英文版课件蒋ch2-physical.ppt

作业 28、31、34 Switching Circuit Switching (a) Circuit switching (b) Packet switching Packet switching A node in a packet switching network incoming links outgoing links Node Memory Message Switching (a) Circuit switching (b) Message switching (c) Packet switching Circuit Switched and Packet-switched Networks CDMA – Code Division Multiple Access A form of spread spectrum communication Bit time Each bit time is subdivided into m short intervals called chips(m=64,128) Chip sequence Each station is assigned a unique m-bit code called a chip sequence Orthogonality 比较站A和站B的片序列分量：一半相同，一半相反 Send data Transmit bit 1: sends its chip sequence Transmit bit 0: sends the complement of its chip sequence No other patterns are permitted CDMA – Code Division Multiple Access (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal Orthogonality A*A=1 A*(~A)=-1 A*B=0 A*(~B)=0 作业 42、43、53 2.7 Cable Television Community Antenna Television An early cable television system A big Antenna: pluck TV signal out of the air Head end: Signal amplifier Coaxial cable: deliver TV signals to houses Since 1974 cable-only TV channels connect cables between cities (net) Internet over Cable Cable television HFC: Hybrid Fiber Coax Internet over Cable (2) The fixed telephone system. Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access Televisions: 54-550MHz (6MHz/channel) 550-750MHz: Downstreams 6MHz/channel, analog modulation QAM-64 , 36Mbps, net payload 27Mbps QAM-256, 48Mbps, net payload 39 Mbps 5-42MHz: Upstreams (QPSK modulation) Cable Modems Typical details of the upstream and downstream channels in North America Modem-to-Computer interface: 10Mbps Ethernet Upstream: minislot, 8 bytes, (contention) Downstream: Fixed Packet size 204 bytes, payload 184 All traffic is encrypted Physical Interface Physical Interface Interface standard between DTE and DCE DTE Data Term