Transcript การสื่อสารข้อมูล

บทที่3
สื่ อที่ใช้ในการสื่ อสารข้อมูล(Media)
ณรงค์ฤทธิ์ มณี จิระปราการ
สาขาคอมพิวเตอร์ธุรกิจ
ภาควิชาพาณิ ชย์ศาสตร์
มหาวิทยาลัยนเรศวร
Data Communication and
Computer Network
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วัตถุประสงค์(Objective)
อธิบายคุณสมบัติของสื่ อประเภทเหนี่ยวนาแบบต่ างๆ
อธิบายคุณสมบัติของสื่ อประเภทกระจายคลืน่
อธิบายความแตกต่ างระหว่างสื่ อเหนี่ยวนาและสื่ อประเภทกระจาย
คลืน่
บอกข้ อดีข้อเสี ยของสื่ อเหนี่ยวนาแบบและสื่ อประเภทกระจาย
คลืน่ ได้
บอกวิธีการพิจารณาเลือกใช้ สื่อทีเ่ หมาะสมกับระบบสื่อสารข้ อมูล
Data Communication and
Computer Network
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Data Communications
สื่อสามารถเแบ่ งออกเป็ นกลุ่มใหญ่ ๆ ได้ 2 กลุ่ม ืือ
1. สื่อประเภทเหนี่ยวนา ได้ แก่ สายืู่บดิ เกลียว,
สายโืแอกเชียล, สายใยแก้ วนาแสง
2. สื่อประเภทกระจายืลื่น ได้ แก่ ืลื่นวิทยุ,
ไมโืรเวฟ
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Introduction
The world of computer networks and data
communications would not exist if there were
no medium by which to transfer data.
The two major categories of media include:
• Conducted media
• Radiation or Wireless media
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Classes of Transmission Media
• Conducted 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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Design Factors for Transmission
Media
• Bandwidth: All other factors remaining
constant, the greater the band-width of a
signal, the higher the data rate that can be
achieved.
• Transmission impairments. Limit the
distance a signal can travel.
• Interference: Competing signals in
overlapping frequency bands can distort or
wipe out a signal.
• Number of receivers: Each attachment
introduces some attenuation and
distortion, limiting distance and/or data
rate.
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Electromagnetic Spectrum for
Transmission Media
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สื่ อประเภทเหนี่ยวนา
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Guided Transmission Media
• Transmission capacity depends on the
distance and on whether the medium is pointto-point or multipoint
• Examples
 twisted pair wires
 coaxial cables
 optical fiber
Coaxial
Fiber optic
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Twisted Pair Wire
Two or more pairs of single conductor wires
that have been twisted around each other.
Twisted pair wire is classified by category.
Twisted pair wire is currently Category 1
through Category 5e.
Twisting the wires helps to eliminate
electromagnetic interference between the two
wires.
Shielding can further help to eliminate
Data Communication and
interference.
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Twisted Pair Wires
• 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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Types of Twisted Pair
• STP (shielded twisted pair)
– the pair is wrapped with metallic foil or
braid to insulate the pair from
electromagnetic interference
• UTP (unshielded twisted pair)
– each wire is insulated with plastic wrap, but the pair is
encased in an outer covering
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Ratings of Twisted Pair
• Category 3 UTP
– data rates of up to 10mbps are achievable
• Category 5 UTP
– data rates of up to 100mbps are achievable
– more tightly twisted than Category 3 cables
– more expensive, but better performance
• STP
– More expensive, harder to work with
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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 (3000Hz)
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Coaxial Cable (or Coax)
• Used for cable television, LANs, telephony
• Has an inner conductor surrounded by a
braided mesh
• Both conductors share a common center
axial, hence the term “co-axial”
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Coaxial Cable
A single wire wrapped in a foam insulation
surrounded by a braided metal shield, then covered
in a plastic jacket. Cable can be thick or thin.
Baseband coaxial technology uses digital signaling
in which the cable carries only one channel of digital
data.
Broadband coaxial technology transmits analog
signals and is capable of supporting multiple
channels of data.
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Coax Layers
outer jacket
(polyethylene)
shield
(braided wire)
insulating material
copper or aluminum
conductor
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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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Fiber Optic Cable
A thin glass cable approximately a little
thicker than a human hair surrounded by a
plastic coating and packaged into an insulated
cable.
A photo diode or laser generates pulses of
light which travel down the fiber optic cable
and are received by a photo receptor.
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Fiber Optic Cable
• Relatively new transmission medium used by
telephone companies in place of long-distance
trunk lines
• Also used by private companies in implementing
local data communications networks
• Require a light source with injection laser diode
(ILD) or light-emitting diodes (LED)
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Fiber Optic Layers
• consists of three concentric sections
plastic jacket
glass or plastic
cladding
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fiber core
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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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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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It is very common to mix fiber with twisted pair in LANs.
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Conducted Media In Action
How do we wire a local area network?
Remember: using Category 5 unshielded twisted
pair, the maximum segment length is 100 meters.
A wall jack is a passive device and does not
regenerate a signal.
Hub to hub connections are often fiber optic
cable.
