计算机网络之Transmission Media（课件PPT）.pptVIP

Computer networks 4(2.4):Transmission Media Overview Guided – wire Unguided – wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the data bandwidth produced by the antenna is more important Key concerns are data rate and distance Design Factors Bandwidth Higher bandwidth gives higher data rate Transmission impairments Attenuation Interference Number of receivers In guided media More receivers (multi-point) introduce more attenuation Electromagnetic Spectrum Guided Transmission Media Twisted Pair Coaxial cable Optical fiber Twisted Pair：双绞线 Twisted Pair - Applications Most common medium Telephone network Between house and local exchange (subscriber loop) Within buildings To private branch exchange (PBX) For local area networks (LAN) 10Mbps or 100Mbps Twisted Pair - Pros and Cons Cheap Easy to work with Low data rate Short range Twisted Pair - Transmission Characteristics Analog Amplifiers every 5km to 6km Digital Use either analog or digital signals repeater every 2km or 3km Limited distance Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise Unshielded and Shielded TP Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference Shielded Twisted Pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy) UTP Categories Cat 3 up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm Cat 4 up to 20 MHz Cat 5 up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm Near End Crosstalk：近端串扰 Coupling of signal from one pair to another Coupling takes place when transmit signal entering the link couples back to receiving pair i.e. near transmitted signal is picked up by near receiving pair Making and Testing 10base-T Coaxial Cable：同轴电缆 Coaxial Cable Applications Most versatile medium：用途最广 Television