bps, then this switch fabric has an overspeed of 100 percent or an overspeed factor of 2. Most switch fabrics offer an overspeed that goes well above 60 percent. Some interesting theoretical modeling work has been done to show the advantages of 100 percent overspeed toward approaching an ideal switch design.5 Regarding the performance of a switch fabric, the industry has not yet agreed upon standard traf- fic patterns. Consequently, the measurements of throughput, delay, variations in delay (jitter), and the probability of loss are not easy to define consistently and compare systematically and objectively among different architectural approaches. Jitter is especially a concern in the voice-over-TDM realm or in the case of voice over IP (VoIP) transmitted over an ATM backbone. This is because even a slight misalignment of the jittered digitized bitstream chunks will likely compromise the integrity of time slots created for it on a trunk connection, thereby leading to potential loss of end-to-end bits, which implies poor link quality issues and consequently customer satisfaction problems. Last but not least, it is important to note that the NPF is in the process of standardizing switch fabric benchmarks. SWITCH FABRIC ARCHITECTURES The term switch fabric has been used liberally SO far. This term has no formal and universally accepted definition; however, in this book, this term refers to a network composed of interconnected switching elements that handles the reliable transfer of packets from its inputs to its outputs. Several helpful ref- 5. Balaji Prabhakar and Nick McKeown, "On the Speedup Required for Combined Input and Output Queued Switching," Computer Systems Technical Report CSL-TR-97-738 (November 1997). 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. SWITCH FABRICS 282 NETWORK PROCESSORS erences on switch fabrics are available such a