Networking media can be defined simply as
the means by which signals (data) are sent from one computer to
another (either by cable or wireless means). There are a wide variety
of networking media in the marketplace. This section will briefly
discuss some of the available media types including two that use
copper (coaxial and twisted-pair), one that uses glass (fiber-optic),
and one that uses waves (wireless) to transmit data.
Coaxial Cable
Coaxial cable is a copper-cored cable surrounded by a heavy
shielding that is used to connect computers in a network. There are several
types of coaxial cable, including thicknet,
thinnet,
RG-59 (standard cable for cable TV), and RG-6 (used in video
distribution). Thicknet is large in diameter, rigid, and thus
difficult to install. In addition, the maximum transmission rate
is only 50 Mbps, significantly less than twisted-pair or
fiber-optic, and its maximum run is 500m. A thinner version, known
as thinnet or cheapernet, is occasionally used in Ethernet
networks. Thinnet has the same transmission rate as thicknet.
Twisted Pair
Twisted-pair is a type of cabling that is used for telephone
communications and most modern Ethernet networks. A pair of wires
forms a circuit that can transmit data. The pairs are twisted to
provide protection against crosstalk, the noise generated by
adjacent pairs. Pairs of copper wires that are encased in
color-coded plastic insulation are twisted together. All the
twisted-pairs are then protected inside an outer jacket. There are
two basic types, Shielded Twisted-Pair (STP) and Unshielded Twisted-Pair (UTP).
There are also categories of twisted-pair wiring:
- STP
– Shielded Twisted-Pair
(STP) cable combines the
techniques of cancellation and the twisting of wires with
shielding. Each pair of wires is wrapped in metallic foil to
further shield the wires from noise. The four pairs of wires are
then wrapped in an overall metallic braid or foil. STP reduces
electrical noise from within the cable (crosstalk) and from
outside the cable (EMI and RFI).
- UTP
–
Unshielded Twisted-Pair (UTP) cable is used in a
variety of networks. It has two or four pairs of wires. This type
of cable relies solely on the cancellation effect produced by the
twisted wire pairs to limit signal degradation caused by Electromagnetic
Interference (EMI) and Radio Frequency Interference (RFI). UTP is the most commonly used cabling in
Ethernet networks.
Although STP prevents interference better than UTP, it is more
expensive and difficult to install. In addition, the metallic
shielding must be grounded at both ends. If improperly grounded,
the shield acts like an antenna picking up unwanted signals. STP
is primarily used in Europe.
Category Rating
UTP comes in several categories that are based on the number of
wires and number of twists in those wires. Category 3 is the
wiring used for telephone connections. It has four pairs of wires
and a maximum data rate of up to 16 Mbps. Category 5 and 5e are
currently the most common Ethernet cables used. They have four
pairs of wires with a maximum data rate of up to 100 and 1000 Mbps
respectively. Category 5e has more twists per foot than Category 5 wiring. These
extra twists further prevent interference from outside sources and
the other wires within the cable. Table
summarizes some useful information about some of the common
category cables and transmission speeds.
The latest is Category 6, which has not yet been ratified by
cabling industry organizations. Category 6 is similar to Category
5/5e except that a plastic divider separates the pairs of wires to
prevent crosstalk. The pairs also have more twists than Category
5e cable.
RS-232
RS-232 supports a connection between two devices
over a single cable. Virtually every computer has one RS-232
serial port. Modems, mice, and serial printers are designed to
connect to the RS-232 port. It is also used to connect modems to
telephones. There are two types of connectors, a 25-pin connector
(DB-25) and a 9-pin connector (DB-9). There are several
limitations of using RS-232. The signaling rates are limited to
only 120 Kbps. The rate is limited because the potential for
crosstalk between signal lines in the cable is very high.
RS-232 is still the most common standard for serial
communication. RS-422 and RS-423 are expected to replace it in the
future. Both support higher data rates and have greater immunity
to electrical interference.
Fiber-Optic Cable
Fiber-optic cable is a networking medium capable of conducting
modulated light transmissions. To modulate light is to manipulate
it in a way that it transmits data. Fiber-optic refers to cabling
that has a core of strands of glass or plastic (instead of
copper), through which light pulses carry signals. Fiber-optic
cable does not carry electrical impulses, as copper wire do.
Instead, signals that represent data are converted into beams of
light. Fiber has many advantages over copper in terms of
transmission bandwidth and signal integrity over distance.
However, it is more difficult to work with and more expensive than
copper cabling. The connectors are expensive as is the labor that
is necessary to terminate the ends of the cables.
Wireless
If the cost of running cables is too high or computers need to be
movable without being tethered to cables, wireless is an alternative
method of connecting a LAN. Wireless networks use radio (RF), laser,
infrared (IR), and satellite/microwaves to carry signals from one
computer to another without a permanent cable connection. Wireless
signals are electromagnetic waves that travel through the air. No
physical medium is necessary for wireless signals, making them a very
versatile way to build a network.
