Data Communications and Networking Chapter 5 Signal Encoding Techniques[数据通信与网络第5章信号编码技术].ppt

Data Communications and Networking Chapter 5 Signal Encoding Techniques References: Book Chapter 5 Data and Computer Communications, 8th edition, by William Stallings Outline Overview Encoding and Modulation Digital data, digital signal Digital data, analog signal Analog data, digital signal Analog data, analog signal Encoding and Modulation Modulation Modulation is the process of encoding source data onto a carrier signal with frequency fc. The frequency of the carrier signal is chosen to be compatible with the transmission medium being used. Modulation techniques involve operation on one or more of the three parameters: amplitude, frequency, and phase According to the input source signal m(t) (either analog or digital), which is called baseband signal (or modulating signal) , the carrier signal fc(t) will be modulated into modulated signal s(t). Encoding/modulation Techniques Digital data, digital signal The equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment. Analog data, digital signal Conversion of analog data (e.g., voice, video) to digital form permits the use of modern digital transmission switching. Digital data, analog signal Optical system and unguided media (wireless system) only propagate analog signals. Analog data, analog signal Baseband: easy and cheap, e.g., in voice-grade telephone lines, voice signals are transmitted over telephone lines at their original spectrum Modulation permits frequency division multiplexing, e.g., AM/FM radios (I) Digital Data, Digital Signal Digital signal is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signal element. Binary data are transmitted by encoding the bit stream into signal elements. In the simplest case, one bit is represented by one signal element. E.g., 1 is represented by a lower voltage level, and 0 is represented by a higher voltage level Terminologies Unipolar If all signal elements have the