Transcript Chapter 1 - Introduction

ITGN 235: Principles of Networking
ITGN 225: Networking
Fall 2007/2008
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Chapter 4
Transmission Media
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Topics Covered
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4.1 Introduction
4.2 Copper Wires
4.3 Glass Fibers
4.4 Radio
4.5 Satellites
4.6 Geosynchronous Satellites
4.7 Low Earth Orbit Satellites
4.8 Low Earth Orbit Satellite Arrays
4.9 Microwave
4.10 Infrared
4.11 Light From A Laser
4.12 Selecting the proper cabling
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4.1 Introduction
• At the lowest level, all computer communication involves
– encoding data in a form of energy
– and sending the energy across a transmission medium
• HW devices attached to a computer perform the encoding and
decoding of data
• This part covers the basics of data transmission:
– This chapter examines the media that are used for transmission in
modern network systems
– The next two chapters explain how data can be transferred across such
media
– Later sections explain how transmission forms the basis of data NW
• Transmission Media = the physical path between transmitter (Tx)
and receiver (Rx).
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Transmission Media
• two major classes
– conducted (wireline) or guided media
• use a conductor such as a wire or a fiber optic cable to move the
signal from sender to receiver
– wireless or unguided media
• use radio waves of different frequencies and do not need a wire or
cable conductor to transmit signals
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Transmission Media
• design factors
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Transmission capacity (throughput)
attenuation: weakening of signal over distances
interference
number of receivers
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Impairments and Capacity
 Impairments exist in all forms of data transmission
 Analog signal impairments result in random
modifications that impair signal quality
 Digital signal impairments result in bit errors (1s and 0s
transposed)
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Attenuation
Attenuation refers to the loss of signal strength as the
signal travels further along a cable.
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Throughput
• Throughput is amount of data the medium can
transmit during a given period of time
– Also called capacity
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Throughput
Throughput measures
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Transmission Flaws
• Electromagnetic interference (EMI)
– Interference that may be caused by motors, power lines,
television, copiers, fluorescent lights, or other sources of
electrical activity
• Radiofrequency interference (RFI)
– Interference that may be generated by motors, power lines,
televisions, copiers, fluorescent lights, or broadcast signals from
radio or TV towers
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Transmission Flaws
Figure 4-11: An analog signal distorted by noise
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Transmission Flaws
Figure 4-12: A digital signal distorted by noise
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Transmission Flaws
•
Attenuation
– Loss of signal strength as transmission travels away from source
– Analog signals pass through an amplifier, which increases not only voltage of a
signal but also noise accumulated
Figure 4-13: An analog signal distorted by noise, and then amplified
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Transmission Flaws
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Regeneration
– Process of retransmitting a digital signal
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Repeater
– Device used to regenerate a signal
Figure 4-14: A digital signal distorted by noise, and then repeated
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Media Characteristics
• Throughput
– Perhaps most significant factor in choosing a transmission medium is
throughput
• Cost
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Cost of installation
Cost of new infrastructure versus reusing existing infrastructure
Cost of maintenance and support
Cost of a lower transmission rate affecting productivity
Cost of obsolescence
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Media Characteristics
• Size and scalability
– Specifications determining size and scalability:
• Maximum nodes per segment
• Maximum segment length
• Maximum network length
– Latency is the delay between the transmission of a signal and
its receipt
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Media Characteristics
• Connectors
– Connects wire to network device
• Noise immunity
– Thicker cables are generally less susceptible to noise
– Possible to use antinoise algorithms to protect data from being
corrupted by noise
– Conduits can protect cabling from noise
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Conducted (guided)
Media
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4.2 Copper Wires (1)
• Conventional computer NW use wires as the primary medium
– NW use copper wire almost exclusively because its low resistance
• The type of wiring used for NW is chosen to minimize interference
• Interference arises because an electrical signal traveling across a
wire acts like a miniature radio station
– the wire emits a small amount of electromagnetic (EM) energy, which
can travel through the air
• Whenever it encounters another wire, an EM wave generates a
small electric current in the wire.
