pt of buffering comes to the rescue of the systems designer SO the sys- tem can ride out this undesirable state. The inter-relationship between these seemingly unrelated and independent factors should be obvious. An interesting approach is the use of credit table mechanisms extensively, for example, by IBM (an interesting example is also discussed in Kung, H. T., T. Blackwell, and A. Chapman. "Credit- Based Flow Control for ATM Networks: Credit Update Protocol, Adaptive Credit Allocation, and Statistical Multiplexing." Proceedings of the ACM SIGCOMM '94 Conference, London, UK [August- September 1994]: 101-114). In this scheme, the fabric issues the input sources a number of credits, which are accounted for by the system. The sources are allowed to access specific output port buffers based on the number of credits they have accumulated. Each time contested resources are used, cred- its are withdrawn from the requesting source's account. Some radical action is taken in terms of assigned priorities when a source runs out of credits before it starts collecting new credits again. In order to accommodate the arbitration of conflicts and the scheduling of transfers, the switch fabric core can be designed to be much faster than the I/O capabilities of the chipset. The fabric core links that traverse the crossbar from inputs to outputs are much faster than the inputs themselves. For example, if two internally competing packets are destined for the same egress port, they will both manage to be transported fast enough through the switch fabric to their destination port where they can be sequenced for output before a new incoming packet shows up at the ingress. Most current com- mercially available fabrics offer an overspeed factor that lies between 60 and 100 percent (these are also sometimes referred to in decimal form such as a factor of 1.6 or 2, respectively). Downloaded from Digital Engineering Library McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any u