Information Technology Reference
In-Depth Information
that may cause the link to fail. The different impedances cause a signal reflection at
the point of intersection that will affect even lower-speed network links. At the
higher speeds, the impedance match is critical to overall circuit performance. The
standards generally allow for the expected variations in impedance from one lot of
cable to the next. A variation of
15% is allowed and more may be acceptable on
short cable runs, although it would technically not meet the standard.
The real problem is that network equipment is designed to expect a cable
impedance of 100 (or 150) ohms. Using a cable with a higher or lower impedance
will cause less power to be coupled to the cable pair and result in a lower received
signal arriving at the other end. If that end also sees an impedance mismatch, even
more power will be lost in the transfer. Receiver performance depends on a certain
minimum signal strength to override the crosstalk interference from the near-end
transmission. If a lower-than-expected signal strength is received, link performance
will suffer and may even cause the link to fail.
One final note of caution: Other twisted-pair cables exist with different imped-
ances than are in the standard. For example, in some parts of the world, 120-ohm
twisted-pair cable is widely available. This cable should not be mixed with 100-ohm
cable in LAN wiring applications. Be sure your cable is the proper impedance for
standard LAN installations.
Fiber-Optic Cable Failure Modes
Fiber-optic cabling always takes a back seat to copper cabling when people are con-
sidering field failures. At one time, this was logical, because only two simple types
of cabling were available, and there were only a small number of potential fiber-
optic connectors, of which one, the ST, was clearly dominant. Nowadays, nothing
could be farther from the truth.
Modern fiber-optic cabling systems are nearly as complex as their copper coun-
terparts, as long as pair-reversals and crossed-pairs are discounted.
Fiber-Optic Installation Errors.
At the initial installation phase, fiber-optic cables
are terminated into fiber-optic patch panels in the telecommunications room. The
installer must verify that the link meets the appropriate transmission performance
standards, including link loss, return loss, and bandwidth. The installer must also
verify that the proper connectors were used, and that the connections were properly
marked.
Marking fiber-optic connections is extremely important. As we covered in the
chapter on fiber-optic technology, each pair of fibers are aligned into an “A” and a
“B” connector position. The reason for this is to ensure that the transmit and receive
