System information

Troubleshooting Tools 2-13

TDRs and OTDRs

Cable testers (scanners) also enable you to check physical connectivity. Cable testers are available

for shielded twisted pair (STP), unshielded twisted pair (UTP), 10BaseT, and coaxial and twinax

cables. A given cable tester might be able to perform any of the following functions:

• Test and report on cable conditions, including near-end crosstalk (NEXT), attenuation, and noise

• Perform 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 this

cable and its installation, ﬁber-optic cable should be tested both before installation (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 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.