Download การสื่อสารข้อมูล

Document related concepts

Low-voltage differential signaling wikipedia , lookup

Computer network wikipedia , lookup

Piggybacking (Internet access) wikipedia , lookup

Network tap wikipedia , lookup

List of wireless community networks by region wikipedia , lookup

Cracking of wireless networks wikipedia , lookup

Airborne Networking wikipedia , lookup

IEEE 1355 wikipedia , lookup

UniPro protocol stack wikipedia , lookup

Transcript
บทที่3
สื่ อที่ใช้ในการสื่ อสารข้อมูล(Media)
ณรงค์ฤทธิ์ มณี จิระปราการ
สาขาคอมพิวเตอร์ธุรกิจ
ภาควิชาพาณิ ชย์ศาสตร์
มหาวิทยาลัยนเรศวร
Data Communication and
Computer Network
1
วัตถุประสงค์(Objective)
อธิบายคุณสมบัติของสื่ อประเภทเหนี่ยวนาแบบต่ างๆ
อธิบายคุณสมบัติของสื่ อประเภทกระจายคลื่น
อธิบายความแตกต่ างระหว่างสื่ อเหนี่ยวนาและสื่ อประเภทกระจาย
คลื่น
บอกข้ อดีข้อเสี ยของสื่ อเหนี่ยวนาแบบและสื่ อประเภทกระจาย
คลื่น ได้
บอกวิธีการพิจารณาเลือกใช้ สื่อทีเ่ หมาะสมกับระบบสื่ อสารข้ อมูล
Data Communication and
Computer Network
2
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
Data Communication and
Computer Network
3
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
Data Communication and
Computer Network
4
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
Data Communication and
Computer Network
5
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.
Data Communication and
Computer Network
6
Electromagnetic Spectrum for
Transmission Media
Data Communication and
Computer Network
7
สื่ อประเภทเหนี่ยวนา
Data Communication and
Computer Network
8
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
Data Communication and
Computer Network
9
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.
Computer Network
10
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
Data Communication and
Computer Network
11
Data Communication and
Computer Network
12
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
Data Communication and
Computer Network
13
Data Communication and
Computer Network
14
Data Communication and
Computer Network
15
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
Data Communication and
Computer Network
16
Data Communication and
Computer Network
17
Twisted Pair Advantages
• Inexpensive and readily available
• Flexible and light weight
• Easy to work with and install
Data Communication and
Computer Network
18
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)
Data Communication and
Computer Network
19
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”
Data Communication and
Computer Network
20
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.
Data Communication and
Computer Network
21
Data Communication and
Computer Network
22
Data Communication and
Computer Network
23
Coax Layers
outer jacket
(polyethylene)
shield
(braided wire)
insulating material
copper or aluminum
conductor
Data Communication and
Computer Network
24
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
Data Communication and
Computer Network
25
Coax Disadvantages
• High attenuation rate makes it expensive
over long distance
• Bulky
Data Communication and
Computer Network
26
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.
Data Communication and
Computer Network
27
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)
Data Communication and
Computer Network
28
Fiber Optic Layers
• consists of three concentric sections
plastic jacket
glass or plastic
cladding
Data Communication and
Computer Network
fiber core
29
Data Communication and
Computer Network
30
Data Communication and
Computer Network
31
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
Data Communication and
Computer Network
32
Fiber Optic Signals
fiber optic multimode
step-index
fiber optic multimode
graded-index
fiber optic single mode
Data Communication and
Computer Network
33
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
Data Communication and
Computer Network
34
Fiber Optic Disadvantages
• expensive over short distance
• requires highly skilled installers
• adding additional nodes is difficult
Data Communication and
Computer Network
35
It is very common to mix fiber with twisted pair in LANs.
Data Communication and
Computer Network
36
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.
Data Communication and
Computer Network
37
Data Communication and
Computer Network
38
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.
Data Communication and
Computer Network
39
Data Communication and
Computer Network
40
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
Data Communication and
Computer Network
41
Wireless Examples
•
•
•
•
terrestrial microwave
satellite microwave
broadcast radio
infrared
Data Communication and
Computer Network
42
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.
Data Communication and
Computer Network
43
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
Data Communication and
Computer Network
44
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
Data Communication and
Computer Network
45
Data Communication and
Computer Network
46
Often the microwave antennas are on towers and buildings.
Data Communication and
Computer Network
47
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
Data Communication and
Computer Network
48
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
Data Communication and
Computer Network
49
Satellite Transmission Process
satellite
transponder
dish
dish
22,300 miles
uplink station
downlink station
Data Communication and
Computer Network
50
Data Communication and
Computer Network
51
Data Communication and
Computer Network
52
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
Data Communication and
Computer Network
53
Satellite Microwave Applications
• Television distribution
• Long-distance telephone transmission
• Private business networks
Data Communication and
Computer Network
54
Microwave Transmission
Disadvantages
• line of sight requirement
• expensive towers and repeaters
• subject to interference such as passing
airplanes and rain
Data Communication and
Computer Network
55
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
Data Communication and
Computer Network
56
Fiber vs Satellite
Data Communication and
Computer Network
57
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.
Data Communication and
Computer Network
58
Data Communication and
Computer Network
59
Data Communication and
Computer Network
60
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)
Data Communication and
Computer Network
61
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)
Data Communication and
Computer Network
62
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.
Data Communication and
Computer Network
63
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)
Data Communication and
Computer Network
64
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.
Data Communication and
Computer Network
65
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.
Data Communication and
Computer Network
66
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.
Data Communication and
Computer Network
67
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.
Data Communication and
Computer Network
68
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?
Data Communication and
Computer Network
69
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)
Data Communication and
Computer Network
70
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.
Data Communication and
Computer Network
71
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.
Data Communication and
Computer Network
72