Misdiagnosing a cabling problem is a serious troubleshooting concern.
Becoming an effective troubleshooter takes years of practice and experience. Determining the nature of the problem and making sure it's cabling related is a good start to your troubleshooting process. But, without the proper diagnostic skills, equipment, and experience, you can easily misdiagnose a cabling problem.
Finding clues to determine the problem. Many associated network problems can appear to be cabling related, but may instead be associated with hardware or software - such as discovering your cabling problem is actually a defective network interface card. Conversely, many cabling problems can appear to be equipment related. For example, "out-of-spec" cabling can result in transmission errors, which will often be manifested as poor performance. The first thing to do when troubleshooting is talk to the user and ask questions.
Once you've exhausted all possible questions, and you still haven't determined what the problem is, begin again with a simple visual inspection.
Look around for anything unusual. Check the work area for anything that suggests intervention. Examine telecommunications closets for clues, such as sloppy wiring or evidence of recent changes. Be alert for signs of recent construction or electrical work.
Use a cable tester to check suspected cabling segments. The results will often provide a good indication of possible causes and the location of a problem. If the tester has an autotest function, be sure to look at the results of the individual tests.
Break the problem into smaller parts. If performing a basic link test, examine each component (the channel and each patch cable) separately. For example, if analyzing a 10Base-2 bus, break the bus at the midpoint and then test each half. If the problem is isolated to one half or the other, break the problem section in half again and repeat the process until you locate the defect.
Look for defective terminations or improperly installed connectors. Loose, improperly installed, or damaged connectors are a major source of cabling problems. Check for intermittent problems by moving or bending the cable under test near the point of termination.
Ensure proper employment of outlets and other connections. For example, outlets designed for Cat. 3 use will probably not meet Cat. 5 specifications.
Pay attention to installation practices. Cabling installers who do not follow proper installation practices can create a multitude of problems - most commonly not maintaining twists close enough to the point of termination and splitting pairs. This is usually due to using the old USOC pinouts instead of T568A or T568B pinouts.
Examine equipment and patch cords carefully - especially in Cat. 5 installations. It's easy to accidentally damage patch cords, especially at or near the point of termination. It's difficult, for example, to maintain the pair twist in close proximity to the modular connector when making Cat. 5 patch cords. Therefore, purchasing a patch cord from a reputable manufacturer rather than making one yourself is a good idea.
Look for missing terminators on 10Base-2 systems. Reinstallation is often faulty during equipment moves and additions.
Look for user intervention. This is a major problem with 10Base-2 systems. Adding sections of cable to a 10Base-2 bus adds a drop cable between a 10Base-2 T-fitting and a network interface card and extends the bus by using an incorrect cable. (For example, cables for IBM 3270 terminals employ the same BNC connectors used by 10Base-2, but are of a different impedance.) A user may have removed one or more computers from the network with the 10Base-2 T-fitting still attached, thus breaking the bus.
Inspect for sources of interference. For example, cable running over fluorescent lights, down elevator shafts, or in parallel with and in close proximity to AC power lines can cause problems.
Check for excessive cabling distances - especially extra-long patch cords. This is a common problem with 10Base-2 systems.
Applying the proper troubleshooting tools. You should also be familiar with the tools required to complete these tests. Let's take a look at some testers so you'll know where and when to make best use of each.
Wire map tester. Wire map testers are low-cost cable testers that test for opens, shorts, crossed pairs, and improper wiring. These units are good for quick, basic tests, but lack the sophisticated diagnostic capabilities of more expensive testers, and are typically for UTP type of cable.
Handheld cable tester. Testers in this category usually provide, at a minimum, the functions of a wire map tester, including that of an optical time domain reflectometer (OTDR). You can use them to measure near-end crosstalk (NEXT), attenuation, and induced noise.
Some even include features for identifying multiple cables and tracing cables in the wall or ceiling. They generally test at frequencies up to 20 MHz; and therefore, are not suitable for certifying Cat. 5 installations. Many are designed to work with UTP, ScTP, STP-A, and coaxial cable. More expensive units store results and allow you to print or export the readings to a database.
Certification test set. These units include all of the functions of the handheld testers described above. They also have features that allow them to verify whether a cabling system meets the transmission performance requirements of EIA/TIA TSB-67.
Certification test units will test a UTP and ScTP cabling system to at least 100 MHz. These units can also measure and record wiremap, length, attenuation, and NEXT.
The autotest feature compares the actual measured values with required values for Cats. 3, 4, or 5 performance and displays pass or fail for the entire battery of tests. The certification test equipment also displays pass or fail and the actual measured values for each test individually. These units are capable of other tests, including impedance, capacitance, and loop resistance. Certification testers can store data and export it to a database or output it to a printer.
Cable tracer. A cable tracer is an accessory for certain cable testers and certification test sets. It consists of a signal generator and signal receiver. The generator connects to one end of the cable and the receiver and traces the path of the cable - through the wall, floor, or ceiling.
Cable-end locator kit. Sometimes called an office locator kit, this is a set of numbered 8-pin modular plugs, each of which the locator can identify. Inserting the plug into an outlet and searching a patch panel to identify the signal is standard practice.
Optical light source and power meter. These are useful in performing loss (attenuation) measurements on optical fiber cable.
Optical fiber flashlight. Use this to determine the continuity of optical fiber strands. It generates a safe light you can check with the naked eye.
Infrared conversion card. This card allows you to visually detect an infrared signal when such a source emits on the card's phosphorus material.
Low-intensity laser. Also known as the hot red light, the low-intensity laser operates in the visible light range. You use the laser to identify individual fibers within a cable by passing a red light down the fiber. When used as a troubleshooting tool, the cable will glow red at the break point.
Multimeter. A multimeter is a combination electrical meter, which at a minimum measures voltage, current, and resistance.
Electromagnetic field strength meter. An electromagnetic field strength meter measures the presence of electromagnetic interference.
Toner/wand. You use a toner to trace and identify cables. The unit operates by placing an audio tone on the cable for you to hear through a telephone test set or identify with a wand.
Telephone test set. Sometimes referred to as a butt set, a telephone test set simulates your telephone equipment and is used to identify circuits.
Miscellaneous tools and equipment. Other equipment you'll probably need to have on hand includes a ladder, two-way radio, fiber or copper talk sets, alcohol and wipes, compressed air, notebook, and inspection microscope.
Developing the knowledge to pinpoint problems on a network and applying the proper tools to troubleshoot a situation is essential to any cabling job. When you go into your next project armed with these troubleshooting tips and background information, you'll enjoy more successful installations every time.
Sidebar: Never Assume Anything
Remember, correlation does not necessarily mean causation. Don't make assumptions. Sometimes, equipment moves, construction work, or other events can be related and may indicate possible causes of problems. To find out, ask questions such as the following:
- Has the basic link or channel in question ever worked?
- Has the basic link or channel in question been tested?
- When and how did the problem begin?
- Is there some specific event that correlates with the start of the problem? If the problem is affecting a single connection, the chances are it is within that specific channel or the equipment directly connected to it. If the problem affects multiple links, it could be related to backbone cabling, a link to a shared service, or with equipment such as a hub or a server.
- What is the scope of the problem? Is it one connection or many?
- Does the problem affect all activity or specific applications? If only specific applications are affected on a network link, chances are the problem is not cabling related. Some applications are more sensitive to cabling variations than others; some cabling problems may only be associated with specific applications.
- Is the cable damaged? Improper installation practices, construction, or remodeling damages installed cable. Also, furniture pushed against a wall plate may create a bend radius smaller than standards allow.
