Chapter 4: Troubleshooting Tools

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Networks are sensitive and require constant monitoring during and after installation. As a network administrator, you should be able to diagnose network problems and take effective remedial actions using troubleshooting tools. This chapter surveys the tools for monitoring and troubleshooting network problems pertaining to Cisco internetworking devices, including routers and switches.

The troubleshooting tools discussed in the chapter include native software products developed by Cisco, Cisco IOS commands, and third-party tools that cater to specific troubleshooting needs. Each tool operates at a particular OSI layer and ascertains systematic diagnosis of network bottlenecks at that layer. However, some tools applied for troubleshooting Cisco devices such as routers and switches may impede their performance. As a result, you need to be judicious about applying a tool in a particular situation, especially in the case of Cisco IOS commands. Following the layered approach, troubleshooting tools have been discussed with respect to the seven layers of the OSI model, starting with the Physical layer.

Connectivity and Cable Testers

Cable testers are used at the Physical layer to check the network for transmission media-related problems such as high EDI rate, attenuation, and insufficient cable length. In addition, specialized equipment is available for testing cables at the lower and higher spectrum of cable testing. Let us discuss these in detail.

Low Spectrum Cable Testers

Low spectrum cable testing involves testing the transmission media for parameters pertaining to the physical connectivity of networks. These parameters are AC and DC voltage, current, capacitance, resistance, and cable continuity. Devices such as volt-ohm meters and digital multimeters check the connectivity at this level. Cable testers also check networks for attenuation, Near-End Crosstalk (NEXT), and noise. In addition, these devices test various types of cables, such as Shielded Twisted Pair (STP), Unshielded Twisted Pair (UTP), coaxial, twinax, and 10BaseT.

Some advanced cable testers provide Time Domain Reflectometers (TDR), traffic monitoring, and wire map functions. In addition, low spectrum cable testers provide limited protocol testing support using diagnostic commands, such as ping.

Low spectrum cable testers (scanners) provide information about the MAC layer, such as actual network utilization and packet error rates. This helps determine the rate of optimal network utilization and detect transit errors of data packets. In addition, cable testers help increase overall network performance by checking and troubleshooting physical connectivity.

Time Domain Reflectometers (TDRs) and Optical Time Domain Reflectometers (OTDRs) are devices that identify the cable break location, impedance mismatches, and other physical connectivity problems. Fiber optic cables require extra precaution before and after installation due to the high cost of setting up a fiber optic-based network. Therefore, it is recommended that the fiber optic cables are tested before installation. This process is called on-the-reel testing. When installation is complete, perform periodic checks on cables, using continuity testing tools such as a light source or a reflectometer. There are various types of reflectometers that provide light at standard wavelengths such as 850 nm, 1300 nm, and 1500 nm. These reflectometers work with optical power meters to measure the specified wavelengths and identify their attenuation and fiber loss. An optical power meter measures the power to and from an optical device. For optimal performance, it is a good idea to calibrate this meter every year.

Note

Fiber optic cables are highly sensitive and should be kept clean. Many problems occur due to dirty cables. Use an optical cleaning kit to keep the optical cable connection clean.

OMNI is a commonly used cable scanner provided by Microtest that has the ability to test all the cables that support a wide range of 100 dB and a bandwidth of 300 MHz.

High Spectrum Cable Testers

High spectrum cable testing involves testing the transmission media to check for impedance mismatch, crimps, and other physical problems in metallic and fiber cables, respectively. Devices such as TDRs and OTDRs check connectivity at this level.

A TDR works by “bouncing” a signal at the end of a cable. Due to various problems in the cable, open or short circuits reflect the signal at different amplitudes. Using a TDR, you can determine the time taken by the signal to reflect after it reaches the end of the cable. The TDR calculates the time based on the following formula:

Distance (d) = Propagation Rate x Time to Measure the Distance to a Cable Fault

In addition to troubleshooting, TDRs measure the length of the cable. This is measured when the signal is returned at very low amplitude after it is reflected from the end of the cable.

OTDRs test fiber optic cables and detect the length of cable breaks, attenuations, and splice losses. In addition, they calculate the number of reflections to compute connector losses.

OTDRs are also used to measure fiber attenuation using pulse reflections, which occur at breaks or joints and uniformly scatter the reflections in a backward direction throughout the fiber optic cable.

Spectrum analyzers analyze the light with respect to the wavelength that helps detect channel crosstalk. It also conducts periodic laser tests on fiber optic cables for better performance and stability.

As a network administrator, you can specify the normal baseline or standard performance of fiber optic cables, based on the normal rate of attenuation and splicing. While monitoring the network, use these baseline statistics to detect cabling problems.

Note

Fixed attenuators can be used to add fixed attenuation levels to connections, including five attenuators with 5 dB at a wavelength of 1310 nm and another 5 with 10 dB at the same wavelength.