计算机网络英文版课件2.pptVIP

江西师大软件学院 Computer networkChapter 2The Physical Layer 郭 斌 E_mail: jxnuit@163.com Physical Layer Provide the means to transmit bits from sender to receiver, defines the mechanical, electrical , and timing interfaces to the network ,involves Transmission media How to use (analog) signals for digital information Techniques such as Modulation techniques (the actual encoding), multiplexing, and switching Topics Basic Concepts of Data Transmission Guided Transmission Wireless Transmission Satellite Public Switched Telephone System Cable Television Basic Concepts of Data Transmission Fourier Analysis Bandwidth the Maximum Data Rate of a Channel Asynchronous and Synchronous Communication Serial and Parallel Communication Data Transmission Multiplexing Fourier Analysis (1) Any reasonably behaved periodic function, g(t) with period T can be constructed as the sum of a (possibly infinite) number of sines and cosines: f = 1/T : the fundamental frequency. an and bn : the sine and cosine amplitudes of the nth harmonics (terms). c : a constant. Such a decomposition is called a Fourier series. From the Fourier series, the function can be reconstructed; that is, if the period, T, is known and the amplitudes are given, the original function of time can be found. Fourier Analysis (2) The results of performing these operations Fourier Analysis (3) No transmission facility can transmit signals without losing some power in the process. If all the Fourier components were equally diminished, the resulting signal would be reduced in amplitude but not distorted Unfortunately, all transmission facilities diminish different Fourier components by different amounts, thus introducing distortion. Fourier Analysis (4) Bandwidth and Bandwidth-Limited Signals the range of frequencies transmitted without being strongly attenuated(衰减) is called the bandwidth. The bandwidth is a physical property of the transmission medium and usually depends on the construction, thickness, and len