1. บทที่3 สื่อที่ใช้ในการสื่อสารข้อมูล(Media)
ณรงค์ฤทธิ์ มณีจิระปราการ
สาขาคอมพิวเตอร์ธุรกิจ
ภาควิชาพาณิชย์ศาสตร์
มหาวิทยาลัยนเรศวร
Data Communication and Computer Network

2. วัตถุประสงค์(Objective)
อธิบายคุณสมบัติของสื่อประเภทเหนี่ยวนำแบบต่างๆ
อธิบายคุณสมบัติของสื่อประเภทกระจายคลื่น
อธิบายความแตกต่างระหว่างสื่อเหนี่ยวนำและสื่อประเภทกระจายคลื่น
บอกข้อดีข้อเสียของสื่อเหนี่ยวนำแบบและสื่อประเภทกระจายคลื่น ได้
บอกวิธีการพิจารณาเลือกใช้สื่อที่เหมาะสมกับระบบสื่อสารข้อมูล
Data Communication and Computer Network

3. Data Communication and Computer Network
Data Communications
สื่อสามารถเแบ่งออกเป็นกลุ่มใหญ่ๆ ได้ 2 กลุ่ม คือ
1. สื่อประเภทเหนี่ยวนำ ได้แก่ สายคู่บิดเกลียว, สายโคแอกเชียล, สายใยแก้วนำแสง
2. สื่อประเภทกระจายคลื่น ได้แก่ คลื่นวิทยุ, ไมโครเวฟ
Data Communication and Computer Network

4. Data Communication and Computer Network
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

5. 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

6. 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

7. Electromagnetic Spectrum for Transmission Media
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8. Data Communication and Computer Network
สื่อประเภทเหนี่ยวนำ
Data Communication and Computer Network

9. Guided Transmission Media
Transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint
Examples
twisted pair wires
coaxial cables
optical fiber
Coaxial
Fiber optic
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10. Data Communication and Computer Network
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 interference.
Data Communication and Computer Network

11. Data Communication and Computer Network
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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12. Data Communication and Computer Network

13. Data Communication and Computer Network
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

14. Data Communication and Computer Network

15. Data Communication and Computer Network

16. 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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17. Data Communication and Computer Network

18. Twisted Pair Advantages
Inexpensive and readily available
Flexible and light weight
Easy to work with and install
Data Communication and Computer Network

19. 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

20. 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

21. Data Communication and Computer Network
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

22. Data Communication and Computer Network

23. Data Communication and Computer Network

24. Data Communication and Computer Network
Coax Layers
outer jacket
(polyethylene)
shield (braided wire)
insulating material
copper or aluminum
conductor
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25. Data Communication and Computer Network
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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26. Data Communication and Computer Network
Coax Disadvantages
High attenuation rate makes it expensive over long distance
Bulky
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27. Data Communication and Computer Network
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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28. Data Communication and Computer Network
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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29. Data Communication and Computer Network
Fiber Optic Layers
consists of three concentric sections
plastic jacket
glass or plastic
cladding
fiber core
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30. Data Communication and Computer Network

31. Data Communication and Computer Network

32. Data Communication and Computer Network
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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33. Data Communication and Computer Network
Fiber Optic Signals
fiber optic multimode
step-index
fiber optic multimode
graded-index
fiber optic single mode
Data Communication and Computer Network

34. 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

35. Fiber Optic Disadvantages
expensive over short distance
requires highly skilled installers
adding additional nodes is difficult
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36. Data Communication and Computer Network
It is very common to mix fiber with twisted pair in LANs.
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37. Data Communication and Computer Network
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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38. Data Communication and Computer Network

39. Data Communication and Computer Network
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

40. Data Communication and Computer Network

41. 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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42. Data Communication and Computer Network
Wireless Examples
terrestrial microwave
satellite microwave
broadcast radio
infrared
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43. Data Communication and Computer Network
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

44. 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

45. Data Communication and Computer Network
Terrestrial Microwave
Land-based, line-of-sight transmission
Approximately 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

46. Data Communication and Computer Network

47. Data Communication and Computer Network
Often the microwave antennas are on towers and buildings.
Data Communication and Computer Network

48. 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

49. 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

50. Satellite Transmission Process
transponder
dish
dish
22,300 miles
uplink station
downlink station
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51. Data Communication and Computer Network

52. Data Communication and Computer Network

53. 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

54. Satellite Microwave Applications
Television distribution
Long-distance telephone transmission
Private business networks
Data Communication and Computer Network

55. 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

56. 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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57. Data Communication and Computer Network
Fiber vs Satellite
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58. Data Communication and Computer Network
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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59. Data Communication and Computer Network

60. Data Communication and Computer Network

61. Data Communication and Computer Network
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

62. Data Communication and Computer Network
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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63. Data Communication and Computer Network
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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64. Data Communication and Computer Network
Wireless LAN (IEEE )
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)
Movies – You go online at home, see which movies are playing, order tickets, download them to home computer, do a wireless transfer over to PDA. Take PDA with you to movies. PDA communicates with ticket booth at movie theatre, you just walk right in.
Or you walk up to your front door and your PDA communicates with your house – unlocks door, turns on lights, sets the heat/cool temperature, turns on the TV, etc.
Other examples?
Data Communication and Computer Network

65. Data Communication and Computer Network
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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66. Data Communication and Computer Network
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.
Movies – You go online at home, see which movies are playing, order tickets, download them to home computer, do a wireless transfer over to PDA. Take PDA with you to movies. PDA communicates with ticket booth at movie theatre, you just walk right in.
Or you walk up to your front door and your PDA communicates with your house – unlocks door, turns on lights, sets the heat/cool temperature, turns on the TV, etc.
Other examples?
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67. Data Communication and Computer Network
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

68. Data Communication and Computer Network
WAP (Wireless Application Protocol)
WAP may be used for applications such as:
- travel directions
- sports scores -
- 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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69. Data Communication and Computer Network
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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70. Data Communication and Computer Network
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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71. Data Communication and Computer Network
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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72. Data Communication and Computer Network
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