本地异步通信技术（RS232）.pptx

Computer Networks and Internets 《计算机网络与因特网》课件 2004-02-6;PART I Data Transmission;6.1 Introduction;;6.2 Sending Signals Across Long Distances;;A continuous oscillating(振荡) signal will propagate farther than other signals. Instead of transmitting an electric current that only changes when the value of a bit changes, long distance communication systems send a continuously oscillating signal,usually a sine wave(正弦波), called a carrier(载波).;The carrier oscillates continuously, even when no data is being sent. To send data, a transmitter modifies the carrier slightly. Such modification are called modulation(调制). The transmitter generates a continuously oscillating carrier signal, which it modulates according to the data being sent. ;The receiver on a long-distance communication link must be configured to recognize the carrier that the sender uses. The receiver monitors the incoming carrier, detects modulation , reconstructs the original data, and discards the carrier.;Network technologies use a variety of modulation techniques, including： amplitude modulation (AM调幅), frequency modulation (FM调频), phase shift modulation (PM调相). ;;;;6.3 Modem Hardware Used For Modulation And Demodulation;Transmission of data across a long distance requires a modulator at one end of the transmission line and a demodulator at the other. Most network systems are full duplex, each location needs both a modulator and a demodulator. manufactures combine both circuits into a single device called a modem(调制解调器).;;;;6.4 Leased Analog Data Circuits租用模拟数据线路;Once a connection has been leased from the phone company,a modem must be installed at each end before communication is possible. After that, the leased line is available to send data. Advantage: data can be sent at any time. Disadvantage: the limited connectivity and cost.;6.5 Optical, Radio Frequency, and Dialup Modems;;6.6 Carrier Frequency And Multiplexing载波频率和多路复用;Computer networks use the prin