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 long- distance 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 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