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Wireless Media
Radio, satellite transmissions, and infrared
light are all different forms of
electromagnetic waves that are used to
transmit data.
Note in the following figure how each source
occupies a different set of frequencies.
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Wireless (Unguided Media) Transmission
• transmission and reception are achieved by
means of an antenna
• Directional
– transmitting antenna puts out focused
beam
– transmitter and receiver must be aligned
• omnidirectional
– signal spreads out in all directions
– can be received by many antennas
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Wireless Examples
•
•
•
•
terrestrial microwave
satellite microwave
broadcast radio
infrared
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Radio
• radio is omnidirectional and microwave is
directional
• Radio is a general term often used to
encompass frequencies in the range 3 kHz
to 300 GHz.
• Mobile telephony occupies several
frequency bands just under 1 GHz.
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Terrestrial Microwave
• used for long-distance telephone service
• uses radio frequency spectrum, from 2 to 40
Ghz
• parabolic dish transmitter, mounted high
• used by common carriers as well as private
networks
• requires unobstructed line of sight between
source and receiver
• curvature of the earth requires stations
(repeaters) ~30 miles apart
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Terrestrial Microwave
Land-based, line-of-sight transmission
Approximately 40-88 Kms maximum between
towers
Transmits data at hundreds of millions of bits per
second
Popular with telephone companies and business
to business transmissions
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Often the microwave antennas are on towers and buildings.
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Satellite
Microwave Transmission
• a microwave relay station in space
• can relay signals over long distances
• 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
– direct broadcast satellite (DBS)
• long-distance telephone transmission
– high-usage international trunks
• private business networks
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Satellite Microwave Applications
• Television distribution
• Long-distance telephone transmission
• Private business networks
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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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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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Fiber vs Satellite
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Mobile Telephone
Wireless telephone service, such as cellular
telephone, cell phone, and PCS.
To support multiple users in a metropolitan area
(market), the market is broken into cells.
Each cell has its own transmission tower and set
of assignable channels.
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Mobile Telephone
AMPS - Advanced Mobile Phone Service - First
popular mobile phone service, uses analog
signals and dynamically assigned frequency
division multiplexing.
D-AMPS - Digital Advanced Mobile Phone
Service - Applies digital time division
multiplexing on top of AMPS.
PCS - Personal Communication Systems - Newer
all-digital mobile phone service (2nd generation)
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Mobile Telephone
PCS phones come in three technologies:
TDMA - Time division multiple access
CDMA - Code division multiple access
GSM - Global system for mobile
communications
3G – Third generation wireless (Internet/data
access)
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Infrared
• Uses transmitters/receivers (transceivers)
that modulate noncoherent infrared light.
• Transceivers must be within line of sight of
each other (directly or via reflection ).
• Unlike microwaves, infrared does not
penetrate walls.
• Very common with remote control devices,
but can also be used for device-to-device
transfers, such as PDA to computer.
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Wireless LAN (IEEE 802.11)
This technology transmits data between
workstations and local area networks using
high speed radio frequencies.
Current technology (and protocol) allows for
11 Mbps data transfer at distances up to
hundreds of feet.
More on this in Chapter Seven (LANs)
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Bluetooth
Bluetooth is a Radio Frequency specification for
short-range, point-to-multipoint voice and data
transfer.
Bluetooth can transmit through solid, non-metal
objects.
Its typical link range is from 10 cm to 10 m, but
can be extended to 100 m by increasing the
power.
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Bluetooth
Bluetooth will enable users to connect to a
wide range of computing and
telecommunication devices without the need
of connecting cables.
Typical uses include phones and pagers,
modems, LAN access devices, headsets,
notebooks, desktop computers, and PDAs.
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WAP (Wireless Application Protocol)
WAP allows wireless devices such as mobile
telephones, PDAs, pagers, and two-way radios to
access the Internet.
WAP is designed to work with small screens and with
limited interactive controls.
WAP incorporates Wireless Markup Language
(WML) which is used to specify the format and
presentation of text on the screen.
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WAP (Wireless Application Protocol)
WAP may be used for applications such as:
- travel directions
- sports scores
- e-mail
- online address books
- traffic alerts
- banking
- news
Possible short-comings of WAP include slow speeds,
security, and a very small user interface.
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Media Selection Criteria
Cost – Initial cost, ROI, maintenance/support
cost
Speed – Data transfer speed, propagation speed
Distance and expandability
Environment – Noise level
Security – Wiretap possible? Need encryption?
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Interconnecting Two Buildings
Two buildings are separated by 400 meters. How
do we interconnect them?
Twisted pair?
(Do we even have access?)
Coaxail cable?
Fiber?
Wireless?
Other? (Chapter 12)
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Wireless Media In Action
DataMining Corporation has one office in
Chicago and one in Los Angeles.
There is a need to transmit large amounts of
data between the two sites.
DataMining is considering using a Very Small
Aperture Terminal satellite system.
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Wireless Media In Action
Cost is proportional to high amount of traffic with very
high reliability.
Speed is high enough to support company’s needs.
Distance can easily expand across the U.S.
Satellite systems are robust in most environments.
Security can be very good with encryption.
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