– The amount of current generated depends on the strength of the EM
wave and the physical position of the wire
• When two wires are placed close together and in parallel, a strong
signal sent on one wire will generate a similar signal on the other
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4.2 Copper Wires (2)
• Problem of interference is severe
– because wires that comprise a NW often are placed in parallel
with many other wires
• To minimize interference, networks use one of three
basic wiring types:
– Unshielded Twisted Pair (UTP)
– Shielded Twisted Pair (STP)
– Coaxial Cable
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4.2 Copper Wires (2)
• See Figure 4.1 for an illustration of twisted-pair cables
• Twists change the electrical properties of the wire:
– First, they limit the EM energy the wire emits:
• So they help prevent electric currents on the wire from radiating
energy that interferes with other wires
– Second, they make the pair of wires less susceptible to EM
energy:
• They help prevent signals on other wires from interfering with the pair
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Twisted-Pair (TP) Cable
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•
Color-coded pairs of insulated
copper wires twisted around
each other and encased in
plastic coating
Twists in wire help reduce effects
of crosstalk
– Number of twists per meter or
foot known as twist ratio
•
Alien Crosstalk
– When signals from adjacent
cables interfere with another
cable’s transmission
Figure 21: Twisted-pair cable
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Unshielded Twisted-Pair
• Consists of one or more insulated wire pairs encased in a plastic
sheath
• Does not contain additional shielding
Figure 4-23: UTP cable
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Unshielded Twisted-Pair
•
To manage network
cabling, it is necessary to
be familiar with standards
used on modern
networks, particularly
Category 3 (CAT3) and
Category 5 (CAT5)
Figure 4-24: A CAT5 UTP cable
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UTP Categories
• Category 1 - traditional telephone cable; can carry voice but not data
• Category 2 - consists of 4 twisted-pairs and is rated for data
transmissions up to 4 Mbps.
• Category 3 - consists of 4 twisted-pairs and 3 twists per foot; rated
for 10 Mbps
• Category 4 - 4 TP, rated for 16 Mbps
• Category 5 - 4 TP, rated for 100 Mbps
» More tightly twisted than category 3 cables
» More expensive, harder to work with
There is also now CAT5E (1Gbps) & CAT6.
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3 twists/foot)
5 twists/foot)
Figure 1.10 Copper wire transmission media: (a) unshielded twisted pair (UTP)
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Shielded Twisted-Pair (STP)
• STP cable consists of twisted wire pairs that are individually
insulated and surrounded by shielding made of metallic
substance
(plastic casting)
(thin metallic shielding: Aluminum foil)
Figure 4-22: STP cable
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STP (Shielded Twisted-Pair)
Figure 3-18: STP cable
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Shielded Twisted-Pair (STP)
Shielded Twisted Pair (STP)
– A shielded twisted pair cable consists of a pair of wires
surrounded by a metal shield
• Each wire is coated with an insulating material,
– so the metal in one wire does not touch the metal in another
• The shield merely forms a barrier that prevents EM
radiation from entering or escaping
• The additional shielding provided by coaxial or shielded
twisted pair cabling is often used
– when wires from a NW pass near equipment that generates
strong electric or magnetic fields
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10BaseT
• Popular Ethernet networking standard
Figure 4-25: A 10BaseT Ethernet network
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10BaseT
• Enterprise-wide
network
– Spans entire
organization
– Often services
needs of many
diverse users
Figure 4-26: Interconnected 10BaseT segments
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100BaseT
• Enables LANs to run at 100-Mbps data transfer rate
• Also known as Fast Ethernet
• Two 100BaseT specifications have competed for popularity as
organizations move to 100-Mbps technology:
– 100BaseTX (using 2 pairs of UTP category 5 cable)
– 100BaseT4 (using 4 pairs of UTP category 3 or above cable)
http://en.wikipedia.org/wiki/100BaseTX
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Comparing STP and UTP
• Throughput
– Both can transmit up to 100 Mbps
• Cost
– Typically, STP is more expensive
• Connector
– Both use RJ-45 connectors and data jacks
• Noise immunity
– STP is more noise-resistant
• Size and scalability
– Maximum segment length for both is 100 meters
RJ-11 for telephone
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Installing Cable
Figure 4-36: A
typical UTP
cabling
installation
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Installing Cable
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Straight-through cable
– Terminations at both ends are identical
•
Crossover cable
– Terminations locations of transmit and receiver wires on one end of cable are
reversed
Figure 4-37:
RJ-45
terminations
on a
crossover
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Coaxial Cables
• Coaxial cables provides even more protection from
interference than twisted pair
– Instead of twisting wires around one another to limit interference,
• a coaxial cable consists of a single wire surrounded by a metal shield
• It’s shown in Figure 4.2
• Metal shield in a coaxial cable forms a flexible cylinder
around the inner wire to provide a barrier for EM radiation
– The barrier isolates the inner wire in two ways:
• it protects the wire from incoming EM energy
• and keeps signals on the inner wire from radiating EM energy
• The cable can be placed parallel to other cables or bent
and twisted around corners
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Coaxial Cable
• Consists of
central copper
core surrounded
by an insulator,
braiding, and
outer cover
called a sheath
Figure 4-15: Coaxial cable
BNC connector)
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Types of Coaxial Cable
• Thin (thinnet)
• Thick (thicknet)
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Thinnet Coax
Thinnet is a flexible coaxial cable about 0.25 inch thick.
