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The purpose of this book is to give the reader two things, to paraphrase Mark Twain: Roots to know the basics of modeling networks and Wings to fly away and attempt modeling other proposed systems of interest. The Internet phenomenon is affecting us all in the way we communicate, conduct business, and access information and entertainment. More unforeseen applications are still to come. All of this is due to the existence of an efficient global hi- performance network that connects millions of users and moves information at a high rate with small delay. High-Performance Networks A high-performance network is characterized by two performance measures ba- width and delay. Traditional network design focused mainly on bandwidth planning; the solution to network problems was to add more bandwidth. Nowadays, we have to consider message delay particularly for delay-sensitive applications such as voice and real-time video. Both bandwidth and delay contribute to the performance of the network. Bandwidth can be easily increased by compressing the data, by using links with higher speed, or by transmitting several bits in parallel using sophisticated modulation techniques. Delay, however, is not so easily improved. It can only be reduced by the use of good scheduling protocols, very fast hardware and switching equipment throughout the network. The increasing use of optical fibers means that the transmission channel is close to ideal with extremely high bandwidth and low delay(speedoflight). Theareasthatneedoptimizationaretheinterfacesanddevices that connect the different links together such as hubs, switches, routers, and bridges.