Computer Networks and Internets《计算机网络与因特网》课件.ppt

Computer Networks and Internets 《计算机网络与因特网》课件 林坤辉 2004-02-6;PART II Packet Transmission;8.2 Direct Point-to-Point Communication直接点对点通信;;Three useful properties: First, each connection is installed independently, appropriate hardware can be used. Second, they have exclusive access, the connected computers can decide exactly how to send data across the connection. Third, because only two computers have access to the channel, it is easy to enforce security and privacy. ;;Disadvantage: The total number of connections grows more rapidly than the total number of computers. The number of connections needed for N computers is proportional to the square of N: Direct connections required=(N2-N)/2 The expense is especially high because many connections follow the same physical path.;8.3 Shared Communication Channels共享通信信道;Each LAN consists of a single shared medium, usually a cable, to which many computers attach. The computers take turns using the medium to send packets. Networks that allow multiple computers to share a communication medium are used for local communication. Point-to-point connections are used for long-distance networks and a few other special cases.;If sharing reduces cost, why are shared networks used only for local communication? the computers attached to a shared network must coordinate use of the network. Because coordination requires communication and the time required to communication depends on distance.;8.4 Significance of LANs and Locality of Reference局域网的重要性与访问的局部性;8.5 LAN Topologies局域网拓扑结构;8.5.1 Star Topology;Star networks seldom have a symmetric shape in which the hub is located an equal distance from all computers.;8.5.2 Ring Topology;A cable connects the first computer to a second computer, another cable connects the second computer to a third, and so on, until a cable connects the final computer back to the first. The computers and connections in a ring network nee