Thinnet coaxial cable can carry a signal up to
approximately 185 meters.
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Thicknet Coax
Thicknet is relatively rigid coaxial cable about 0.5 inch in
diameter. The copper core is thicker than a thinnet core.
Because of the thicker copper core, thicknet can carry a
signal for up to 500 meters.
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Coaxial Cable
Table 4-2: Some types of coaxial cable
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Thicknet vs Thinnet
Important Characteristics of Coaxial Cable
Thinnet
Thicknet
Maximum Cable
Length
185 meters (607
feet)
500 meters (1640
feet)
Bandwidth
10 Mbps
10 Mbps
Bend Radius
360 degrees/foot
30 degrees/foot
Cost
More than twisted
pair
More than thinnet,
less than fiber
Interference
Susceptibility
Good, better than
twisted pair
Best of all
electrical cable
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4.3 Glass Fibers (1)
• NW also use flexible glass fibers to transmit data
– known as an optical fiber
• Medium uses light to transport data
• The miniature glass fiber is encased in a plastic jacket
– which allows the fiber to bend without breaking
• A transmitter at one end of a fiber uses
– a light emitting diode (LED) or a laser to send pulses of light
• A receiver at the other end uses
– a light sensitive transistor to detect the pulses
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Fiber-Optic Cable
• Contains one or
several glass fibers
at its core
– Surrounding the
fibers is a layer of
glass called
cladding
Figure 4-28: A fiber-optic cable
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Figure 1.11 Optical fiber transmission media: (a) cable structures
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Fiber-Optic Cable
• Single-mode fiber
– Carries light pulses
along single path
• Multimode fiber
– Many pulses of light
generated by LED
travel at different
angles
Core = 9 microns in diameter
Core = 50 or 62.5 microns in diameter
Single-mode and multimode
fiber-optic cables
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Fiber-Optic Cable
• Two popular connectors used with fiber-optic cable:
– ST connectors
– SC connectors
ST and SC fiber connectors
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4.3 Glass Fibers (2)
Main advantages fiber optics over wires:
• First, because they use light,
– neither cause electrical interference in other cables
– nor are they susceptible to electrical interference
• Second, because glass fibers can be manufactured to reflect most
of the light inward
– a fiber can carry a pulse of light much farther than a copper wire can
carry a signal
• Third, because light can encode more information than electrical
signals
– an optical fiber can carry more information than a wire
• Fourth, unlike electricity, which always requires a pair of wires
connected into a complete circuit,
– light can travel from one computer to another over a single fiber
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4.3 Glass Fibers (3)
Optical fibers do have some disadvantages
• First, installing a fiber requires special equipment
– that polishes the ends to allow light to pass through
• Second, if a fiber breaks inside the plastic jacket:
– finding the location of the problem is difficult
• Third, repairing a broken fiber is difficult
– special equipment is needed to join two fibers
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10BaseF and 100BaseFX
• 10BaseF
– Physical layer standard for networks specifying baseband
transmission, multimode fiber cabling, and 10-Mbps throughput
• 100BaseFX
– Physical layer standard for networks specifying baseband
transmission, multimode fiber cabling, and 100-Mbps throughput
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Submarine Optical Cable
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Broken Submarine Cable
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Physical Layer Networking Standards
Voice-grade
Table 4-3: Physical layer networking standards
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Wireless Media
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4.4 Radio
• In Radio Frequency (RF) transmissions
– each participating computer attaches to an antenna
– Antenna can both transmit and receive RF
• Physically, the antennas used with RF networks may be
large or small, depending on the range desired:
– An antenna designed to propagate signals several miles
• may consist of a metal pole approximately two meters long that is
mounted vertically on top of a building
– An antenna designed to permit communication within a building
• may be small enough to fit inside a portable computer (e.g., less
