【数据通信与网络】Lecture3-SignalsEncoding分解.ppt

Data Communications and Networking;;3.1 Analog-to-Digital convertion;Coding of analog signals For example voice and video Analog signal is measured at equal intervals Sampling Quantization in time PAM—pulse amplitude modulation 脉冲幅度调制; Sampling (采样);The sampling rate must be at least twice the highest frequency in the analog signal This frequency is often called the Nyquist frequency, or Nyquist rate Theoretically, no information is gained by sampling at a higher rate;Aliasing– 混叠[?eili?si?] ; Data width of sample —— how many bits per sample (样本数据宽度); Conversion Bit Rate (转换比特率);3.2 Pulse Code Modulation (PCM) ------ basic method of analog-to-digital conversion;Quantized PAM Signal;PCM;PCM Examples;Improvement of PCM: DPCM and ADPCM;PCM: From Analog to Digital;3.3 Waveform Encoding;3.3.1 Unipolar Encoding　 (单极性编码) ---- the most primitive and simplest encoding; problems:;3.3.2 Polar Encoding （极化编码）; A、NRZ: non return to zero ;NRZ-L: NRZ level coding (非归零电平编码); B、RZ: return to zero —— coding method to solve synchronization problem thoroughly. including synchronization information in encoded signal; Solution using three levels: positive, negative, zero signal changing not between bits but during each bit;Manchester: at middle of each bit interval, negative-to-positive (jump up)---------”1” positive-to-negative (jump down)---------”0” The transition at the middle of a bit is used for both synchronization and bit representation. It occupies less bandwidth than RZ and applies for Ethernet. ;Two signal levels Higher pulse rate requires larger bandwidth;Need only detect ”transition” or ”no transition”;3.3.3 Bipolar Encoding　(双极性编码);AMI ( Alternate Mark Inv