System information

Troubleshooting Fiber Distributed Data Interface 5-71
Troubleshooting FDDI
• Scrambling—When no data is being sent, FDDI transmits an idle pattern that consists of a string
of binary ones. When this signal is sent over twisted-pair wire, the EMI is concentrated at the
fundamental frequency spectrum of the idle pattern, resulting in a peak in the frequency spectrum
of the radiated interference. By scrambling FDDI data with a pseudo-random sequence prior to
transmission, repetitive patterns are eliminated. The elimination of repetitive patterns results in a
spectral peak that is distributed more evenly over the spectrum of the transmitted signal.
• Encoding—Signal strength is stronger, and EMI is lower when transmission occurs over
twisted-pair wire at lower frequencies. MLT3 is an encoding scheme that reduces the frequency
of the transmitted signal. MLT3 switches between three output voltage levels so that peak power
is shifted to less than 20 MHz.
• Equalization—Equalization boosts the higher frequency signals for transmission over UTP.
Equalization can be done on the transmitter (predistortion), at the receiver (postcompensation),
or both. One advantage of equalization at the receiver is the ability to adjust compensation as a
function of cable length.
Troubleshooting FDDI
This section provides troubleshooting procedures for common FDDI media problems.
Table 5-1 outlines problems commonly encountered on FDDI networks and offers general
guidelines for solving those problems.
Table 5-1 Media Problems: FDDI
Media Problem Suggested Actions
Nonfunctional FDDI ring
Step 1 Use the show interfaces fddi exec command to determine
the status of the router’s FDDI interfaces.
Step 2 If the show interfaces fddi command indicates that the
interface and line protocol are up, use the ping command
between routers to test connectivity.
Step 3 If the interface and line protocol are up, make sure the MAC
addresses of upstream and downstream neighbors are as
expected.
Step 4 If all zeros appear in either of the address fields for these
neighbors, there is probably a physical connection problem.
In this case (or if the status line does not indicate that the
interface and line protocol are up), check patch-panel
connections or use an OTDR
1
or light meter to check
connectivity between neighbors. Ensure that signal strength
is within specifications.
1 OTDR = optical time-domain reflectometer
Upstream neighbor has failed and bypass
switch is installed
Bypass switches can cause signal degradation because they do not
repeat signals as a normal transceiver does.
Step 1 Check upstream neighbor to determine whether it is
operational.
Step 2 If the node is down and a bypass switch is in place, resolve
any problems found in the upstream neighbor.