Equipment Used to Troubleshoot the Physical Layer

Many problems that are encountered occur at the bottom of the OSI Reference model at the Physical layer. This is the layer where you convert the data to bits and place them on the physical wire in the network. Problems at the Physical layer can be some of the simplest to solve; for example, by seeing a break in the cable or a cable not plugged in.

When the network's Physical layer is intact, the Data Link layer will be where you next want to start looking for your problem.

Physical layer problems can be extremely difficult to solve, however, especially if the problems reside in the hardware. These problems can include things such as a NIC card connecting to the network or bad cabling in the wall. You also have those pesky rodents who like to chew on your sweet tasting cabling in the roof and walls. So many factors can contribute to errors and loss of connectivity that isolating the exact problem may be hard at times. Tools are available that can help in testing the physical infrastructure of a network. You should know the different types of troubleshooting equipment available to you, and how and when to use each type; this knowledge can alleviate a lot of headaches associated with network troubleshooting. The next few sections take a look at some of the tools available to test the physical layer of the network, including its cabling.

Basic Cable Testers

Noise levels, heat, insects , animals, seismic events, static, or humidity can all make subtle changes that affect cabling or devices. Some of the tools used to test network cabling are volt-ohm and digital meter. These devices measure parameters such as AC and DC voltage, currents, resistance, and cable continuity. Time domain reflectors (TDRs) are another type of cable tester; they can isolate a break in the cabling.

A cable tester is a must for anyone who works with the infrastructure in the network. Since there are so many potential Layer 1 problems, you need to be able to isolate such problems and fix them quickly. Cable testers can be used to test for physical connectivity on the existing wiring. Different types of cable testers exist; some are for general use, while others focus on a specific type of cabling.

One of the most common types of cable testers focuses on 10BaseT or 100BaseT UTP for Ethernet networks. Specific testers are made for fiber- optic cabling, but they're expensive, and so they are used mainly by companies that install fiber. Using a cable tester for Ethernet, you can find problems with physical connectivity, test the wire mapping, and report cable conditions, including near-end crosstalk (NEXT), attenuation, and noise.

If you are using an advanced cable tester, you can configure the tester with an IP address to test for LAN connectivity, as well as display MAC addresses, provide information about LAN traffic, and provide a generic look at network utilization. You also can perform tests such as ping and arp to test the LAN connectivity; these tests report the time a packet takes to reach a destination. In a large campus environment, cable tester devices are helpful in testing the basic equipment, such as data jacks and connectivity.

One of my favorite cable testers is the Pentascanner 350 shown in Figure 7.1. This tester uses multiple automated tests and displays a very clear picture of the cable's continuity and problems. By connecting two ends of a copper cable such as CAT 3, 4, 5 or 6 cable to both parts of the tester and pressing Test, the cable will be tested and the scanner will display the results. The results can also be saved and labeled for later viewing or porting to a spreadsheet.

Figure 7.1. A Pentascanner 350 cable tester.

Time Domain Reflectors (TDRs)

Time Domain Reflectors, or TDRs, are sophisticated and complex cable testers. Some of the problems TDRs look for are open circuits, short circuits, crimped wires, and anything that may inhibit the connectivity on the wire.

TDRs not only report problems with a cable, but also can isolate the location of the problem on the cable.

TDRs work with a principle similar to ping . A TDR sends an electronic signal down the physical wire, then waits until the same signal returns to the device, reporting the time intervals from sending and receiving. When the TDR receives the signal back, it looks at the signal for any abnormalities with the signal strength or other possible errors. If the TDR detects a problem, it attempts to determine where the problem could be located. Depending on the signal, the TDR reports whether the problem is a short, a crimp, open cabling, or other cabling issue. The TDR reviews the time factor of the received signal to determine where on the cable the signal may be failing. TDRs usually can determine within a foot or two the location of the problem on the cable. This type of equipment is imperative to have when you are looking at thousands of wires, each of which may be 100 to 200 feet long.

Optical Time Domain Reflectors (OTDRs)

OTDRs, or Optical Time Domain Reflectors , are advanced cable testers that analyze fiber by sending pulses. OTDRs test exclusively for fiber-optic cabling using optical pulses rather than electronic signals. OTDRs use the same testing principle used by TDRs; after sending and receiving a pulse, the testers analyze the pulse for possible problems.

Protocol Analyzers

A protocol analyzer, sometimes called a sniffer, is a software-based application that works to find faults in the network, as well as test the performance. This software enables a network administrator to capture data packets, review the contents of a data packet, decode data from the captured packets, and collect and view network traffic statistics. More advanced protocol analyzers can include features, such as traffic generators, to create traffic flows to test the network, can perform fault analysis, can provide network isolation tools, and have the ability to add remote management interfaces.