Lesson 3:Network Testing and Monitoring Tools
Not all of the tools used by network administrators are purely informational. There are also physical tools (beyond the standard screwdrivers and pliers) that can come in handy. Most of these specialized tools are used to install and troubleshoot cables, primarily because this is one component of the network that has no means of displaying error messages.
After this lesson, you will be able to
- Describe the troubleshooting functions of crossover cables and loopback connectors
- Understand the uses of a tone generator and locator
- List the capabilities of more elaborate cable testing equipment
Estimated lesson time: 20 minutes
Crossover Cables
A crossover cable, which is used to connect UTP Ethernet computers without a hub, is also a good tool for eliminating the hub and the cables as a possible source of a network communications problem. If you have two computers that seem to be properly connected using a hub and prefabricated cables (or an internal cable run and patch cables), and they are not communicating, try connecting the computers with a crossover cable that you know works properly. If the computers are able to communicate using the crossover cable, you know that you have a problem in either your hub or the cables connecting the computer to the hub. If the computers fail to communicate using the crossover cable, the problem lies in one or both of the computers or network interface adapters.
A crossover cable is a UTP cable in which the transmit contacts in each of the RJ-45 connectors is connected to the receive contacts in the other connector, eliminating the need for a hub, which normally supplies the crossover circuit. For more information, see Lesson 2: Making Connections, in Chapter 15, "Installing a Network."
Hardware Loopback Connectors
A loopback connector is an inexpensive device that you plug into a jack. This connector redirects the outgoing signals from the device right back into it. You can purchase loopback connectors for parallel and serial ports, for example, that work in conjunction with diagnostic software to check the transmission and reception capabilities of the ports. In the same way, you can purchase a loopback connector that plugs into a UTP network interface adapter's RJ-45 port. Many adapters have a diagnostic utility built into their configuration programs. After plugging the loopback connector into the adapter port, you run the diagnostic program and it transmits a series of signals out through the adapter. If the adapter receives the signals back in exactly the same format as they were sent, the adapter passes the test.
Be aware that running a test using a loopback connector is completely different from transmitting packets to the TCP/IP loopback address (127.0.0.1). Even though using that address causes all transmitted traffic to return to the incoming buffers of the same computer, the signals never actually reach the network interface adapter. The loopback address is a feature of the IP protocol, and packets sent to it never travel below the network layer of the OSI reference model. In a loopback connector test, the packets travel all the way down to the physical layer and out of the computer, only to be routed immediately back in by the loopback connector.
Tone Generators and Tone Locators
When you install UTP cable internally, testing each of your connections is an absolute requirement. The last thing you will want to do—after you've pulled all of your cables, secured them all in the walls and ceilings, punched them all down, installed all the wall plates, and cleaned everything up—is tear everything apart again because of an improperly wired connection.
One of the most basic ways to identify and test a cable connection is to use a tone generator and locator (see Figure 18.25), also known as a "fox and hound" cable tester. The tone generator is a device that you connect to a cable at one end. It then transmits a signal over the cable. The tone locator is a separate device that has a probe capable of detecting the generator's signal, either by touching it to the conductor in the cable, or simply by touching it to the insulation on the outside of the cable. When the locator detects the generator's signal, it emits an audible tone. You can use this type of device to test an entire cable or to test the individual wire connections inside a UTP cable.
Figure 18.25 A tone generator and locator
Tone generators and locators are most commonly used to identify the cable belonging to a particular connection. For example, if you're performing an internal cable installation, and you forget to label one of your cables, you can connect the tone generator at the wall plate end and touch the probe to each of the cables at the patch panel end until you find the one that produces a tone. Some cable installers omit the labeling process entirely and rely completely on this method for identifying their cable runs, but this is not recommended. The tool is also valuable for identifying one particular cable in a bundle in the middle of the connection.
