数据通信ch04.pptVIP

Digital transmission needs a low-pass channel. A digital signal theoretically needs a bandwidth between 0 and infinity. The lower limit (0) is fixed; the upper limit (infinity) can be relaxed if we lower our standards by accepting a limited number of harmonics. Note: Analog transmission can use a band-pass channel. The bandwidth of an analog signal can always be shifted. For example, we can always shift a signal with a bandwidth from f1 to f2 to a signal with a bandwidth from f3 to f4 as long as the width of the bandwidth remains the same. The bandwidth of a medium can be divided into several band-pass channels to carry several analog transmissions. Note: Data Rate Limit Noiseless Channel: Nyquist Bit Rate Noisy Channel: Shannon Capacity Using Both Limits Data rate depends on three factors: 1. The bandwidth available 2. The levels of signals we can use 3. The quality of the channel (the level of the noise). Two theoretical formulas were developed to calculate the data rate: Nyquist for a noiseless channel Shannon for a noisy channel Data Rate Limit An analog signal is best represented in the frequency domain. Note: Time and Frequency Domain 7 1 s 12 12 时域图：显示了信号振幅随时间的改变。 频域图：可以表现振幅或相位于频率之间的关系。 最大振幅－频率图（在数据通信中更常用） 相位－频率图 A single-frequency sine wave is not useful in data communications; we need to change one or more of its characteristics to make it useful. Note: When we change one or more characteristics of a single-frequency signal, it becomes a composite signal made of many frequencies. Note: According to Fourier analysis, any composite signal can be represented as a combination of simple sine waves with different frequencies, phases, and amplitudes. Note: Fourier showed that any composite signal is a sum of a set of sine waves of different frequencies, phases, and amplitudes. In other words, we can write a composite signal as s(t) = A1 sin (2πf1t + φ1) + A2 sin (2πf2t + φ2) + A3 sin (2πf3t + φ3) + . . . Note: Square wave For example, let us c