Transcript William Stallings Data and Computer

William Stallings
Data and Computer
Communications
Chapter 4
Transmission Media
1
Overview
Guided - wire
Unguided - wireless
Characteristics and quality determined by
medium and signal
For guided, the medium is more important
For unguided, the bandwidth produced by the
antenna is more important
Key concerns are data rate and distance
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Design Factors
Bandwidth
Higher bandwidth gives higher data rate
Transmission impairments
Attenuation
Interference
Number of receivers
Major factor in guided media
More receivers (multi-point) introduce more
attenuation
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Electromagnetic Spectrum
4
Guided Transmission Media
the transmission capacity depends on the
distance and on whether the medium is pointto-point or multipoint
e.g.,
Twisted Pair
Coaxial cable
Optical fiber
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Twisted Pair
 consists of two insulated copper wires arranged in a
regular spiral pattern to minimize the electromagnetic
interference between adjacent pairs
 often used at customer facilities and also over distances
to carry voice as well as data communications
 low frequency transmission medium
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Twisted Pair - Applications
Most common medium
Telephone network
Between house and local exchange (subscriber loop)
Within buildings
To private branch exchange (PBX)
For local area networks (LAN)
10Mbps or 100Mbps
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Twisted Pair - Pros and Cons
Cheap
Easy to work with
Low data rate
Short range
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Twisted Pair - Transmission
Characteristics
Analog
Amplifiers every 5km to 6km
Digital
Use either analog or digital signals
repeater every 2km or 3km
Limited distance
Limited bandwidth (1MHz)
Limited data rate (100MHz) using different
modulation & signaling techniques
Susceptible to interference and noise
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Unshielded and Shielded TP
Unshielded Twisted Pair (UTP)
Ordinary telephone wire
Cheapest
Easiest to install
Suffers from external electromagnetic interference
(EM)
Shielded Twisted Pair (STP)
the pair is wrapped with metallic foil or braid to
insulate the pair from electromagnetic interference
More expensive
Harder to handle (thick, heavy)
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UTP Categories
Cat 3
up to 16MHz
Voice grade found in most offices
Twist length of 7.5 cm to 10 cm
Cat 4 (least common)
up to 20 MHz
Cat 5
up to 100MHz
Commonly pre-installed in new office buildings
Twist length 0.6 cm to 0.85 cm
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Twisted Pair Advantages
inexpensive and readily available
flexible and light weight
easy to work with and install
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Twisted Pair Disadvantages
susceptibility to interference and noise
attenuation problem
For analog, repeaters needed every 5-6km
For digital, repeaters needed every 2-3km
relatively low bandwidth
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Coaxial Cable
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Coaxial Cable Applications
Most versatile medium
Television distribution
Aerial to TV
Cable TV
Long distance telephone transmission
Can carry 10,000 voice calls simultaneously
Being replaced by fiber optic
Short distance computer systems links
Local area networks
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Coaxial Cable - Transmission
Characteristics
Analog
Amplifiers every few km
Closer if higher frequency
Up to 500MHz
Digital
Repeater every 1km
Closer for higher data rates
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Coax Advantages
higher bandwidth
400 to 600Mhz
up to 10,800 voice conversations
can be tapped easily (pros and cons)
much less susceptible to interference than
twisted pair
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Coax Disadvantages
high attenuation rate makes it expensive over
long distance
bulky
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Optical Fiber
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Optical Fiber - Benefits
Greater capacity
Data rates of hundreds of Gbps
Smaller size & weight
Lower attenuation
Electromagnetic isolation
Greater repeater spacing
10s of km at least
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Optical Fiber - Applications
Long-haul trunks
Metropolitan trunks
Rural exchange trunks
Subscriber loops
LANs
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Optical Fiber - Transmission
Characteristics
Act as wave guide for 1014 to 1015 Hz
Portions of infrared and visible spectrum
Light Emitting Diode (LED)
Cheaper
Wider operating temp range
Last longer
Injection Laser Diode (ILD)
More efficient
Greater data rate
Wavelength Division Multiplexing
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Fiber Optic Types
multimode step-index fiber
the reflective walls of the fiber move the light pulses
to the receiver
multimode graded-index fiber
acts to refract the light toward the center of the fiber
by variations in the density
single mode fiber
the light is guided down the center of an extremely
narrow core
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Optical Fiber Transmission
Modes
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Fiber Optic Signals
fiber optic multimode
step-index
fiber optic multimode
graded-index
fiber optic single mode
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Fiber Optic Advantages
greater capacity (bandwidth of up to 2 Gbps)
smaller size and lighter weight
lower attenuation
immunity to environmental interference
highly secure due to tap difficulty and lack of
signal radiation
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Fiber Optic Disadvantages
expensive over short distance
requires highly skilled installers
adding additional nodes is difficult
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Wireless Transmission
Unguided media
Transmission and reception via antenna
Two techniques are used:
Directional
Focused beam
Careful alignment required
 Omnidirectional
Signal spreads in all directions
Can be received by many antennas