than twenty centimeters)
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microwave
4.4 Radio
300Mhz
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The Wireless Spectrum
Figure 3-37: The wireless spectrum
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Characteristics of Wireless Transmission
Figure 3-38: Wireless transmission and reception
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Wireless LAN Architecture (continued)
Figure 3-41: An infrastructure WLAN
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Wireless LAN Architecture (continued)
Figure 3-42: Wireless LAN interconnection
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4.5 Satellites
• RF technology can be combined with satellites
– to provide communication across longer distances
• Figure 4.3 illustrates a satellite in orbit
• The satellite contains a transponder
– that consists of a radio receiver and transmitter
• The transponder
– accepts an incoming radio transmission
– amplifies it
– and transmits the signal back toward the ground at a slightly
different angle than it arrived
• A single satellite usually contains multiple transponders
– Each transponder uses a different radio frequency (i.e., channel)
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Satellite Point to Point Link
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Satellite Broadcast Link
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Satellite systems: (a) broadcast television; (b) data communications
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4.6 Geostationary Satellites (1)
• Communication satellites can be grouped into categories according to the
height at which they orbit:
– The easiest is geosynchronous or geostationary satellites
– The name arises because a geosynchronous satellite is placed in an orbit that
is exactly synchronized with the rotation of the earth.
– Such an orbit is classified as a Geostationary Earth Orbit (GEO)
– When viewed from the ground,
• satellite appears to remain at exactly the same point in the sky at all times
• Laws of physics determine the exact distance from the earth that a
satellite must orbit to remain synchronized with the earth's rotation
– See Kepler's Law for details
– The distance is 35,785 kilometers or 22,236 miles
• An antenna can point in a fixed direction and maintain a link with the
satellite
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4.6 Geosynchronous Satellites (2)
• GEO is about one tenth of the distance to the moon
– Engineers refer the distance as “high earth orbit”
• There is a limited amount of ``space'' available in the
GEO above the equator
– because satellites using a given frequency must be separated
from one another to avoid interference
• The minimum separation depends on the power of the transmitters
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• Sample geostationary
satellite coverage
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• Orbital positions of the 165 geostationary satellites orbit
Earth today
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4.7 Low Earth Orbit Satellites (1)
• Second category of satellites operate in what is called
Low Earth Orbit (LEO)
– which means that they orbit a few hundred miles above the earth
(typically 200 to 400 miles)
• The chief disadvantage of a LEO lies in the rate at which
a satellite must travel
– Their period of rotation is faster than the rotation of the earth
• LEOs do not stay above a single point on the earth's surface
• An observer, who stands on the earth looking upward through a
telescope, sees LEOs move across the sky
• A single satellite can complete an entire orbit in
approximately 1.5 hours
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4.7 Low Earth Orbit Satellites (2)
From a communication provider's point of view:
• having a satellite that does not appear to remain
stationary causes problems:
– First, the satellite can only be used during the time
• that its orbit passes between two ground stations
– Second, maximal utilization requires complex control systems
• that continuously move the ground stations so they point directly at
the satellite
International space station
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4.8 Low Earth Orbit Satellite Arrays (1)
• A group of LEO satellites working in concert is known as
a satellite constellation.
• Such a constellation can be considered to be a number
of satellites with coordinated ground coverage, operating
together under shared control, synchronised so that they
overlap well in coverage and complement rather than
interfere with other satellites' coverage.
