Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 2 Troubleshooting Overview
Troubleshooting Tools
Low-End Cable Test Equipment
At the low-technology end of the spectrum of test equipment are volt-ohm meters and digital
multimeters. These devices measure parameters such as AC and DC voltage, current, resistance,
capacitance, and cable continuity. They can be used to check physical connectivity.
Cable testers (that is, scanners) can also be used to check physical connectivity. Cable testers give users
access to physical-layer information and are available for shielded twisted-pair (STP), unshielded
twisted-pair (UTP), 10BaseT, and coaxial and twinax cables.
Cable testers can perform one or more of the following functions:
• Test and report on cable conditions, including near-end crosstalk (NEXT), attenuation, and noise
• Perform time domain reflectometry (TDR), traffic monitoring, and wire map functions
• Display media access control (MAC) layer information about LAN traffic, provide statistics such as
network utilization and packet error rates, and perform limited protocol testing (for example,
TCP/IP tests such as ping)
Similar testing equipment is available for fiber-optic cable. Due to the relatively high cost of fiber cable
and its installation, it is recommended that fiber-optic cable be tested before installation (that is,
on-the-reel testing) and after installation. Continuity testing of the fiber requires either a visible light
source or a reflectometer. Light sources capable of providing light at the three predominant
wavelengths---850 nm, 1300 nm, and 1550 nm---are used with power meters that can measure the same
wavelengths and test attenuation and return loss in the fiber.
One of the cable scanners available from Microtest, for example, is the OMNI Scanner. The OMNI
Scanner has the functionality to test cables complying with current and upcoming standards with an
extremely wide dynamic range of 100 dB and the ability to support up to 300 MHz bandwidth. The
OMNI Scanner can test all the way up to 300 MHz on Category 7 cables.
High-End Cable Testers
At the most technologically advanced end of the cable testing spectrum are time domain reflectometers
(TDRs). These devices can quickly locate opens, shorts, crimps, kinks, sharp bends, impedance
mismatches, and other defects in metallic cables.
A TDR works by reflecting a signal from the end of the cable. Opens, shorts, and other problems reflect
the signal back at different amplitudes, depending on the problem. A TDR measures how much time it
takes for the signal to reflect and calculates the distance to a fault in the cable. When a signal reaches
the end of a cable, it reflects at a very low amplitude, so TDRs can also be used to measure the length of
a cable. Some TDRs can also calculate the propagation rate based on a configured cable length.
Fiber-optic measurement is performed by an optical TDR (OTDR). These devices can accurately
measure the length of the fiber, locate cable breaks, measure the fiber attenuation, and measure splice or
connector losses by measuring the reflections that occur. Pulse reflections that are generated at breaks
or joints, and backscatter reflections that are generated uniformly throughout the cable, are used to
measure the fiber attenuation. One way in which the OTDR can be put to good use is to take the signature
of a particular installation, noting attenuation and splice losses. This baseline measurement can then be
compared with future signatures when a problem in the system is suspected.