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Frequencies
2GHz to 40GHz
Microwave
Highly directional
Point to point
Satellite
30MHz to 1GHz
Omnidirectional
Broadcast radio
3 x 1011 to 2 x 1014
Infrared
Local
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Wireless Examples
terrestrial microwave transmission
satellite transmission
broadcast radio
infrared
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Terrestrial Microwave
 uses the radio frequency spectrum, commonly from 2 to
40 Ghz
 transmitter is a parabolic dish, mounted as high as
possible
 used by common carriers as well as by private networks
 requires unobstructed line of sight between source and
receiver
 curvature of the earth requires stations (called
repeaters) to be ~30 miles apart
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Microwave Transmission
Applications
long-haul telecommunications service for both
voice and television transmission
short point-to-point links between buildings for
closed-circuit TV or a data link between LANs
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Microwave Transmission
Advantages
no cabling needed between sites
wide bandwidth
multichannel transmissions
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Microwave Transmission
Disadvantages
line of sight requirement
expensive towers and repeaters
subject to interference such as passing airplanes
and rain
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Satellite Microwave
a microwave relay station in space
Satellite receives on one frequency, amplifies or
repeats signal and transmits on another
frequency
geostationary satellites
remain above the equator at a height of 22,300 miles
(geosynchronous orbit)
travel around the earth in exactly the time the earth
takes to rotate
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Satellite Transmission Links
earth stations communicate by sending signals
to the satellite on an uplink
the satellite then repeats those signals on a
downlink
the broadcast nature of the downlink makes it
attractive for services such as the distribution of
television programming
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Satellite Transmission Process
satellite
transponder
dish
dish
22,300 miles
uplink station
downlink station
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Satellite Transmission
Applications
television distribution
a network provides programming from a central
location using direct broadcast satellites (DBS)
long-distance telephone transmission
high-usage international trunks
private business networks
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Principal Satellite Transmission
Bands
C band: 4(downlink) - 6(uplink) GHz
the first to be designated
Ku band: 12(downlink) -14(uplink) GHz
rain interference is the major problem
Ka band: 19(downlink) - 29(uplink) GHz
equipment needed to use the band is still very
expensive
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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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Broadcast Radio
Omnidirectional
FM radio
UHF and VHF television
Requires line of sight
Suffers from multipath interference
Reflections
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Infrared
Achieved using tranceivers that modulate
noncoherent infrared light
Requires line of sight (or reflection)
Blocked by walls
e.g. TV remote control, Infrared port
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Common Carriers
a government-regulated private company
involved in the sale of infrastructure services in
transportation and communications
required to serve all clients indiscriminately
services and prices from common carriers are
described in tariffs
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Leased (or Dedicated) Lines
permanently or semi-permanently connect
between two points
economical in high volume calls between two
points
no delay associated with switching times
can assure consistently high-quality connections
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Leased (or Dedicated) Lines
voice grade channels
normal telephone lines
in the range of 300 Hertz to 3300 Hertz
conditioning or equalizing
reduces the amount of noise on the line, providing
lower error rates and increased speed for data
communications
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T-1 Carrier
also referred to as DS-1 signaling
provides digital full-duplex transmission rates of
1.544Mbps
usually created by multiplexing 24 64-Kbps voice
or 56-Kbps data lines
higher speeds are available with T-3 (45Mbps)
[sometimes referred to a DS-3 lines; can be
multiplexed into 28 T-1 signals; T-3 consists of
672 individual channels, each of which supports
64-Kbps] and T-4 services (274Mbps)
in Europe, E-1 (2.048Mbps) is used instead of T-1
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Integrated Services Digital
Network (ISDN)
all-digital transmission facility that is designed to
replace the analog PSTN
basic ISDN (basic rate access)
two 64Kbps bearer channels + 16Kbps data channel
(2B+D) = 144 Kbps
broadband ISDN (primary rate access)
twenty-three 64Kbps bearer channels + 64 data
channels (23B+D) = 1.536 Mbps
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Past Criticism of ISDN
“Innovations Subscribers Don’t Need”
“It Still Doesn’t Network”
“It Still Does Nothing”
Why so much criticism?
overhyping of services before delivery
high price of equipment
delay in implementing infrastructure
incompatibility between providers' equipment.
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ISDN Channel Definitions
B (bearer) channels
64 kbps channels that may be used to carry voice,
data, facsimile, or image
D (demand) channels
mainly intended for carrying signaling, billing and
management information to control ISDN services
(out-of-band control messages)
may be either 16 or 64 kbps
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Two Levels of ISDN Service
basic rate interface (BRI)
2B (64 kbps) + D (16 kbps) = 144 kbps
primary rate interface (PRI)
23B (64 kbps) + D (64 kbps) = 1.536 Mbps
North American standard
30B (64 kbps) + D (64 kbps) = 1.984 Mbps
European standard
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