计算机第2章e.ppt

coder：data? adapt to transport signal ——identify、error correct modulator：signal? adapt to transport format——according to frequency 、amplitude、phase decoder：transport signal?primitive data demodulator：receive wave-shape?digital signal sequence Two direction signal share link(链路) bandwidth: 1）Link has two independent physical transport line，or 2）bandwidth one divides into two(一分为二)，individual uses differ direction signal transport c）Noise ????? Thermal noise：caused by conductor thermal disturbance , or called white-noise(白噪声)。 ??????Crosstalk(串扰)：Not necessary coupling (耦合) produced between channel。For：multi-pair twisted-pair(多对双绞线) ??????Pulse noise：not succession、random、bigger amplitude。More produced by exterior (外部) electromagnetic disturbance（lighting、high-power(大功率) electric motor start-up(启动) etc.）。 Noise will demolish(破坏) signal，produce error code。 Time lasts 0.01s disturbance can demolish about 560 bits（56Kbps）。 Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP. Coaxial Cable (同轴电缆) p93 A coaxial cable. Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection. Light ray transmission system：light source、medium、light detector light source: 850nm/1300nm/1500nm LEDs (light emitting diodes(发光二极管) / Semiconductor laser medium: fiber light detector: photoelectricty diodes (光电二极管)PIN/avalanche diodes(雪崩二极管APD) ?single transmission，bidirection(双向) needs two fiber Transmission of Light through Fiber Attenuation of light through fiber in the infrared region. Basic principle：use digital signal modulates carrier wave different parameters。 Modems (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation It needs to convert voice signal to digital signal for voice signal transmission in the digital channel 。The three steps are ： 1）Sampling——according to the interval to sample to voice signal