数据与计算机通信第12章.ppt

William StallingsData and Computer Communications7th Edition Chapter 12 Routing 内容 12.1 Routing in Circuit –switching Networks 12.2 Routing in Packet-Switching Networks 12.3 Least-Cost Algorthms 12.1 Routing in Circuit Switched Network Many connections will need paths through more than one switch Need to find a route Efficiency Resilience(回弹力) Public telephone switches are a tree structure Static routing uses the same approach all the time Dynamic routing allows for changes in routing depending on traffic Uses a peer structure for nodes Alternate Routing Possible routes between end offices predefined Originating switch selects appropriate route Routes listed in preference order Different sets of routes may be used at different times AlternateRoutingDiagram 12.2 Routing in Packet Switched Network Complex, crucial (至关重要的) aspect of packet switched networks Characteristics (特性) required Correctness(正确性) Simplicity(简洁行) Robustness(稳健性) Stability (稳定性) Fairness(公平性) Optimality(最优性) Efficiency(高效性) Performance Criteria Used for selection of route Minimum hop (跳数,途径结点的数量) Least cost See Stallings appendix 10A for routing algorithms Example Packet Switched Network Decision Time and Place Time Packet or virtual circuit basis PlacePlace(refers to whcich node or nodes in the network are responsible for the routing decsion,是指应该由哪一个或者哪一些结点来负责路由选择的判决) Distributed Made by each node Centralized Source Network Information Source and Update Timing (网络信息资源和更新定时) Routing decisions usually based on knowledge of network ((not always，大多数情况下，路由会要网络的拓扑结构、通信负荷量和链路费用等信息)) Distributed routing (分布式路由) Nodes use local knowledge May collect info from adjacent nodes May collect info from all nodes on a potential route Central routing (集中式路由) Collect info from all nodes Update timing(更新定时) When is network info held by nodes updated Fixed - never updated Adaptive - regular updates Routing Strategies Fixed(固定式) Flooding(洪泛式) Random(随机式) Adaptive(自适应式) Fixed Routing Single permanent route for