The GPS constellation calls for 24 satellites
to be distributed equally among six circular
orbital planes
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4.8 Low Earth Orbit Satellite Arrays (2)
• Instead of focusing on one satellite,
– the scheme requires a communication company to launch a set of
satellites into low earth orbits
• Although a given satellite orbits quickly,
– the set of orbits is chosen so that each point on the ground has at least
one satellite overhead at any time
– sixty-six (66) satellites are required to provide service over the entire
surface of the earth
• From the point of view of an observer on earth,
– it appears that a satellite emerges from a point on the horizon
– flies overhead
– and then disappears into a point on the opposite horizon
• The key to the scheme lies in the set of orbits
– guarantees at least one satellite is available at any time
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4.8 Low Earth Orbit Satellite Arrays (3)
• In addition to transponders used to communicate with
ground stations
– an array of satellites in low earth orbit contains radio equipment
used to communicate with other satellites in the array
• As they move through their orbits
– the satellites communicate with one another and agree to
forward data
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Satellite Advantages
• can reach a large geographical area
• high bandwidth
• cheaper over long distances
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Satellite Disadvantages
• high initial cost
• susceptible to noise and interference
• propagation delay
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4.9 Microwave
• Many long-distance telephone companies use
microwave (MW) to carry telephone conversations
– A few large companies have also installed MW systems as part
of the company's NW
• MW are merely a higher frequency (300 megahertz and
300 gigahertz) version of radio waves, but they behave
differently
– Instead of broadcasting in all directions,
• a MW transmission can be aimed in a single direction, preventing
others from intercepting
– In addition, MW transmission can carry more information than
lower frequency RF transmissions
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4.9 Microwave
• MW cannot penetrate metal structures:
– transmission works best when a clear path exists between two
parties
– most MW installations consist of two towers
• that are taller than the surrounding buildings and vegetation
– each MW transmitter aimed directly at a MW receiver on the
other
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4.9 Microwave Frequency Bands
Designation
Frequency range
L band
1 to 2 GHz
S band
2 to 4 GHz
C band
4 to 8 GHz
X band
8 to 12 GHz
Ku band
12 to 18 GHz
K band
18 to 26.5 GHz
Ka band
26.5 to 40 GHz
Q band
30 to 50 GHz
U band
40 to 60 GHz
V band
50 to 75 GHz
E band
60 to 90 GHz
W band
75 to 110 GHz
F band
90 to 140 GHz
D band
110 to 170 GHz
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4.10 Infrared
• Infrared is limited to a small area (e.g., a single room)
• Usually requires that the transmitter be pointed toward
the receiver
• Infrared HW
– is inexpensive compared to other mechanisms,
– and does not require an antenna
• It is possible to equip a large room with a single infrared
connection
– that provides NW access to all computers
– computers can remain in contact with the NW while they are
moved within the room
• Infrared NW are especially convenient for small, portable
computers
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4.11 Light From A Laser
(FSO: free space optic)
• A beam of light can also be used to carry data through the air
• A communication link that uses light consists of two sites that each
have a transmitter and receiver
– equipment is mounted in a fixed position, often on a tower
– aligned so the transmitter at one location sends its beam of light directly
to the receiver at the other
• The transmitter uses a laser to generate the beam of light
– because a coherent laser beam will stay focused over a long distance
• Light from a laser must travel in a straight line and must not be
blocked
• A laser beam cannot penetrate vegetation or weather conditions
such as snow and fog:
– Thus, laser transmission has limited use
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4.11 Light From A Laser
(FSO: free space optic)
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4.11 Light From A Laser
(Applications)
– LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit
Ethernet speeds.
– LAN-to-LAN connections in a city. example, Metropolitan area network.
– To cross a road or other barriers.
– Speedy service delivery of high bandwidth access to fiber networks.
– Converged Voice-Data-Connection.
– Two solar-powered satellites communicating optically in space via
lasers.
– Temporary network installation (for events or other purposes).
– Reestablish high-speed connection quickly (disaster recovery).
– As an alternative or upgrade add-on to existing wireless technologies.
– As a safety add-on for important fiber connections (redundancy).
– For communications between spacecraft, including elements of a
satellite constellation.
– The light beam can be very narrow, which makes FSO hard to intercept,
improving security. FSO provides vastly improved EMI behavior using
light instead of microwaves.
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Choosing the Right Transmission Media
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Areas of high EMI or RFI
Corners and small spaces
Distance
Security
Existing infrastructure
Growth
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Selecting Cabling
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How heavy will the network traffic be?
What are the security needs of the network?
What are the distances that the cable must cover?
What are the cable options?
What is the budget?
installation logistics (how easy to install?)
shielding (what level?)
noise and crosstalk (any power lines? motors?)
transmission speed (how fast?)
cost (how much?)
attenuation
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Cable Comparison Summary
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Cable cost
Usable length
Transmission Rates
Flexibility
Ease of installation
Susceptibility to interference
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