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The evolution of the telecommunications in general and of the packet sWitching netWorks in particular is driven by many factors among Which tWo of them Worth emphasiZing: technologies and applications. Communication technologies have realiZed these last years considerable progress With: the maturing of neW transmission media and specially of optical? ber. High speed rates can noW be sustained With very loW bit error rates. the universal use of digital technologies Within private and public telecommunications netWorks. The increase in communication capacity is generating more attractive tariffs and large bandWidths are economi cally more and more attractive. On the other hand, in relation With these neW emerging technologies, many potential applications that Were not possible before are noW becoming accessible and attractive. In this environment, three generic requirements are expressed by the …