Information Technology Reference
In-Depth Information
•
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.
Table5-1
Media Problems: FDDI
Media Problem
Suggested Actions
Nonfunctional
FDDI ring
1.
Use the
show interfaces fddi exec
command to determine the
status of the router's FDDI interfaces.
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.
3.
If the interface and line protocol are up, make sure the MAC
addresses of upstream and downstream neighbors are as
expected.
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.
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.
1.
Check upstream neighbor to determine whether it is
operational.
2.
If the node is down and a bypass switch is in place, resolve any
problems found in the upstream neighbor.
1.
OTDR = optical time-domain reflectometer
