what are the Types of Communications Media

Four major types of communications media (cabling) are available for data networking today:
unshielded twisted pair (UTP), shielded or screened twisted pair (STP or ScTP), coaxial, and
fiber optic (FO). It is important to distinguish between backbone cables and horizontal cables.
Backbone cables connect network equipment such as servers, switches, and routers and connect
equipment rooms and communication closets. Horizontal cables run from the communication
closets to the wall outlets. For new installations, multistrand fiber-optic cable is
essentially universal as backbone cable. For the horizontal, UTP reigns supreme. Much of the
focus of this book is on UTP cable.
Twisted-Pair Cable
By far the most economical and widely installed cabling today is twisted-pair wiring. Not only
is twisted-pair wiring less expensive than other media, installation is also simpler, and the tools
required to install it are not as costly. Unshielded twisted pair (UTP) and shielded twisted pair
(STP) are the two primary varieties of twisted pair on the market today. Screened twisted pair
(ScTP) is a variant of STP.
Unshielded Twisted Pair (UTP)
Though it has been used for many years for telephone systems, unshielded twisted pair (UTP)
for LANs first became common in the late 1980s with the advent of Ethernet over twisted-pair
wiring and the 10Base-T standard. UTP is cost effective and simple to install, and its bandwidth
capabilities are continually being improved.
UTP cabling typically has only an outer covering (jacket) consisting of some type of nonconducting
material. This jacket covers one or more pairs of wire that are twisted together. In
this chapter, as well as throughout much of the rest of the book, assume unless specified otherwise
that UTP cable is a four-pair cable. Four-pair cable is the most commonly used horizontal
cable in network installations today. The characteristic impedance of UTP cable is 100
ohms plus or minus 15 percent, though 120-ohm UTP cable is sometimes used in Europe and
is allowed by the ISO/IEC 11801 cabling Standard.
This simple cable consists of a jacket that surrounds
four twisted pairs. Each wire is covered by an insulation material with good
dielectric
properties. For data cables, this means that in addition to being electrically nonconductive, it
must also have certain properties that allow good signal propagation.
UTP cabling seems to generate the lowest expectations of twisted-pair cable. Its great popularity
is mostly due to the cost and ease of installation. With every new generation of UTP
cable, network engineers think they have reached the limits of the UTP cable’s bandwidth and
capabilities. However, cable manufacturers continue to extend its capabilities. During the
development of 10Base-T and a number of pre-10Base-T proprietary UTP Ethernet systems,
critics said that UTP would never support data speeds of 10Mbps. Later, the skeptics said that
UTP would never support data rates at 100Mbps. In July 1999, the IEEE approved the
1000Base-T standard, which allows Gigabit Ethernet to run over Category 5 cable