信息通信Chapter 06.ppt

6.1 Digital Data Transmission 6.2 DTE-DCE Interface 6.3 Other Interface Standards 6.4 Modems 6.5 Summary 6.4 Modems The most familiar type of DCE is a modem. Modems stands for modulator/demodulator. A modulator converts a digital signal to an analog signal. A demodulator converts an analog signal to a digital signal. Fig 6.19 6.4.1 Modulator/demodulator 6.4 Modems Every line, can accept a certain range of signal changes per second. If it is too slow, it cannot over the capacitance of the line, if it is too fast, it can be impeded by the inductance of the line. Every line have an upper limit and a lower limit on the frequencies of the signals it can carry. The limit range is called the bandwidth (of the line). 6.4 Modems Traditional telephone line BW for telephone line=3000Hz efficient bandwidth for data transmission =2400 Hz Fig 6.20 6.4 Modems 6.4.2 Modem Speed The speed of modem is depend upon its signal modulating methods. 1. ASK baud rate for half-duplex ASK 6.4 Modems baud rate for full-duplex ASK Fig 6.22 6.4 Modems 2. FSK baud rate for half-duplex FSK Fig 6.23 3. PSK and QAM The minimum bandwidth required is the same as for ASK. 6.4 Modems baud rate for full-duplex FSK 2400/2 -(fc0-fc1) 6.4 Modems 6.4.2 Modem Standards Two modem standards: Bell modems and ITU-T modems 6.4.2.1 Bell modems standards Bell Telephone Company 1970s 6.4.2.2 ITU-T modem standards Many popular modems are based on standards published by the ITU-T. 6.4 Modems Intelligent modems(智能调制解调器): contain software to support a number of additional functions ,such as automatic answering(自动应答) and dialing(拨号). 6.1 Digital Data Transmission 6.2 DTE-DCE Interface 6.3 Other Interface Standards 6.4 Modems 6.5 Summary 6.5 Summary 1. Digital transmission can be either parallel or serial in mode 2. In parallel mode a group of bits is sent simultaneously with