网络课件 双语教学 模拟实验
计算机网络教研室

1 The first section
2
Exercises
3 Online lecture
Department of Computer
Networking Application

Chapter 8 Transmission Media
8.1 INTRODUCTION
Media transmission
Conducted or guided media
A type of media that provides some type of physical path , such as wire, cable, or optical fiber , along which the signal moves from end to end .
Radiated or unguided media most commonly known today as wireless ,such as radio waves,microwaves,or infrared waves ,through air,water,or a vacuum .
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Chapter 8 Transmission Media
8.2 CONDUCTED MEDIA
8.2.1 Unshielded Twisted Pair Wire
Figure 8-1 Twisted pair wires are the most commonly used medium for communications transmission.
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Chapter 8 Transmission Media
Twisting a pair of wires together has the effect of electrically canceling the signals radiating from each wire .
The Electrical Industries
Association(EIA) has defined six categories of UTP for telephone and data transmission use. P150 www.gxmu.edu.cn

Chapter 8 Transmission Media
Cat 3 Cat 5
Wire containing at least three twists per foot and that is about the same as normal telephone cable installed in most office buildings is called CAT 3 .
Company Logo
CAT 5 cable normally contains four pairs of wires and is the type that is most commonly being installed in new homes and businesses, particularly when it is known that a computer network will be used.
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Chapter 8 Transmission Media
When UTP is use in a data application it is normally terminated with a RJ-45 jack . P151
The purpose of a terminating block or a punchdown block is to be responsible for providing and maintaining all wiring within the building. P152 www.gxmu.edu.cn

Chapter 8 Transmission Media
Figure 8-2 Wires installed outside may be buried or hung from poles above ground.
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Chapter 8 Transmission Media
8.2.2 Shielded Twisted Pair Wire （ STP ）
Twisted pair wire is placed inside a thin metallic shielding,similar to aluminum foil and then enclosed in an outer plastic casing .
STP cable is infrequently used because it is difficult to work with. www.gxmu.edu.cn

Chapter 8 Transmission Media
8.2.3
Figure 8-3 Parts of coaxial cable.
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Chapter 8 Transmission Media
Coaxial cable is advantageous when taps in the cable need to be made—it is easier to tap than optical fiber.
The most common connector used in networking applications is the British
Naval Connector(BNC) .
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Chapter 8 Transmission Media
8.3
OPTICAL FIBER
The optical fiber is a very thin glass fiber of high purity .
The core of an optical fiber is surrounded by another type of glass called the cladding .
Data are placed on an optical fiber with a light source, either an LED or a laser.
The total diameter of an optical fiber is less than that of a human hair.
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Chapter 8 Transmission Media
8.3
OPTICAL FIBER
光发射机原理示意图 光接收机光电转换原理示意图 www.gxmu.edu.cn

Chapter 8 Transmission Media
Figure 8-4 Parts of optical fiber cable.
Single mode optical fiber is more expensive than multimode fiber.
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Chapter 8 Transmission Media
Structure of Optical Fiber Cable www.gxmu.edu.cn

Chapter 8 Transmission Media
8.3
OPTICAL FIBER
The characteristic of optical fiber include:
 high bandwidth ;
 difficult to tap and splice ;
 not subject to electrical interference ;
 cable is very small diameter and lightweight .
 little loss of signal strength ; www.gxmu.edu.cn

Chapter 8 Transmission Media
8.4
WIRELESS MEDIA
8.4.1 Microwave Radio
Microwave radio signals travel in a straight line ,and cann’t follow the curvature of the earth so it is necessary for the transmitting and receiving antennas to be in a direct LOS.
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Chapter 8 Transmission Media
8.4.2 Satellite
Satellites in geosynchronous orbits circle the earth at 22,300 miles.
The radio signal beamed from the Earth at a satellite is called the uplink ， where it is received, amplified ,and then rebroadcast on a different frequency,called the downlink .
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Chapter 8 Transmission Media
Figure 8-5 Satellite transmission.
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Chapter 8 Transmission Media
Propagation delay is the delay that occurs because light and radio signals only travel at 186,000 miles per second.
A type of satellite commonly used to send television programming into homes is called a direct broadcast satellite .
Satellites in medium earth orbits(MEO) are approximately 6,000 miles high.
Low earth orbit(LEO) satellites orbit at between 300 and 1000 miles high.
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Chapter 8 Transmission Media
8.4.3
Infrared transmissions require the sender and receiver to be in direct line of sight
.
8.4.4 Free Space Optics
The use of modulated laser beams to send data between locations that are within LOS of each other is called Free space optics (FSO).
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Chapter 8 Transmission Media
8.6 SUMMARY OF MEDIA CHARACTERISTICS www.gxmu.edu.cn

Chapter 8 Transmission Media
1.The radio signal beamed from the Earth at a satellite is called the （ A ） .
√ A 、 uplink
× B 、 downlink
× C 、 data link
× D 、 control link
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Chapter 8 Transmission Media
2.The most popular cable for networking being installed in new office buildings is （ D ） .
× A 、 STP
×
B 、 optical fiber
× C 、 CAT 3
√ D 、 CAT 5
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Chapter 8 Transmission Media
3.Multiplexing on optical fibers is performed by using （ B ） .
× A 、 FSK
√ B 、 WDM
× C 、 FDM
× D 、 PAM
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Chapter 8 Transmission Media
4. The type of media that provides some type of physical path is called （ D ） .
×
A 、 unguided media
×
B 、 physical media
× C 、 wireless media
√ D 、 conducted media
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Chapter 8 Transmission Media
5.Twisting a pair of wires together has the effect of （ C ） .
× A 、 shielding the wire from outside interference
×
B 、 strengthening the pair against breakage
√ C 、 electrically canceling the signals radiating from each wire
× D 、 amplifying the signal flowing through the wire
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Chapter 8 Transmission Media
Answer
7.The most appropriate media to use when there is a need for
Answer
Answer

Chapter 8 Transmission Media difficult to tap and splice; not subject to electrical interference, cable is very small diameter and lightweight.
Answer
10.A type of satellite commonly used to send television
Answer

Chapter 8 Transmission Media www.gxmu.edu.cn