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Passage Three
Network Transmission
Media
Training target:
In this part，you should try your best to form
good reading habits. In order to avoid your ill
habits such as regression，you should read with
the card showed to you in the preface of this book.
Keep reading the paragraphs from the beginning
to the end without stopping.
Text
• Transmission media can be transferred in
wired or wireless method. The basic wired
media are twisted pair，coaxial cable，and
fiber optics. Wireless media have terrestrial
microwave，satellite microwave and
broadcast radio.
1.Twisted pair
• Twisted pair（Pic6.9）is made of two copper wires
which are covered by plastics or materials that are
similar. The two insulated wires are twisted together
to form the communication line. The purpose of
twisting the wires is to avoid the electrical
interference. Twisted pair should be familiar to us，
since it is widely used in our telephone
communication either in the home or the office.
Twisted pair can be combined into a cable that may
consist of two，four，fifty，or hundreds of pairs.
Twisted pair is the basis of transmission media. It
supports most of the local telephone network.
Pic 6.9
Twisted pair
2.Coaxial cable
• Coaxial cable（Pic6.10）contains two
conductors，which are insulated from one
another. The inner conductor is covered by the
dielectric. The outer conductor is covered by a
shield.
• There are many applications for coaxial cable. For instance，
the cable TV is one of the applications of coaxial cable. Local
network，short-run computer system links，and long-distance
telephone transmission are also the important applications of
coaxial cable. In local network，coaxial cable can support
different types of data communication including a single
building or a complex facility. In the transmission of data
between different computers，coaxial cable can survey high
speed input and output channels for computer systems. By
using the frequency division multiplexing（FDM），a
coaxial cable can carry more than 10，000 channels at the
same time.
Pic 6.10 Coaxial cable
3.Optical fiber
• Optical fiber（Pic6.11）is the best transfer
media at present. The optical fiber contains
three parts: core，cladding and jacket. The
inner most part is the core，which contains
one or more thin fibers that are made of plastic
or glass. Each of the fiber in the core is
covered by the cladding. The cladding is a
glass or plastic coating which has different
optical properties from the core. The outer
most part of the optical fiber is made of plastic
or other materials，which is called jacket.
Pic 6.11 Optical Fiber
• The purpose of the jacket is to prevent the destruction
from the environment. There are a light-emitting
diode（light source）in one side of the fiber and a
photodiode（light-sensitive receptor）in the other
side of the fiber. The fiber optics equipment transmits
the light pulses from one side to the other through the
plastic or glass. Because of the better performance
and the greater data rate of optical fiber，the
application of optical fiber has increased dramatically
in long-distance communication and military
application.
• 4.Terrestrial microwave
•
Terrestrial（Pic6.12）microwave transmission is
another way of transmitting data. A pair of
microwave antennas which are equipped on the top of
towers will transmit the signals through the air. The
shape of microwave antenna is similar to a dish and is
about ten feet wide. The microwave antennas are
usually equipped in higher place in order to achieve
better transmission effect. Terrestrial microwave is
used as an alternatives of coaxial cable and optical
fiber，since terrestrial microwave transmission needs
less repeaters than that of coaxial cable. The voice
transmission，television transmission，and the short
point-to-point links between building will be the
major application of terrestrial microwave
transmission.
•
Pic6.12 Terrestrial Microwave
• 5.Satellite microwave
•
A satellite consists of several frequency bands
which receive signals from one frequency band，this
action is called uplink. After this action，the satellite
will amplify or repeat the signals to the other
frequency band，this action is called downlink. A
satellite link is like a microwave link. Satellite will
link two or more ground stations，like point-to-point
link or broadcast link. The most important
applications for satellite communication will be longdistance telephone transmission，private business
networks，and television distribution.
• 6.Broadcast radio
•
The major difference between microwave and
broadcast radio transmission is that the broadcast
radio can have many directions，and microwave is
only point to point transmission. In other words，the
broadcast radio can transmit data to many
destinations at the same time. The broadcast radio
covers the range of frequency from 3 kHz to 300kHz.
From 30MHz to 1GHz，this range is most effective
for broadcast radio to transmit data.
• Comparison among twisted pair，coaxial cable，
and optical fiber:
• The coaxial cable has higher data rate and
bandwidth than that of twisted pair（4 Mbps for
twisted pair and 500 Mbps for coaxial
cable）.The repeater spacing of twisted pair and
coaxial cable is similar.
• However，the fiber optics has superior
performance compared with twisted pair
and coaxial cable. The fiber optics has
2GHz of bandwidth（350MHz for coaxial
cable），and the repeater spacing is10～
100km for fiber optics（1～10 km for
coaxial cable）.There is a very important
characteristic of optical fiber needed to be
mentioned.
• The optical fiber is not affected by the
electromagnetism，but the twisted pair and the
coaxial cable are affected by the electromagnetism
because their conductors will produce electrical
coupling with the electric field that produced by
others. These characteristics of twisted pair，coaxial
cable，and fiber optics will provide some
information when the user，who may also be a
manager，to choose the transmission media to meet
their needs.
Key words
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twisted pair 双绞线
optical fiber 光纤
antenna 天线
conductor导体
uplink 上行
FDM 频分多路复用
coaxial cable同轴电缆
insulate绝缘的
downlink下行
terrestrial microwave地面微波
broadcast radio广播无线电波
The End