Information Technology Reference
In-Depth Information
reflectometer. Light sources capable of providing light at the three predominant
wavelengths—850 nanometers (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.
TDRs and OTDRs
At the top end of the cable testing spectrum are
TDRs
. These devices can quickly locate open and short
circuits, crimps, kinks, sharp bends, impedance mismatches, and other defects in metallic cables.
A TDR works by bouncing a signal off 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. 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 OTDR. OTDRs can accurately measure the length of the
fiber, locate cable breaks, measure the fiber attenuation, and measure splice or connector losses. An
OTDR can be used 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.
Breakout Boxes, Fox Boxes, and BERTs/BLERTs
Breakout boxes, fox boxes,
and
bit/block error rate testers
(
BERTs/BLERTs
) are digital interface testing
tools used to measure the digital signals present at PCs, printers, modems, the channel service
unit/digital service unit (CSU/DSU), and other peripheral interfaces. These devices can monitor data line
conditions, analyze and trap data, and diagnose problems common to data communication systems.
Traffic from data terminal equipment (DTE) through data communications equipment (DCE) can be
examined to help isolate problems, identify bit patterns, and ensure that the proper cabling has been
installed. These devices cannot test media signals such as Ethernet, Token Ring, or FDDI.
Network Monitors
Network monitors
continuously track packets crossing a network, providing an accurate picture of
network activity at any moment, or a historical record of network activity over a period of time. They do
not decode the contents of frames. Monitors are useful for baselining, in which the activity on a network
is sampled over a period of time to establish a normal performance profile, or baseline.
Monitors collect information such as packet sizes, the number of packets, error packets, overall usage of
a connection, the number of hosts and their MAC addresses, and details about communications between
hosts and other devices. This data can be used to create profiles of LAN traffic as well as to assist in
locating traffic overloads, planning for network expansion, detecting intruders, establishing baseline
performance, and distributing traffic more efficiently.
Network Analyzers
A
network analyzer
(also called a
protocol analyzer
) decodes the various protocol layers in a recorded
frame and presents them as readable abbreviations or summaries, detailing which layer is involved
(physical, data link, and so forth) and what function each byte or byte content serves.
