Hardware Reference
In-Depth Information
more effective way to prevent interference from disrupting transmissions. As a result, earlier
cabling scenarios relied on shielded cables rather than the unshielded cables more commonly
in use today.
Shielded cables also have some special grounding concerns because one, and only one, end of
a shielded cable should be connected to an earth ground; issues arose when people
inadvertently caused grounding loops to occur by connecting both ends or caused the shield to
act as an antenna because it wasn't grounded.
Grounding loops
are created when two grounds are tied together. This is a bad situation
because each ground can have a slightly different potential, resulting in a circuit that has low
voltage but infinite amperage. This causes undue stress on electrical components and can be a
fire hazard.
Most Ethernet installations that use twisted-pair cabling use UTP because the physical flexibility and
small size of the cable and connectors makes routing it easy. However, its lack of electrical insulation
can make interference from fluorescent lighting, elevators, and alarm systems (among other devices) a
major problem. If you use UTP in installations where interference can be a problem, you need to
route the cable away from the interference, use an external shield, or substitute STP for UTP near
interference sources.
Four standard types of UTP cabling exist and are still used to varying degrees:
•
Category 3 cable
—The original type of UTP cable used for Ethernet networks was also the
same as that used for business telephone wiring. This is known as Category 3, or voice-grade
UTP cable, and it is measured according to a scale that quantifies the cable's data-transmission
capabilities. The cable itself is 24 AWG (American Wire Gauge, a standard for measuring the
diameter of a wire) and copper-tinned with solid conductors, with 100-105 ohm characteristic
impedance and a minimum of two twists per foot. Category 3 cable is largely obsolete because
it is only adequate for networks running at up to 16Mbps, so it cannot be used with Fast or
Gigabit Ethernet.
•
Category 5 cable
—The faster network types require greater performance levels. Fast Ethernet
(100BASE-TX) uses the same two-wire pairs as 10BASE-T, but Fast Ethernet needs a greater
resistance to signal crosstalk and attenuation. Therefore, the use of Category 5 UTP cabling is
essential with 100BASE-TX Fast Ethernet. Although the 100BASE-T4 version of Fast Ethernet
can use all four-wire pairs of Category 3 cable, this flavor of Fast Ethernet is not widely
supported and has practically vanished from the marketplace. If you try to run Fast Ethernet
100BASE-TX over Category 3 cable, you will have a slow and unreliable network. Category 5
cable is commonly called CAT 5 and is also referred to as Class D cable.
Most cable vendors also sell an enhanced form of Category 5 cable called Category 5e
(specified by Addendum 5 of the ANSI/TIA/EIA-568-A cabling standard). Category 5e or 6
cable can be used in place of Category 5 cable and is especially well suited for use in Fast
Ethernet networks that might be upgraded to Gigabit Ethernet in the future. Category 5e cabling
must pass several tests not required for Category 5 cabling and, unlike Category 5 cable, can be
certified for Gigabit Ethernet networks.
•
Category 6 cable
—Category 6 cabling (also called CAT 6 or Class E) can be used in place of
CAT 5 or 5e cabling and uses the same 8P8C (RJ-45) connectors as CAT 5 and 5e. CAT 6