You can also use a tone generator and locator to test the individual wire connections inside a UTP cable. You connect the generator to a single wire or connector contact using alligator clips, and then touch the locator to each wire or contact at the other end of the cable. Using this method, you can test for any major wiring faults that affect internal UTP cable installations. For example, if you fail to detect a signal on the contact to which you have the generator connected at the other end, you have an open circuit. If you detect a signal on the wrong contact, you have punched down the wires to the wrong contacts, resulting in transposed wires. If you detect a signal on two or more wires, you have a short.
The tone generator and locator is the simplest and most inexpensive type of cable tester (at approximately $100), but this method of testing UTP cable connections is relatively unreliable and incredibly time-consuming. Testing each of the wires in a UTP cable individually is a slow process as prone to error as connecting them in the first place. You also must have two people to use the equipment—one at the generator end and one at the locator end—who are in constant contact. Or you can do this by yourself if you don't mind running back and forth from one end of your cable connections to the other. For troubleshooting a single cable connection, it's a useful tool. For testing a large number of newly installed cable runs, you can purchase a wire map tester instead that detects all the same faults by testing all of the wire connections in the cable at once.
Wire Map Testers
A wire map tester is a device that is similar in principle to the tone generator and locator, except that it tests all the wire connections in a UTP cable at once. This device also consists of two parts that you connect to the opposite ends of a cable. The unit at one end transmits signals over all the wires, which are detected by the unit at the other end. A wire map tester can detect transposed wires, opens, and shorts, just as a tone generator and locater can, but it does all the tests simultaneously and provides you with a simple readout telling you what, if anything, is wrong. The one common cable fault that a typical stand-alone wire map tester can't detect is a split pair.
A split pair is a wiring fault in which the wires are connected to the wrong contacts at both ends of the cable in exactly the same way. Each of the contacts is wired straight through to its corresponding contact at the other end, yielding a connection that appears to be correct to a normal wire map test. However, the wires that are actually carrying the signals are improperly paired. Normally, a UTP cable has one transmit wire and one receive wire, each of which is twisted into a separate pair with its corresponding ground wire. In a split pair situation, the transmit and receive wires can be twisted into one pair and their two ground wires into another pair. Having the two signal wires twisted into the same pair generates an excessive amount of crosstalk, which negatively affects communications. A wire map tester knows only that the signals it has transmitted over each wire have reached the other end of the cable at the correct contact. You need a device that can measure crosstalk, such as a multifunction cable tester, to detect split pairs.
Wire map testers are available as relatively inexpensive ($200 to $300) stand-alone devices. You can also find the same functions as part of a multifunction cable tester, which costs a great deal more. For a small to medium-sized internal cable installation, a wire map tester is a good investment, both for installation and for troubleshooting purposes later. You can use the tester to check your prefabricated cables for faults as well. For large installations or professional cable installers, a multifunction cable tester is a better idea.
Multifunction Cable Testers
Multifunction cable testers, also called media testers or certifiers, are handheld devices, like the one shown in Figure 18.26, that perform a variety of tests on a cable connection and compare the results to standard values that have been programmed into the unit. As a result, anyone can use these devices. You simply connect the unit to the cable, press a button, and the device comes up with a list of pass or fail ratings for the individual tests.
Figure 18.26 A multifunction cable tester
In addition to the basic wire mapping tests described earlier, multifunction cable testers can also test any of the following:
- Length. The most common method for determining the length of a cable is called time domain reflectometry (TDR), in which the tester transmits a signal over the cable and measures how long it takes for the signal's reflection to return. Using the nominal velocity of propagation (NVP) for the cable, which is the speed at which signals travel through the cable (supplied by the manufacturer), you can compute the length of the cable. This function also enables you to determine the location of a break in a cable.
- Attenuation. By comparing the strength of a signal at the far end of a cable to its strength when transmitted, the tester determines the cable's attenuation (measured in decibels).
