order of a specific packet in the transmitted sequence. As a result, the intermittent network gear would know where a packet was coming from and where it was going. The packet sequence could be transmitted through switched virtual circuits or permanent con- nections. If a switch ran into problems and went out of operation, for example, another link would be set up around the affected link to reestablish connectivity. This would enable the carrier to deliver the packets to their destinations reliably. X.25 was designed with reliability in mind. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. THE EVOLUTION OF NETWORK TECHNOLOGY: DISTRIBUTED COMPUTING AND THE CONVERGENCE OF NETWORKS THE EVOLUTION OF NETWORK TECHNOLOGY 9 The OSI model has been analyzed in depth in several publications (see, for example, Radia Perlman's book Interconnections: Bridges, Routers, Switches, and Internetworking Protocols),3 SO we will not elaborate on it here. However, we will use the numbering system of its layers in numerous places in this book, SO the reader should be familiar with its fundamental premises. X.25 was a success worldwide, but its performance limit of 64 Kbps quickly became a huge imped- iment for the improved transmission of data. As a result of the ongoing semiconductor technology evolution, computers, bridges, and switches became increasingly faster. It was impossible to accept that the global network infrastructure would keep things strapped down to low speeds. This was the impetus for the next step in the evolution of networks-frame relay (FR). The reliability mechanisms of X.25 were stripped down and replaced by newer and less noisy trans- mission media (such as fiber optics). Clever bit-setting mechanisms in frames (a new formal name for the evolution of packets) were also introduced to signal advance congestion notification. These changes led t