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Cabling Issues
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Half-duplex Fast Ethernet implementations limit the Multi Mode fiber distance
to 400 meters to allow for the round-trip time of the packet transmission.
Cable Problems
Cable problems may appear as intermittent issues or as a single failure. Clearly, the
intermittent issues provide greater challenges, especially if the duration of the
problem is very short. An intermittent cable problem may appear as slow perfor-
mance or failure of the workstation, which requires a reboot.
An analyzer may be the best method for finding cable problems, and
administrators should be familiar with the operation of an available cable
tester, time domain reflectometer (TDR), or handheld analyzer. Even when
certified by the cable installer, cables may break or develop problems during
subsequent activity in the conduit or at the jack.
In addition to a tester, it is a good idea to have spare cables and a crimp
set to quickly re-terminate circuits when troubleshooting.
Cross-Over Cables
A surprising number of network administrators have not used cross-over
cables, particularly when their previous experience is from the workstation
installation and configuration segments of Information Services or other
Information Technology departments. In other companies, such cables are
used only when absolutely necessary and with a great deal of documenta-
tion, including highly recommended color-coding.
Normally, a workstation is connected to a hub that does not require the
cross-over of the transmit and receive pairs in the wire. However, there are
times when a connection is needed and the pairs must be crossed. This occurs
when connecting two 10BaseT workstations together without a hub or when
connecting two network devices. Note that some devices provide a button or
other administrator-selectable setting to enable or disable the function.
Small hubs frequently provide this with an "uplink" port.
Connectivity problems may result when the wrong type of cable is installed or
when a selectable port is set incorrectly. This may be masked by link lights and
other indications that the connection is correct. The only way to isolate this prob-
lem is to look at the colors in the cube (the RJ-45 connector) and verify that they
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