases. This becomes a problem since computers communicate in this instance via two values; high values and low values. Since the high value becomes increasingly similar to a low value, the signal will eventually become worthless for communication.
Several factors create attenuation- mostly resistance in the copper wire and leaked signal energy. So to fix the problem, we try to stick with the recommended cable lengths. If you indeed need a longer cable run, you may clean the signal by adding a repeater, switch, router, or other devices to your network so that the signal can be regenerated.
It’s also important to note that as frequency (the number of cycles a second) increases, so does the noise and interference.
Cable Testing Devices
It’s generally considered vital to test a cable after it is made, repaired, or otherwise interfered with. We can do this via several different types of devices.
One of the easiest solutions to testing a cable is to look at a wire map. The device will output the wire map on a screen, so that you may review it and check for the correct wiring. A wire map can also tell us is there are any short-circuits, opens, or reversed-pair faults within the wiring. If one of these faults are indeed found, you’ll need to cut off the connector and reapply a new one- this time paying more attention to the wiring process. Below you can see some of the common wiring mistakes for a straight-through cable, as viewed from a wire map.
Other more advanced devices may test for thing such as propagation delay. Propagation delay is a measurement of how long a signal takes to get from one point to another on a cable. Obviously if there is an abnormally long wait time, we will need to adjust the length of the cable accordingly. However, most wiring jobs do not necessarily need tests such as these, and a wire map will suffice.
One last thing to keep in mind about cable testers is that they can indeed test for crosstalk. There are several types of crosstalk, each particularly harmful to your network. It is generally a good idea to test for crosstalk, although the skilled cable maker will know how to properly install a connector, and thus, this test isn’t as vital.
Notes on Fiber Optic Media
If you’ve worked with fiber optics, you know that optical fiber is resistant to noise and outside interference- unlike copper. Thus, we only have one concern when dealing with fiber optic cable: keeping the signal strength strong. Since we don’t have to worry about interference, we can cable fiber optic media much farther than twisted-pair cable.
We noted earlier that computers communicate with high values and low values. However, it would be more accurate to claim that they instead communicate via “on” and “off” indications. Fiber optic media uses light to signal this on or off state.
You’ll generally want to use fiber optic cable when copper media proves to be too limited for long distances, or noisy environments. It may cost a little more, but the speed and efficiency fiber optic cable provides is well worth it. And in longer cable runs, fiber optic cable will actually cost less than copper media.
Cisco doesn’t put a lot of emphasis on cable testing, but many of their courses require a student to know how to make a proper cable. The CCNA 1 Module 4 exam also covers a lot of the material mentioned above.