Chapter 3 Communications Media
A communication network cannot exist w/o a medium to connect the source
and receiver and provide a path over which messages can be sent. Two
types of medium:
1. conducted media - some form of physical
wire or cable. An electrical or optical device sends, or conducts,
signals
down the copper wire or glass cable.
2. radiated media - cannot be see,
airwaves; the signal is radiated through the air by means of a transmitter
Types of Conducted Media
1. twisted pair wire - its core consists of two wires twisted
together a specific number of times to create a magnetic field.
More twists = less EMI. Two categories of TP:
a. shielded twisted pair - twisted wire cores are placed into
cladding material, which is wrapped with wires to
absorb any interference. The wires are then wrapped in a rubber outer shell.
b. unshielded twisted pair - lacks the cladding of shielded twisted
pair wiring and is more prone to EMI; UTP
categories p3-3
T-carrier circuits
- leased from long-distance carriers; made up of two pairs of twisted pair
wires, coaxial cable,
fiber-optic cable,
or
microwave signals, and transmit data in a digital rather than analog form.
Advantages of TP : less expensive than coax and fiber, and easier to install
2. Coaxial cable - composed of a single copper wire, surrounded
by an insulating cladding material called a shell. The shell
then is surrounded by a
second conductor which gives the cable the ability to transmit much more
data than a twisted pair
wire. Finally, the
entire cable is enclosed in a rubber outer cladding called the jacket.
3. Fiber Optic Cable - uses light to transmit data signals.
The core of a fiber-optic cable is composed of one or more thin
tubes of either very pure
glass or plastic. Each tube, called an optical fiber, is as thin
as a human hair. Millions of these
optical fibers can be bundled
together in a sheathing that could hold only thousands of coaxial cables.
Use a light emitting
diode (LED) is a low-powered
light created by an electrical diode (used in digital clocks and watches).
Three typesof Fiber Optic
Cable:
a. multimode step index - uses a plastic coating or a mirror-like
coating around the core to reflect the light from the
laser or LED. As the light is reflected off the sides of the cable,
it moves down the cable to its destination
b. multimode graded index - core of the cable varies in density,
which bends the light and causes it to move forward
to the receiver
c. single-mode cable - fastest; uses a very thin core and sends
light straight down the cable so it does not need to
bounce off the cable walls.
Advantages:
a. Speed
b. Security - hard to tap into, the heat-fused cladding of
multimode graded index and single-mode cable makes
them practically impossible to tap. Even if a tap succeeds, it often
detected b/c the match on the fiber-optic
must be perfect in order for there to be no disruption in the light transmission.
c. Elimination of crosstalk - occurs when the signals being transmitted
on the wires interfere with each other, in
which case neither signal is transmitted properly.
Two types of transmission along a medium:
1. broadband - a single cable is divided electrically into many
channels, each of which can carry a different transmission.
2. baseband - only a single signal is transmitted over the cable.
Types of Radiated Media (wireless media)
1. Broadcast Radio - sending signals through the air between
transmitters at frequencies ranging from 30 - 300 MHz; they are
omnidirectional -
the antenna that is used to receive the broadcast signal does not have
to be positioned in or pointed in a
specific direction to receive
the signal
a. AM and FM
b. Shortwave radio - broadcast signals can be transmitted by only
licensed operators and are restricted to specific
frequency ranges
c. Citizens band (CB) radio - uses low frequencies; its broadcast
signals can be sent and received by anyone who
buys the transmitting/receiving equipment.
d.Two basic types of television frequencies are used:
1. Ultra-high frequency (UHF) - require a special antenna; reception
can be unpredictable depending on
conditions in the atmosphere; transmit on one of 70 channels above13
2 Very-high frequency (VHF) - travel longer distances then UHF and
produce a stronger signal; channels 2-13
* Newer technology is high-definition television (HDTV) or digital television
(DTV) - broadcasts a signal that
provides a much clearer picture than current television and CD-quality
sound.
2. Microwave - uses very-high frequency signals (3,000
MHz to 30 GHz) to transmit signals between stations. High
frequency permits large amount of data; transmissions
are focused and unidirectional. The signals produced by a microwave
station use line-of-sight transmission meaning that
signals travel in a straight line and that the antennas used for transmission
must be pointed directly at one another signals
to be sent and received. Can be affected by rain and snow and by
obstacles
between the microwave stations. Two types:
a. terrestrial - transmissions are sent between two microwave stations
on the earth; most common form of long-distance
communication today
b. satellite - transmissions involves sending microwave transmissions
between two or more earth-based microwave
stations and a satellite. The signal from a satellite can reach only
a cretion part of the earth - an area that is called a
footprint.
3. Cellular Radio - Cellular telephones are radio devices
that use cellular radio signals to transmit voice and data messages.
Cellular radio is a form of broadcast radio w/ restrictions
on how far the signal is transmitted. The broadcast areas for
cellular radio system is divided into cells,
each of which has its own transmitting antenna. Transmitters operate
at a very low
power so that transmitters in adjacent cells can
broadcast on the same frequency and not interfere with one another.
4. Spread Spectrum Radio (SSR) - involves transmitting
radio signals spread over a wide range of the spectrum, thus avoiding
a concentration of power in a single narrow frequency
band. Originally developed by the military but now is the most widely
used transmission technique for wireless LANs. Two
main methods are used to send SSR signals:
a. frequency hopping spread spectrum (FHSS) -first type of SSR developed;
broadcasts signal over a seemingly
random series of radio frequencies - transmitting a short burst on one
frequency, "hopping" to another frequency for
another short burst and so on. Source and destination are synchronized
so they are on the same frequency at the
same time
b. direct sequence spread spectrum (DSSS) - spreads the signal over
several frequencies simultaneously. Transmitters
spread their transmissions by adding redundant data bits called "chips".
Receiver maps these chips back into a bit,
thus re-creating the data. Receiver and transmitter must be synchronized.
Most LANs used DSSS.
5. Infrared Transmission (IR) - sends electromagnetic light
signals at a frequency between visible light and radio waves.
Line-of-sight technology; LANs must be configured
so that the transmission is direct or reflected off of some surface.
Things to Consider when Selecting Media
1. Cost - cheapest: UTP, most expensive: fiber-optic
2. Speed - p3-21
3. Rate of Errors - combination of high speed and low error rate
4. Security - ease with which someone can steal data; encryption
and compression
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