 component. These system concepts can be applied in a switch at multiple levels of abstraction depending on the architectural intentions, design goals, and overall development context. For example, if we discuss concepts that we address later in this chapter, such as the scheduling of switching, arbitration of the crossbar, or backpressure, as a means of notifying upstream logic about congestion occurring downstream, it is instructive to note the similarities with the case that we would encounter when dealing with the issues surrounding the action of decelerat- ing a vehicle. An individual can decelerate a vehicle in several ways, including the following: Taking his or her foot off the gas pedal. Hitting the brake pedal. Pulling the handbrake. Shifting the gear-box down. Driving temporarily uphill without applying more gas. Ramming into the rear of another vehicle in the front that moves slower or crashing altogether onto a massive object, tree, or wall that does not move at all. All these varied techniques undoubtedly achieve deceleration as a result, but from the vehicle designer and driver's points of view, they all tackle the problem in very different ways. Each approach involves trade-offs in efficiency and sometimes undeniable or undesirable side effects. A typical stu- dent driver does not need a degree in mechanical engineering to know how the depicted multiple sce- narios function inside the decelerating vehicle. He or she only needs basic driver training (at the car's system level) and some common sense to decelerate the car. 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 SWITCH FABRICS 265 The same thing occurs with switch fabrics. A switch user should have systems-level knowledge in order to understand the most important macro-issues. However, learning these basics does not need to be boring or frightening for the