Workshop

This is called looping back, because you create a loop at one end of the cable. Loopback is a powerful
diagnostic technique that can also pinpoint which part of your hardware is broken. When you’re in a
situation where you cannot swap a network card (suppose you only have one high-speed network card in
your server), you’ll probably want to run the diagnostics program that comes with the network card. This
program will likely ask you for a loopback plug—one that makes the network card talk to itself by
connecting its send pins directly to its receive pins.
Ethernet uses an RJ-45 plug, where only four pins out of eight really matter. Some Token-Ring cards use
a DB-9 plug, where only four pins out of nine really matter. Some can use either a DB-9 or an RJ-45.
If your network card seems okay when you connect the loopback plug to it, you can rule out cable
problems by moving the loopback plug farther up the line. For example, you can connect the
workstation’s cable that normally plugs into the wall in between the network card and the loopback plug.
You might need a gender-changer to do this (which looks very much like a telephone coupler for an RJ-45
plug). You plug the loopback into the coupler, the coupler to the network cable, and the network cable
into the network card, and then run the diagnostic test again.
If that seems okay, you can connect the loopback plug to the end of the cable that normally plugs into the
hub and then test again. This rules out the entire cable run as a source of transmission or reception
problems.
Out of Gas on the Information Superhighway
Running out of system resources is enough to make any operating system sing the blues—and stop you
from operating properly. Knowing how to rule out (or rule in) a resource problem gets you closer to a
solution.