【数据通信与网络】Lecture13-WideAreaNetwork.ppt

13.2 Switched Networks How to switch information from one link to the other? Store-and-forward Synchronous switching Time slots How to select the transmission path (route) through the network? routing How to ensure that links do not get overloaded? congestion control (1) Circuit Switching Establishment Channels over internal links TDMA, FDMA, WDMA At each node: Make routing decision Reserve resources TDM Circuit Switching With Time-Slot Interchange Properties of Circuit Switching Fixed data rate with low delay and small delay variations Suitable for voice and video Delay before connection can be used Resources are permanently allocated to connection Much idle time ? low utilisation Variable bit rate traffic ? channel has to be allocated for peak rate (maximum rate) Packet Switching Data transmitted in short packets Large messages broken up into a number of small packets Store and forward switching Receive-store-transmit Properties of Packet Switching Link capacity dynamically shared Traffic with changing bit rate Connections with different bandwidth needs Packets stored and delayed at switching nodes Larger and variable (non-deterministic) traffic delay Voice and video transmission more difficult Packet Switch Buffer Designs How to transmit a sequence of packets? Datagram each packet is treated independently (connectionless) each packet contains full address information Virtual Circuit Switching a connection is set up prior to data transfer (connection oriented) each packet only contains a virtual connection (VC) identifier Datagram Versus Virtual Circuit Performance Comparison Routing Table Routing information stored at each switch: next hop for each destination In this example: Next hop specified by link endpoints (from, to) Routing Strategies (cont’d) Optimality criteria path with the fewest number of links The shortest output queue The lowest end to end delay Administrative policies A router computes least cost path to all other routers Dijkstra’s algorithm