Information Technology Reference
In-Depth Information
direction of the optical signal, but merely acts similar to pin numbers in a conven-
tional connector.
Furthermore, because every fiber cable and adapter coupling reverses the A/B
order, you could not tell which fiber carried which signal, even if you knew how the
signals started out, unless you traced every fiber through all the polarity reversals.
As a user, the only thing that you would know is that the reversals had been
arranged in such a way that whatever signal started out on the A position at one end
of the link will be on the B position at the other end. This is done to create the trans-
mit/receive reversal that must be done for a fiber link to function.
Remember that some fiber-optic links may use laser illumination that is poten-
tially hazardous. Most LAN connections that you encounter will use optical power
levels far below that which would actually cause a problem, but you should still use
reasonable precautions when working around fiber. As we cover in Chapters 8 and
12, you should never look directly into an open fiber connector (or an unterminated
fiber that might be part of an active circuit). The light wavelengths used for these
signals are in the infrared part of the spectrum and are quite invisible to the eye.
Some fiber transmitters that are intended for long distances may actually have lev-
els of light that would be hazardous, so just don't get into a bad habit.
Category 7 (Class F) Patches and Cords.
The new Category 7 (Class F) cabling
standards extend the frequency performance of twisted-pair cable to 600 MHz (typ-
ically characterized to 750 MHz). Category 7 has been a long time coming. The
cable that forms the basis for Category 7 operation (see Chapter 5) is a dramatic
departure from tried-and-true UTP. The same can be said for the patches and cords
that are used for Category 7.
The first thing to understand about Category 7 is that the cabling system uses
individually shielded
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pairs, bundled as four pairs within an overall shield. From
the standpoint of patches and patch cords, this means that both must also be
shielded, to get the proper performance from a Category 7 system.
The second major feature of Category 7 is that it allows two radically differ-
ent types of connectors, known as “RJ” and “non-RJ” types. As you might guess,
the RJ-type is a fairly conventional 8-pin modular jack and plug similar to the
famous RJ-45 connector. However, both mating connectors must be shielded to
accommodate the shield connection of Category 7 cable. There are several varia-
tions of this scheme, but the basic idea is to try to be backward compatible with
older Category 5e/6/AC6 cabling components and equipment interfaces.
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The terms shielded and screened are used rather interchangeably, although specific standards may
prefer one term over the other. Foils, braids, or both may be used to increase the immunity to electro-
magnetic fields.