- Near end crosstalk (NEXT). Testing for NEXT is a matter of transmitting a signal over one of a cable's wires and then detecting the strength of the signal that bleeds over into the other wires near the end of the cable where the transmitter is located.
- Power sum NEXT (PSNEXT). This is a measurement of the crosstalk generated when three of the four wire pairs are carrying signals at once. This test is intended for networks using technologies like Gigabit Ethernet that transmit signals over several wire pairs simultaneously.
- Equal level far end crosstalk (ELFEXT). This is a measurement of the crosstalk at the opposite end of the cable from the transmitter, corrected to account for the amount of attenuation in the connection.
- Power sum ELFEXT (PSELFEXT). This is a measurement of the crosstalk generated at the far end of the cable by three signal-carrying wire pairs, corrected for attenuation.
- Propagation delay. This indicates the amount of time required for a signal to travel from one end of a cable to the other.
- Delay skew. This is the difference between the lowest and the highest propagation delay measurements for the wires in a cable. Because the wire pairs inside a UTP cable are twisted at different rates, their relative lengths can differ, and the delay skew measurement quantifies that difference.
- Return loss. This is a measurement of the accumulated signal reflection caused by variations in the cable's impedance along its length. These impedance variations are typically caused by untwisting too much of the wire pairs when making connections.
The tests listed here are those typically found in copper cable testers. Fiber optic cables require completely different testing methodologies and usually require a different type of tester unit.
Not all of these tests are required for every cable installation, but knowing the lengths of your cables and other measurements can help you ensure that your cable installation is within the guidelines established for the protocol you will be using. Measuring elements such as attenuation and delay skew are also useful for testing cables before you install them, so that you can be sure that you've gotten the cable grade that you paid for.
Multifunction cable testers can, in some ways, be dangerous because of the very strengths they advertise. The implication in much of the marketing material for these devices is that you don't really have to know what all of these measurements mean; you can just plug your cables in and rely on the device to tell you if they're installed correctly. This is true, as long as the tester is calibrated to the proper standards. If you don't know what the various tests represent, you're relying on the manufacturer of the device to set it to the proper standards, and, in some cases, official standards for certain cable types have not yet been ratified.
It's also possible to reprogram the device with your own baseline standards, which can be a problem if you're relying on someone else's tester to tell you that your installation has been performed properly. For example, an unscrupulous cable installer could make a few simple changes to the tester's settings, such as changing the NVP rating for the cable, and cause a network that would previously have failed certain tests to pass them. The bottom line for using these devices is that you should not trust the tester of an untrustworthy person, and if you purchase a tester of your own, you should familiarize yourself with all of its tests and the standards against which it compares its results.
The other drawback of multifunction cable testers is that most of them carry extremely high price tags. Prices running to several thousand dollars are common, with top-of-the-line units (such as those that combine copper and fiber optic testing capabilities) costing $5,000 or more.
Exercise 1: Network Testing Equipment
For each of the devices listed in the left column, specify which of the faults in the right column it is capable of detecting.
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Lesson Review
- A fox and hound tester is another term for what device?
- A crossover cable
- A tone generator and locator
- A wire map tester
- A multifunction cable tester
- Which of the following cabling faults can a wire map tester not detect?
- Open pairs
- Split pairs
- Transposed pairs
- Shorts
- Which of the following types of cable tester is most expensive?
- Fox and hound
- Wire map
- Multifunction
- Loopback
Lesson Summary
- Crossover cables can eliminate cable runs and hubs as possible sources of communication problems.
- A loopback connector tests the functionality of a network interface adapter by redirecting its outgoing signals back into it.
- A tone generator and locator is a simple cable-testing device that determines whether a cable is carrying a signal.
- Wire map testers test all four of the wire pairs in a UTP cable at the same time.
- Multifunction cable testers perform a comprehensive battery of tests on a cable connection and compare the results to established standards.
EAN: 2147483647
Pages: 105