Digital_Transmission_Fundamentals汇总.ppt

* Coaxial Cable Cylindrical braided outer conductor surrounds insulated inner wire conductor High interference immunity Higher bandwidth than twisted pair Hundreds of MHz 35 30 10 25 20 5 15 Attenuation (dB/km) 0.1 1.0 10 100 f (MHz) 2.6/9.5 mm 1.2/4.4 mm 0.7/2.9 mm * Optical Fiber Light sources (lasers, LEDs) generate pulses of light that are transmitted on optical fiber Very long distances (1000 km) Very high speeds (40 Gbps/wavelength) Nearly error-free (BER of 10-15) Profound influence on network architecture Optical fiber Optical source Modulator Electrical signal Receiver Electrical signal * Core Cladding Jacket Light ?c Geometry of optical fiber Total Internal Reflection in optical fiber Transmission in Optical Fiber Core has higher index of refraction than cladding Light rays incident at less than critical angle qc is completely reflected back into the core * Multimode: Thicker core, Rays on different paths interfere causing dispersion limiting bit rate Single mode: Very thin core, More expensive lasers, but achieves very high speeds Multimode fiber: multiple rays follow different paths Single-mode fiber: only direct path propagates in fiber Direct path Reflected path Multimode Single-mode Fiber * Optical Fiber Properties Advantages Very low attenuation Noise immunity Extremely high bandwidth Security: Very difficult to tap without breaking No corrosion More compact lighter than copper wire Disadvantages New types of optical signal impairments dispersion Limited bend radius Difficult to splice * 100 50 10 5 1 0.5 0.1 0.05 0.01 0.8 1.0 1.2 1.4 1.6 1.8 Wavelength (?m) Loss (dB/km) Infrared absorption Rayleigh scattering Very Low Attenuation 850 nm Low-cost LEDs LANs 1300 nm Metropolitan Area Networks “Short Haul” 1550 nm Long Distance Networks “Long Haul Water Vapor Absorption (removed in new fiber designs) * 100 50 10 5 1 0.5 0.1 0.8 1.0 1.2 1.4 1.6 1.8 Loss (dB/km) Huge Available Bandwidth Optical range from λ1 to λ1 +Δλ contains bandwidth